VENTURA COUNTY GENERAL PLAN
HAZARDS APPENDIX
Last Amended by the Ventura County Board of Supervisors
on
January 27, 2004
Ventura County General Plan
HAZARDS APPENDIX
2004 Decision- Makers and Contributors
Ventura County Board of Supervisors Ventura County Planning Commission
Steve Bennett First District Leo Molitor
Linda Parks Second District Nora Aidukas
Kathy I Long Third District Bill Bartels
Judy Mikels Fourth District Michael Wesner
John K. Flynn Fifth District Selma Dressler
Ventura County Planning Division
Chris Stephens, Division Manager
Bruce Smith, General Plan Section Manager
Nancy Settle, Regional Plans Section Manager
Dennis Hawkins, Project Manager
Other Ventura County Agencies
Jim O’Tousa, Public Works Agency, Development and Inspection Services Division
Dale Carnathan, Sheriff’s Department, Office of Emergency Services
Consultants
Jim O’Tousa, RJR Consultants
Chris Hitchcock, William Lettis & Associates
Lynn Kada, Planning Consultant
Word Processing Graphics
Susanna Miller Kay Clark, Graphics Supervisor
Jose Moreno
Gloria Hennety
This update was made possible by grants from the Federal Emergency Management Agency
( FEMA) and the State Resources Agency Coastal Resource Grant Program.
County of Ventura
Resource Management Agency
Planning Division
800 South Victoria Avenue
Ventura, CA 93009- 1740
( 805) 654- 2494 FAX ( 805) 654- 2509
HAZARDS APPENDIX
Table Of Contents
2.1 Introduction .................................................................................................. 1
2.2 Fault Rupture................................................................................................ 2
2.2.1 General Effects Of The Hazard................................................................................................ 2
2.2.2 General Inventory Of The Hazard ............................................................................................ 3
2.2.3 Local Resources Affected By The Hazard ............................................................................... 8
2.2.4 Definition Of Fault Hazard Zones........................................................................................... 10
2.2.5 Nature Of The Information ..................................................................................................... 11
2.2.6 Alleviation Of The Hazard ...................................................................................................... 12
2.2.7 Conclusion..................................................................................................................... ........ 12
2.3 Ground Shaking ......................................................................................... 19
2.3.1 General........................................................................................................................ .......... 19
2.3.2 Location Of The Hazard ......................................................................................................... 20
2.3.3 Conclusion..................................................................................................................... ........ 21
2.4 Liquefaction................................................................................................ 25
2.4.1 General........................................................................................................................ .......... 25
2.4.2 General Effects Of The Hazard.............................................................................................. 26
2.4.3 General Inventory Of The Hazard .......................................................................................... 27
2.4.4 Nature Of The Information ..................................................................................................... 27
2.4.5 Conclusion..................................................................................................................... ........ 28
2.5 Seiche ......................................................................................................... 31
2.5.1 Nature Of The Hazard............................................................................................................ 31
2.5.2 History Of The Hazard............................................................................................................ 31
2.5.3 Conclusion..................................................................................................................... ........ 31
2.6 Tsunami ...................................................................................................... 32
2.6.1 Local History........................................................................................................................ .. 32
2.6.2 Location Of The Hazard ......................................................................................................... 32
2.6.3 Alleviation Of The Hazard ...................................................................................................... 33
2.6.4 Ongoing Research ................................................................................................................. 35
2.6.5 Conclusion..................................................................................................................... ........ 35
2.7 Landslides/ Mudslides................................................................................ 37
2.7.1 General........................................................................................................................ .......... 37
2.7.2 Nature Of The Information ..................................................................................................... 38
2.7.3 General Inventory Of The Hazard .......................................................................................... 38
2.7.4 Conclusion..................................................................................................................... ........ 39
2.8 Subsidence................................................................................................. 44
2.8.1 Location Of The Hazard ......................................................................................................... 44
2.8.2 General Effects Of The Hazard.............................................................................................. 44
2.8.3 Nature Of The Information ..................................................................................................... 45
2.8.4 Alleviation Of The Hazard ...................................................................................................... 45
2.8.5 Conclusion..................................................................................................................... ........ 46
2.9 Expansive Soils.......................................................................................... 48
2.9.1 General........................................................................................................................ .......... 48
2.9.2 General Inventory Of The Hazard .......................................................................................... 48
2.9.3 Alleviation Of The Hazard ...................................................................................................... 48
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2.9.4 Conclusion..................................................................................................................... ........ 49
2.10 Flood Hazards ............................................................................................ 50
2.10.1 General Effects Of The Hazard.............................................................................................. 50
2.10.2 Local History........................................................................................................................ .. 50
2.10.3 Location Of The Hazard ......................................................................................................... 50
2.10.4 Alleviation Of The Hazard ...................................................................................................... 50
2.10.5 Conclusion..................................................................................................................... ........ 51
2.11 Inundation From Dam Failure ................................................................... 53
2.11.1 Nature Of The Hazard............................................................................................................ 53
2.11.2 Conclusion..................................................................................................................... ........ 53
2.12 Coastal Wave And Beach Erosion............................................................ 57
2.12.1 General Inventory Of The Hazard .......................................................................................... 57
2.12.2 Alleviation Of The Hazard ...................................................................................................... 58
2.12.3 Conclusion..................................................................................................................... ........ 60
2.13 Fire Hazards................................................................................................ 61
2.13.1 Nature Of The Hazard............................................................................................................ 61
2.13.2 General Effects Of The Hazard.............................................................................................. 61
2.13.3 Fire Hazard Management ...................................................................................................... 62
2.13.4 Emergency Response............................................................................................................ 63
2.13.5 High Fire Hazard Areas.......................................................................................................... 64
2.13.6 Conclusion..................................................................................................................... ........ 64
2.14 Transportation Related Hazards............................................................... 70
2.14.1 General Effects Of The Hazard.............................................................................................. 70
2.14.2 Alleviation Of The Hazard ...................................................................................................... 73
2.14.3 Conclusion..................................................................................................................... ........ 74
2.15 Hazardous Materials And Waste............................................................... 79
2.15.1 General Inventory Of The Hazard .......................................................................................... 79
2.15.2 Nature Of The Hazard............................................................................................................ 79
2.15.3 Hazardous Materials/ Waste Planning.................................................................................... 80
2.15.4 Conclusion..................................................................................................................... ........ 83
2.16 Noise ........................................................................................................... 85
2.16.1 Introduction................................................................................................................... ......... 85
2.16.2 Definitions.................................................................................................................... .......... 85
2.16.3 Noise Characteristics ............................................................................................................. 87
2.16.4 Noise Effects .......................................................................................................................... 88
2.16.5 Noise Criteria, Measurement And Evaluation ........................................................................ 89
2.16.6 Noise In Ventura County ........................................................................................................ 92
2.16.7 Acoustic Measurements....................................................................................................... 100
2.16.8 Noise Impacts....................................................................................................................... 102
2.16.9 Mitigation Strategies............................................................................................................. 104
2.17 Civil Disturbance...................................................................................... 130
2.17.1 General Effects Of The Hazard............................................................................................ 130
2.17.2 Nature Of The Hazard.......................................................................................................... 132
2.17.3 Alleviation Of The Hazard .................................................................................................... 134
2.17.4 Conclusion..................................................................................................................... ...... 134
Bibliography.......................................................................................................... 136
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List Of Figures
Figure 2.2.1a Earthquake Faults Map - North Half................................................................................ 14
Figure 2.2.1b Earthquake Faults Map - South Half ............................................................................... 15
Figure 2.2.2 Resources Affected By Faults And Fault Zones................................................................ 16
Figure 2.2.3a Earthquake Fault Hazard Zones Map- North Half........................................................... 17
Figure 2.2.3b Earthquake Fault Hazard Zones Map - South Half ......................................................... 18
Figure 2.3a Ground Shaking Map - North Half ...................................................................................... 23
Figure 2.3b Ground Shaking Map - South Half...................................................................................... 24
Figure 2.4a Liquefaction Areas Map - North Half .................................................................................. 29
Figure 2.4b Liquefaction Areas Map - South Half.................................................................................. 30
Figure 2.6 Tsunami Inundation Hazard Areas Map............................................................................... 36
Figure 2.7.1a Mapped Landslides Map - North Half.............................................................................. 41
Figure 2.7.1b Mapped Landslides Map - South Half ............................................................................. 42
Figure 2.7.2 Potential Earthquake Induced Landslide Areas Map....................................................... 43
Figure 2.8 Subsidence Zones Map........................................................................................................ 47
Figure 2.10 100- Year Flood Zone Map.................................................................................................. 52
Figure 2.11.1 Dams With Inundation Potential Table ............................................................................ 54
Figure 2.11.2 Dam Inundation Areas Map............................................................................................. 56
Figure 2.13.1 1952 To 2000 Fires Over 1,000 Acres ............................................................................ 65
Figure 2.13.2a Fire Hazard Map - North Half ........................................................................................ 66
Figure 2.13.2b Fire Hazard Map - South Half........................................................................................ 67
Figure 2.13.3a Fire History Map - North Half......................................................................................... 68
Figure 2.13.3b Fire History Map - South Half ........................................................................................ 69
Figure 2.14.1 Airport Hazard Zones Map .............................................................................................. 76
Figure 2.14.2a Railroads and Truck Routes Subject to Heavy Loads and Hazardous Materials Map
- North Half ............................................................................................................................... ..... 77
Figure 2.14.2b Railroads and Truck Routes Subject to Heavy Loads and Hazardous Materials Map
- South Half........................................................................................................................... ......... 78
Figure 2.16.1 Fletcher- Munson Curves Graph .................................................................................... 105
Figure 2.16.2 California Noise Control Guidelines .............................................................................. 106
Figure 2.16.3 Oxnard Airport CNEL Map............................................................................................. 107
Figure 2.16.4 Camarillo Airport CNEL Map ......................................................................................... 108
Figure 2.16.5 Point Mugu CNEL Map.................................................................................................. 109
Figure 2.16.6 Santa Paula Airport CNEL Map..................................................................................... 110
Figure 2.16.7 2010 Noise Contours Map for South Half...................................................................... 111
Figure 2.16.8 Existing And Year 2010 CNEL Noise Contours For County Roads.............................. 112
Figure 2.16.9 Year 2010 CNEL Noise Contours for Proposed Highway Links ................................... 119
Figure 2.16.10 Existing Noise Levels at Nine Selected Locations Graphs ......................................... 120
Figure 2.16.11 Existing R. R. Train Noise at First St., Simi Valley Graphs .......................................... 129
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2. HAZARDS APPENDIX
2.1 Introduction
The purpose of this appendix is to provide additional background information and technical details
regarding individual hazards addressed in the General Plan Goals, Policies and Programs. The
physical, social and other effects of the hazards are discussed, and more detailed information is
provided regarding the location of hazards zones and areas. Where appropriate, the factors that
cause or produce certain hazards are discussed. In the case of geologically related hazards, the
tectonic and other geologic forces that produce such hazards are described. Information regarding
the local history of specific hazards is included where appropriate, and any pertinent ongoing
research is also mentioned. Measures used or recommended for alleviation of individual hazards
are also included. In some cases, table and other data, as well as copies of pertinent documents,
have also been appended.
The following hazards are discussed in this appendix, in the order shown:
2.2 Fault Rupture
2.3 Ground Shaking
2.4 Liquefaction
2.5 Seiche
2.6 Tsunami
2.7 Landslides/ Mudslides
2.8 Subsidence
2.9 Expansive Soils
2.10 Flood Hazard
2.11 Inundation from Dam Failure
2.12 Coastal Wave and Beach Erosion
2.13 Fire Hazards
2.14 Transportation Related Hazards
2.15 Hazardous Materials and Waste
2.16 Noise
2.17 Civil Disturbance
The first five hazards in the above list are so grouped because they all tend to be seismically
related, although it should be recognized and understood that some hazards not associated with
earthquakes may be triggered or exacerbated by earthquakes.
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2.2 Fault Rupture
An earthquake resulting in catastrophic effects, having a magnitude ( M) of 8.3 on the south- central
San Andreas fault is likely before the end of the twenty- first century and is estimated to have a
current annual probability of occurrence between two and five per cent. It is based on a repeat
occurrence of the great Ft. Tejon earthquake of January 9, 1857, and other geophysical
observations. As geologists know, at least eight major earthquakes have occurred in this area,
with an average spacing in time of 140 years, plus or minus 30 years. New faults within the region
are continuously being discovered. Scientists have identified almost 100 faults in the Los Angeles
area known to be capable of a magnitude 6.0 or greater earthquake. The January 17, 1994
magnitude 6.7 Northridge Earthquake ( thrust fault), which produced severe ground motions,
caused 57 deaths, 9,253 injuries and left over 20,000 displaced. Scientists have stated that such
devastating shaking should be considered the norm near any large thrust earthquake.
Recent reports from scientists of the U. S. Geological Survey and the Southern California
Earthquake Center say that the Los Angeles Area could expect one earthquake every year of
magnitude 5.0 or more for the foreseeable future.
A major earthquake occurring in or near this jurisdiction may cause many deaths and casualties,
extensive property damage, fires and hazardous material spills and other ensuing hazards. The
effects could be aggravated by aftershocks and by the secondary affects of fire, hazardous
material/ chemical accidents and possible failure of the waterways and dams. The time of day and
season of the year would have a profound effect on the number of dead and injured and the
amount of property damage sustained. Such an earthquake would be catastrophic in its affect
upon the population and could exceed the response capabilities of the individual cities, Los
Angeles County Operational Area and the State of California Emergency Services. Damage
control and disaster relief support would be required from other local governmental and private
organizations, and from the state and federal governments.
Extensive search and rescue operations would be required to assist trapped or injured persons.
Injured or displaced persons could require emergency medical care, food and temporary shelter.
Identification and burial of many dead persons would pose difficult problems; public health would
be a major concern. Mass evacuation may be essential to save lives, particularly in areas
downwind from hazardous material releases. Many families would be separated particularly if the
earthquake should occur during working hours, and a personal inquiry or locator system could be
essential to maintain morale. Emergency operations could be seriously hampered by the loss of
communications and damage to transportation routes within, and to and from, the disaster area
and by the disruption of public utilities and services.
The economic impact on the County of Ventura from a major earthquake would be considerable in
terms of loss of employment and loss of tax base. Also, a major earthquake could cause serious
damage and/ or outage of computer facilities. The loss of such facilities could curtail or seriously
disrupt the operations of banks, insurance companies and other elements of the financial
community. In turn, this could affect the ability of local government, business and the population to
make payments and purchases.
2.2.1 General Effects Of The Hazard
Nearly all man- made structures are susceptible to damage ranging from severe to total when
affected by displacement along faults passing beneath their foundations. The San Fernando
Earthquake of 1971 has shown that no structures designed under present standards are safe from
severe damage or destruction as a result of surface fault displacement of foundations. It is widely
acknowledged that design of most structures, such as single- family homes or larger structures,
roads, bridges, pipelines, or other conduits, to resist fault displacement is generally not feasible.
Only massive earth structures such as earth fill dams can be designed to remain functional after
several feet of displacement along an underlying fault.
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Permanent effects of surface displacement along faults also can include:
1. Abrupt elevation or depression of ground surfaces of several feet for distances of many
hundreds of feet along the fault;
2. Disruption of surface drainage;
3. Changes in groundwater levels in wells;
4. Blockage and surface seepage of groundwater flow;
5. Changes in survey benchmark elevations;
6. Dislocations of street alignments and property lines of many feet if lateral ( horizontal)
displacement also occurs along a fault;
7. Displacement of drainage channels and drains.
Secondary effects of surface displacements along faults within an urban area could include:
1. Disruption of movement along roadways due to abrupt depressions or elevation of
pavement surfaces;
2. Possible flooding due to disruption of drainage channel and storm drain flow;
3. Disruption of utility services such as water, gas, fuel, telephone and electric power lines;
4. Temporary impact on industry and commerce similar to that resulting from the occurrence
of most kinds of regional natural catastrophic events such as hurricanes or floods.
2.2.2 General Inventory Of The Hazard
The State Division of Mines and Geology ( CDMG) 1 indicates that on a statewide basis the potential
hazard to structures from the surface displacement of faults is low compared to other geologic
phenomena such as earthquake ground shaking and landslides ( Urban Geology Master Plan for
California, 1973, Bull. 198).
Although questions remain regarding distributed, aseismic deformation, moment balance and slip
rate calculations indicate that most of the motion in the upper 15 kilometers of the Southern
California plate boundary zone occurs as earthquakes on active faults ( Stein, R. S. & Hanks, T. C.,
1999). Major structural losses due to fault displacement within Southern California occurred during
the San Fernando Earthquake of 1971. As a result of these losses, the enactment of the Alquist-
Priolo Earthquake Fault Zoning Act occurred in 1972. Structural losses due to fault displacement in
other major earthquakes in California are unknown but were probably small. Most of the losses
incurred during the 1994 Northridge earthquake were a result of ground shaking. The Northridge
earthquake was a surprise to geologists in two ways: first, it occurred on a previously unknown
thrust fault and second, it produced virtually no surface rupture.
The greatest potential for fault rupture is along any of the active faults that lie within the several
major fault systems that transect Ventura County from east to west. The 1971 San Fernando
Earthquake and the previously unknown blind thrust fault responsible for the 1994 Northridge
earthquake occurred along two of these major fault systems and illustrate the high level of activity
that some faults within these systems may have, and they also suggest a potential for the
occurrence for other such earthquakes in the Los Angeles and Ventura regions.
The San Fernando earthquake of 1971 is an example of the typical surface rupture that could occur
along some of the east- west trending faults that transect Ventura County. It is most likely that
surface fault displacement within the County will be sudden, occurring in less than one minute.
1 The CDMG has recently changed its name to the California Geological Survey ( CGS) and for the purposes
of this appendix CDMG and CGS are the same State Agency and references to CDMG and CGS are
dependent on the date of the reference relative to the name change date.
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The surface displacement would be accompanied by severe ground shaking lasting perhaps
several tens of seconds.
The Northridge earthquake of 1994 resulted in a different type of surface rupture resulting from
ground deformation. Blind thrust faults, so- called because they lack surface fault rupture, may be
accompanied by fault- propagation folds or fault- bend folds. Fault- propagation folds are folds within
the rock that are caused by the growth of buried fault( s) toward the surface. Fault- bend folds are
folds within the rock that are caused by the change in inclination of the fault surface. Blind thrust
faults have gained recognition as damaging earthquake sources within the Southern California
region since the 1987 Whittier Narrows earthquake and have been highlighted by the 1994
Northridge earthquake.
Many of the faults in the County are associated with major fault systems extending beyond County
boundaries. For example, the recent 1994 Northridge earthquake in the San Fernando Valley, Los
Angeles County, is interpreted to be an eastward continuation of the Oakridge Fault from the
Ventura Basin. The Ventura Basin is considered a large syncline ( trough) that extends east- west,
from the San Gabriel Mountains to the Pacific Ocean. The portion of the basin east of the San
Gabriel Fault is referred to as the Soledad Basin. The Ventura Basin is geologically noted for a
remarkably thick section of marine sedimentary rocks that total more than 58,000 feet. The axis of
the trough generally coincides with the Santa Clara River valley and the offshore Santa Barbara
Channel. The northern boundary of the basin is the Santa Ynez Fault and the southern boundary
is the Simi Hills, Mountclef Ridge, and along the western edge of the Santa Monica Mountains into
the Pacific Ocean. Several of the fault systems, and blind thrust systems are considered active,
but additional information must be assembled to determine the potential for, as well as the nature
of, activity of most of the faults systems within Ventura County.
Unfortunately, the majority of new earthquake information is developed following an earthquake
with the coordination of earthquake science in Southern California by several government agencies
and the Southern California Earthquake Center ( SCEC). These data will provide consistent
scientific judgments for public policy decisions in earthquake risk management. The present level
of knowledge of the recency of surface or near surface movement along the faults and fault
systems within Ventura County does not provide sufficient data on which to base a determination of
the " degree" of fault activity. There is some evidence that some of the known faults have displaced
at least late Quaternary terrace sediments, indicating possible movement younger than 11,000
years ago. This is the primary basis for designating the faults as “ active”, as these could have the
higher potential of future surface rupture.
All of the fault designations within the County are subject to change as further evidence is received,
providing either clearer proof of potential for activity or convincing geologic evidence of inactivity.
The Simi- Santa Rosa, Springville, and Camarillo faults have been zoned as “ active” under the
State of California Alquist- Priolo Earthquake Fault Zoning Act. The following is a description of the
major active and potentially active faults and fault systems within Ventura County ( also see Figures
2.2.1a and 2.2.1b).
Malibu Coast Fault System
The Malibu Coast Fault system is considered as the southern boundary of the Transverse Ranges
and this fault system includes the Malibu Coast, Santa Monica and Hollywood Faults. This fault
system is believed to consist of a series of major north- dipping reverse or thrust faults that extend
from offshore along the southern Ventura County coast and onshore in Los Angeles County for a
total of over 40 miles and perhaps a much greater distance offshore in the Santa Barbara Channel.
It begins in the Hollywood area and extends along the southern base of the Santa Monica
Mountains and passes offshore a few miles west of Point Dume.
Geologic evidence for activity of the fault system during recent geologic time up through the
present are faulted terrace and near surface sedimentary deposits, and the 1973 Point Mugu
earthquake, which is believed to have originated on this fault system.
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The fault system is an oblique left lateral reverse fault with a slip rate of 0.1 to 0.5 mm/ yr, a
potential maximum moment magnitude of 6.7 and a recurrence interval of 2908 years ( CGS, 1996).
The faults within this system are considered active. This fault is zoned by the State of California as
active and details pertaining to the fault designation may be obtained by reviewing the State of
California Division of Mines and Geology, Fault Evaluation Report FER 229 ( Treiman, 1994) for
onshore portions of the fault within Los Angeles County.
Simi- Santa Rosa Fault System
This fault system extends from the Santa Susana Mountains westward along the northerly margin
of the Simi and Tierra Rejada Valleys, along the south slope and crest of the Las Posas Hills to
their westerly termination. The presence of the Springville and Camarillo Faults, short distances to
the north and south, respectively, of the westerly projection of Simi- Santa Rosa Fault could be
considered branches of the Simi- Santa Rosa Fault and project into the Oxnard Plain along the
same trend.
Surface evidence north of Simi Valley and within the Santa Rosa Valley indicates that this fault has
been active during Holocene time ( 0 to 11,000 years before present). This fault is zoned by the
State of California as an active fault.
The Simi- Santa Rosa fault system is a reverse fault with a slip rate of 1.0 mm/ yr, a potential
maximum moment magnitude of 6.7 and a recurrence interval of 933 years ( CDMG, 1996).
This fault is zoned by the State of California as active and details pertaining to the fault designation
may be obtained by reviewing the State of California Division of Mines and Geology, Fault
Evaluation Report FER 244 ( Treiman, 1998).
Bailey Fault
This fault marks the boundary between the western Santa Monica Mountains and the Oxnard Plain.
It extends from the Mugu Lagoon area northerly to an apparent intersection with the Camarillo
Fault near Calleguas Creek and State Highway 101. The presence of the fault is based primarily
upon water well data.
No evidence of surface expression of the fault is known nor have any earthquakes been recorded
as having originated on it. The fault trace is obscured by geologically young alluvium over its entire
length. Available information is insufficient to conclude that the fault has not been active during
Pleistocene or more recent time.
The fault is considered as potentially active until more information is available for evaluation.
Information pertaining to this fault is anticipated to be difficult to obtain because the depth of the
Oxnard Plain surficial sediments.
Camarillo Fault
This fault extends in an east- west direction immediately south of Camarillo from Calleguas Creek to
Camarillo Airport. The present of the fault is based primarily upon the abrupt uplifted sediments
along the north side of the fault that resulted in the long linear hill of the southern portion of
Camarillo just south of the 101 Freeway.
The apparent uplift along the north side of the fault is believed to be a surface expression of fault
propagation folds. The fault surface trace, however, is obscured by geologically young alluvium
over its entire length.
Sycamore Canyon and Boney Mountain Faults
These faults are the most prominent of a series of northeast trending breaks extending from the
Point Mugu and south coast area to the Thousand Oaks area. Surface evidence of displacement
of sedimentary and volcanic rocks of Miocene age indicates that these faults have been active after
the formation of these rocks. Rocks younger than Miocene age are not known to have been
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displaced by the faults. However, no detailed specific investigations have been conducted for the
purposes of addressing the activity of these faults. Special areas of concern would be in the
Potrero, Conejo, and Hidden Valleys and the Thousand Oaks area.
The faults are considered as potentially active until more information is available for evaluation.
Oak Ridge Fault System
The Oak Ridge Fault System is a steep ( 65- degrees) southerly- dipping reverse fault that extends
from the Santa Susana Mountains where it has been overridden by the north- dipping Santa
Susana Thrust Fault, westward along the southerly side of the Santa Clara River Valley and thence
into the Oxnard Plain. The relationship of possible westerly extension of the fault to the McGrath
and offshore faults is unclear and may be complex. None of the faults beyond the westerly
terminus of South Mountain have surface expression nor have any been shown to cut near- surface
sediments ( alluvium). It is conceivable that past movement of these faults in the Oxnard Plain area
has not resulted in surface displacements but, instead, has resulted in only broad warping or tilting
of the near- surface alluvial sediments. The lack of surface evidence of fault displacement in the
Oxnard Plain is not necessarily indicative of activity in the recent geologic past as surface features
could easily have been obscured by fluvial processes ( erosion or deposition of alluvium). Several
recorded earthquake epicenters in the offshore as well as mainland area during historic time may
have been associated with the Oak Ridge Fault System or within close proximity and associated
with it.
The Oak Ridge Fault System probably contains many branching faults and is believed to be
associated with one or more faults of similar trend present in the Santa Barbara Channel west of
the Oxnard Plain. The system is over 50 miles long on the mainland and may extend an equal or
greater distance offshore.
The rugged, steep terrain of the north slope of South Mountain as well as displacement of young
alluvial sediments indicates that portions of the Oak Ridge Fault System are active.
The Oak Ridge Fault System is a reverse fault with a slip rate of 4.0 mm/ yr, a maximum moment
magnitude of 6.9 and a recurrence interval of 299 years ( CDMG, 1996).
The faults within this system are considered active. Portions of this fault are zoned by the State of
California as active and details pertaining to the fault designation may be obtained by reviewing the
State of California Division of Mines and Geology, Fault Evaluation Report FER 54 ( Smith, T. C.,
1977 and FER 219 and 2 Supplements dated 1998 and 1999 ( Treiman, 1990).
Ventura and Pitas Point Faults
The Ventura- Pitas Point Fault extends along the base of the foothills on the north side of the Santa
Clara River from Santa Paula westerly to the mouth of the Ventura River, thence westerly into the
Santa Barbara Channel area. The fault is a north dipping reverse fault.
Evidence for the existence of the Ventura Fault is based mainly upon minor faulting of terrace
deposits north of San Buenaventura and evidence of faulting from the Tidewater Oil Company
corehole # 5. The fault is believed to be north- dipping.
The Ventura- Pitas Point fault system is an oblique left lateral reverse fault with a slip rate of 1.0
mm/ yr, a maximum moment magnitude of 6.8 and a recurrence interval of 1,112 years ( CDMG,
1996).
San Cayetano- Red Mountain- Santa Susana Fault System
This fault system consists of a major series of north- dipping reverse faults that extend over 150
miles from Santa Barbara County into Los Angeles County.
The San Cayetano fault is a major, north dipping reverse fault that extends for 40 km along the
northern portion of the Ventura Basin. The fault has been mapped in detail both at the surface and
in the subsurface and can be separated into two sections defined by a right step in the fault zone
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near the City of Fillmore. The eastern section surface trace of the San Cayetano fault dies out
several km east of the Town of Piru, where the details regarding the mechanics of slip transfer are
unclear, but slip may be transferred onto the Santa Susana Fault ( Dolan and Rockwell, 2000). The
western surface trace lies well above the base of the slope of the Sespe Mountains and the surface
trace ends just east of the City of Ojai. The western section may transfer slip onto the Sisar blind
thrust fault and then ultimately to the Red Mountain fault.
Geologic evidence that each of the fault systems are considered active throughout their length is
shown by location of earthquake epicenters ( including the San Fernando Earthquake of 1971),
groundwater barriers, and displaced alluvial sediments. In addition, age determinations from
detrital charcoal recovered from faulted sections of the San Cayetano fault indicate surface rupture
occurred after A. D. 1660 ( Dolan and Rockwell, 2000). This event was on the eastern section of the
fault and generated at least 4.3 m of surface displacement.
The Red Mountain Fault, San Cayetano, and Santa Susana fault systems are zoned by the State
of California as active faults.
The Red Mountain fault system is a reverse fault with a slip rate of 2.0 mm/ yr, a maximum moment
magnitude of 6.8 and a recurrence interval of 507 years ( CDMG, 1996).
The San Cayetano fault system is a reverse fault with a slip rate of 6.0 mm/ yr, a maximum moment
magnitude of 6.8 and a recurrence interval of 150 years ( CDMG, 1996). Recent studies ( Dolan,
J. F. and Rockwell, T. K., 2000), suggest the San Cayetano fault system is capable of producing a
moment magnitude greater than 7 with reverse slip rates over the past million years of 10- 12
mm/ yr.
The Santa Susana fault system is a reverse fault with a slip rate of 5.0 mm/ yr, a maximum moment
magnitude of 6.6 and a recurrence interval of 138 years ( CDMG, 1996).
Lion Mountain- Big Canyon- Sisar Fault System
These faults and several others present in the eight- mile gap between the Red Mountain and San
Cayetano Faults dip southerly beneath Sulphur Mountain. The general area is complexly broken
and folded by faulting which may be associated with the high fluid pressures present in the Ventura
Oil Field to the south.
Although the general area of these faults has not experienced earthquake activity during historic
time, their position within the San Cayetano- Red Mountain- Santa Susana Fault System and the
possible displacement of terrace deposits ( Pleistocene time) indicates that they should be
considered at least potentially active.
Mission Ridge- Arroyo Parida- Santa Ana Fault System
This fault system extends from Montecito, Santa Barbara County, to the Ventura River and
probably along the south side of Ojai Valley, Ventura County.
Although no earthquake activity has been recorded during historic time, the fault does apparently
form a groundwater barrier in the alluvium beneath the Ventura River. On this basis, it should be
considered potentially active. Future information may require reclassification.
Santa Ynez Fault
This fault extends from Point Conception in Santa Barbara County, across the central portion of
Ventura County, to near the east County line. It is considered to be one of the major faults in the
region and is about 90 miles long. Past displacement has been about 10,000 feet of relative
uplifting of the south side of the fault. The fault lies about 4 miles north of Ojai.
Left lateral displacement of streams crossing this fault has been cited as evidence for recent fault
movement. Several earthquake epicenters have been located along this fault and one or two of
these were in Ventura County. The 1927 earthquake centered west of Point Conception may have
originated on the westerly, offshore extension of this fault.
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This fault is considered potentially active until additional information is available for evaluation.
Faults Between the Santa Ynez and North County Line
Several large faults occur in the mountainous area north of the Santa Ynez Fault and within
Ventura County. The most significant of these faults are the Tule Creek, Munson Creek, Agua
Blanca, Frazier Mountain and Big Pine Faults. Of these the more important appear to be the Pine
Mountain Thrust and Big Pine Faults ( 9 and 16 miles north of Ojai, respectively). The Pine
Mountain Thrust is north- dipping and favorably oriented for generating earthquakes in response to
the north- south compressive forces which have triggered activity along such similar faults as the
Malibu, San Fernando and San Cayetano.
Terrace deposits and stream channels have been offset by geologically recent movement along the
Big Pine Fault. More importantly, it is reported to have ruptured the ground surface for a distance
of 30 miles along its length during the northern Ventura County earthquakes of November 1852.
Both of these faults are considered active. The rest of the faults in the north half are in the
Potentially Active Fault Hazard Zone.
San Andreas Fault
The San Andreas is the longest and most important fault in California. Due to clearly established
historical earthquake activity, this fault has been designated as active by the State Division of
Mines and Geology. The last major earthquake on this fault near the County was in 1857. The
earthquake is estimated to have been on the order of magnitude 8.0 ( Richter Scale) and would
have caused considerable damage if there had been structures in the southern County area. The
occurrence of another such major earthquake along this fault is considered possible within the near
future.
2.2.3 Local Resources Affected By The Hazard
Movement along faults may substantially impact the unincorporated areas of the County. The
effects are summarized in the accompanying table ( Figure 2.2.2), following the text. All
transmission lines from power sources cross or enter into at least one fault zone. In addition, all oil
and gas transmission pipelines, fiber optic cables, telephone transmission lines, some cellular
tower antennas, and oil and gas processing and production facilities are within or cross fault zones.
One school is in the Mission Ridge- Arroyo Parida- Santa Ana Fault Zone, as is the water
transmission line from Lake Casitas to Ventura and the Ojai area, which also crosses the fault
zone. Sewer mains in the Meiners Oaks area and in Villanova Road are also located in the fault
zone.
The Red Mountain- Padre Juan Fault System extends from Highway 33 near Canada Larga Road
to the north coastal area of the County into the Pacific Ocean and Santa Barbara County. The
coastal communities of La Conchita and Solimar are located along this zone, as is a county fire
station on Highway 101 that is very close to or on the Padre Juan fault. Power transmission lines
going to Santa Barbara County cross this fault zone as do many major gas lines, oil pipelines, fiber
optic cables, oil and gas processing and production facilities, communication towers, water
transmission pipes, sewer mains, Highway 101 and the Ventura- Santa Barbara road lines.
North of Red Mountain- Padre Juan Fault System extends another secondary potentially active fault
zone encompassing a portion of Highway 33, power transmission lines to Santa Barbara County
and their associated substations in the area, gas mains, oil and gas pipelines and storage tanks,
water transmission lines and sewer mains between Ventura and Ojai.
The Lion Mountain Fault Zone is located between Lake Casitas and the San Cayetano fault zone
east of Ojai. This zone contains the major portion of the Oak View Community including Oak View
and Sunset schools and a County fire station. Major electrical transmission lines, gas mains, water
transmission lines, telecommunication lines and towers, and sewer mains between Oak View and
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Ojai transverse this zone. The major gas transmission line from Kern County also crosses this fault
zone.
The San Cayetano Fault Zone is located between Ojai and the Los Angeles County Line extending
north of Santa Paula and Fillmore and entering into portions of Piru. Summit and Piru Schools are
located in the fault zone as well as a fire station near Summit School on Highway 150 and portions
of Highway 150 itself. The State Fish Hatchery east of Fillmore is also within the hazard zone.
Main power lines from Los Angeles County to the Santa Clara Transmission Station enter into this
fault zone, as well as gas mains, water transmission lines, and sewer mains east of the cities of
Ojai and Fillmore that are in the hazard zone. Portions of Highway 126 near Piru are also located
along the fault. There are major oil/ gas pipelines, processing, production, and storage facilities
within this zone. Telecommunication lines and towers are also nearby.
The Big Canyon- Sisar Fault System extends west of the San Cayetano Fault Zone, south of
Highway 150. Because it is located in hilly portions of the County, resources affected by this fault
hazard include oil/ gas pipelines and production, water and communication transmission lines.
Few major resources are located in the Santa Susana Fault Zone, which is between the Oak Ridge
Fault Zone and the Los Angeles County Line. However, major electrical transmission lines from
Los Angeles County to Moorpark and a 34" gas main penetrates the fault zone. There are also oil
and gas pipelines, production, processing and storage facilities near this zone.
The Simi Fault Zone extends from the Los Angeles County Line, north of Simi Valley and ends up
in the Virginia Colony area. A portion of a housing development is located within the fault zone as
well as major power lines, gas mains north of Simi Valley, a water transmission line to Moorpark
and a sewer main west of Simi Valley. Also, the Simi Valley Adventist Hospital is located within the
fault zone. There is a sewer treatment plant and landfills within the area as well as communication
lines and towers, and utilities ( water, sewer, and gas).
No major resources are located within the Canada de la Brea Fault Zone, which reaches out from
the Simi Fault. However, power transmission lines, an 8" gas main and a sewer line do enter into
an extension fault zone of the Canada de la Brea fault. The extension of a landfill is also planned
into this fault zone.
Another fault exists between Moorpark and Fillmore with Highway 23 bisecting the zone. Besides
the roadway, only a six- inch gas line is located within the fault zone.
The Sycamore Canyon Fault rises from the Pacific Ocean near Mugu Lagoon, through the Santa
Monica Mountains to Newbury Park, past the 101 Freeway into Thousand Oaks. Going through
the Newbury Park area, four schools plus several utility facilities such as power lines to Lake
Sherwood, gas mains near the 101 Freeway and Lynn Road, water transmission lines,
communication lines and towers, and sewer mains for housing developments are located in the
fault zone.
The eastern extension breaks from the Sycamore Canyon Fault south of Borchard Road, touching
Ventu Park Road and continuing on past the 101 Freeway further into Thousand Oaks. Except for
the general urban area that is located within the fault, there appear to be no major resources
located within the area.
No major resources except for a power line to the Thousand Oaks sub- station appear to lie in the
Boney Mountain Fault Zone, which is located south of the eastern extension of the Sycamore
Canyon Fault. Utility lines ( water, gas, sewer) and communication lines are located in this area.
The Bailey Fault Zone extends from Mugu Lagoon towards Camarillo and intersects the Santa
Rosa fault zone east of that city. Major power lines from Ormond Beach power plant, as well as
gas lines, water transmission pipes, and the Camrosa Wastewater Treatment Plant mains east of
Camarillo are situated within the fault zone.
An extension fault zone is located due east of the Bailey Fault through the California State
University, Channel Islands, ( formerly Camarillo State Hospital), into surrounding hills.
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The Santa Rosa Fault Zone starts east of the Springville Fault at Las Posas Road, following the
north side of the Santa Rosa Valley and along the north side of Simi Valley to meeting the Santa
Susana Fault at the County of Ventura Line. Within the unincorporated areas, major power lines
from Ormond Beach power plant as well as utility ( gas, water, sewer) mains, and communication
lines and towers are located in this fault zone.
The Springville Fault Zone extends along the base of the Camarillo Hills north of Camarillo, through
Oxnard to the Pacific Ocean. Near Camarillo, two schools are located within the fault zone as well
as the Mandalay Power plant in Oxnard. Utility ( water, sewer, gas) and electric lines cross the fault
zone. Oil/ gas production and storage facilities and water storage tanks are located near the zone.
The Oak Ridge Fault basically follows the Santa Clara River bed with a southern deviation near
Fillmore. Due to this, utilities such as power, water, gas and sewer lines are resources that cross
the zone. Oil/ gas production, processing, and storage facilities are located nearby.
Communication lines and towers are also within the fault zone.
The accompanying table ( Figure 2.2.2), " Resources Affected by Faults and Fault Zones,"
summarizes the information presented above.
2.2.4 Definition Of Fault Hazard Zones
The " fault hazard zones" define a boundary where active or potentially active faults are believed to
be located.
Locally Identified Fault Hazard Zones
The Fault Hazard Zones identified by the Alquist- Priolo Earthquake Fault Zoning Act are
designated ” active” by the State of California and based on available geologic mapping and
judgment of the County Engineering Geologist. Other faults and extensions of the active faults are
shown on Figures 2.2.1a and 2.2.1b.
The extent of Fault Hazard Zone boundaries are controlled by the traces of active faults that are
based on the best data available at the time the map was compiled ( January, 2002). However, the
faults shown on the maps were not field- checked during the map compilation.
In many places the zone boundaries have been tentatively extended beyond the mapped limits of
faults, such as occurs westerly of Camarillo and westerly of Saticoy. These zone extensions are
considered necessary because, even though faults have not been mapped in these areas, it is
considered likely that extensions of known faults or branches of faults do extend into these area.
Future investigation or studies would be required for confirmation of any fault extensions.
The earthquake fault hazard zone designates areas that are believed to contain active faults. The
potentially active fault hazard zones include those faults for which less evidence is available
concerning their potential for activity, however, their approximate surface trace has been mapped.
No degree of relative potential for future surface displacement or degree of hazard is implied for the
faults shown.
A " fault" is defined as a fracture or zone of closely associated fractures along which rocks on one
side have been displaced with respect to those on the other side. Most faults are the result of
repeated displacement that may have taken place suddenly and/ or by slow creep. A " fault zone" is
a zone of related faults that commonly are braided and subparallel, but may be branching and
divergent. It has significant width ( with respect to the scale at which the fault is being considered,
portrayed, or investigated), ranging from a few feet to several miles.
State Special Studies Zones
In 1972, the California State Legislature enacted to the Alquist- Priolo Special Studies Zones Act.
Pursuant to this act, the " State Geologist shall delineate... special studies zones to encompass all
potentially and recently active traces of the San Andreas, Calaveras, Hayward, and San Jacinto
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Faults, and such other faults...( that) constitute a potential hazard to structures from surface faulting
or fault creep."
The Act also requires the State Geologist to compile maps of special studies zones and submit
them to local jurisdictions. The State Earthquake Fault Hazards Zones are shown on Figures
2.2.3a and 2.2.3b. Special Studies Zones are delineated on topographic base maps at a scale of
1: 24,000 ( 1 inch equals 2000 feet).
Faults other than those depicted on the maps may be present within the Special Studies Zones.
The zone boundaries delimit the area that the State Geologist believes warrants special geologic
investigations to detect the presence or absence of active faults.
The State of California Earthquake Fault Hazard Maps are on file in the County Planning Division.
The intent of the zone is to provide for public safety from the hazard of fault rupture by avoiding, to
the extent possible, the construction of structures for human occupancy across active faults. The
California Geologic Survey has adopted policies and criteria for development within these zones.
The complete text of the Policies and Criteria is included herein. Its most significant criterion is that
no habitable structure may be built across the trace of an active fault. Furthermore, the area within
fifty feet of an active fault shall be assumed to be underlain by active branches and therefore,
before any structure can be built within the zone, a geologic investigation and submission of a
report by a Registered Geologist in the State of California is required. In addition, the County may
impose more restrictive policies. Upon approval of the report, the County is required to submit a
copy of the report to the State Geologist.
Uses and Limitations of the Earthquake Fault Hazard Zones
The best use of the fault zones is to define areas where special geologic studies would be required
prior to building structures for human occupancy. Such a criterion may require a developer or
builder to evaluate specific sites within the zone to determine if a potential hazard from any fault
exists with regard to proposed structures.
Such studies are required for Earthquake Fault Hazard Zones. The County Geologist may require
fault evaluation based on more recent data or for fault areas for which little information is presently
known. Future studies could result in the designation of some of these areas to active fault zones.
Users of the maps should be fully aware that the zones are delineated to define those areas within
which special studies may be required prior to building structures for human occupancy. Traces of
all faults are shown on the maps mainly to justify the locations of zone boundaries and to provide
fault data beyond zone boundaries. These fault traces are plotted as accurately as the sources of
data permit; yet the plots are not sufficiently accurate to be used as the basis for setback
requirements.
The fault information shown on the map is not sufficient to meet the requirement for special studies.
It is the County's responsibility to require the developer to evaluate specific sites within the
Earthquake Fault Hazard Zones to determine if a potential hazard from fault rupture, exists with
regard to proposed structures and their occupants.
Fault studies should, however, continue to be made of any fault suspected of recent activity or to
be an extension off faults already zoned prior to approval of any individual residential or other
permanent developments which may be proposed over or in the near vicinity of this fault.
2.2.5 Nature Of The Information
The geologic information relating to the location of faults and their potential for activity is based
largely upon regional geologic studies conducted by Universities, Petroleum and Engineering
Geologists, as well as information compiled by the California Geological Survey and the County
Department of Public Works.
The evaluative system utilized in estimating the potential or past activity of individual faults and
fault systems is discussed under " General Inventory of the Hazard." The basis and method of
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designation of the Earthquake Fault Hazard Zones is similar to that used by the California
Geological Survey in establishing the Earthquake Hazard Zones along active faults within the
State.
Research and experience dealing with the nature and mechanism of faults movement and fault
activity is being conducted by various Federal and State agencies as well as by universities and
professional organizations. Much of this work is being conducted on a statewide basis; however,
indirect benefit to Ventura County will be gained through developed technology. Additional
investigation is being conducted on a continuing basis by:
• Private Geologic Consultants who conduct and provide original information during
investigations for public and private developments.
• Ventura County Public Works Agency that provides review and evaluation of Geologic and
Soils and Foundation Engineering reports prepared for private projects within the
unincorporated area of the County.
Presently, there is no way to prevent or accurately predict when an earthquake and surface
displacement may occur along a fault. The state of the art is such that at best only the recency of
past activity can be determined along some faults. In the southern California area, those faults that
have general east- west trends or are associated with the northwesterly- trending San Andreas Fault
are considered the most active.
2.2.6 Alleviation Of The Hazard
Alleviation of the hazard is largely accomplished through land use controls. The agencies,
departments and legislative bodies making land use decisions have the primary responsibility for
alleviating the hazard. Decisions concerning adoption of these recommendations within the
unincorporated areas of the County of Ventura rest ultimately with the Planning Commission and
the Board of Supervisors. Other bodies making land use decisions include Port Districts and their
associated cities, redevelopment agencies, and special districts.
Alleviation of existing hazards can be effected by removal of structures located over active faults.
Determination of whether structures are hazardously located would require detailed investigation of
geologic conditions and of the potential for activity along any faults found.
Present information is not considered sufficiently accurate to warrant special investigation for most
existing development. Consideration should, however, be given to reconfirming the safety of
critical facilities, including public structures and those where large numbers of people may gather,
where such facilities are over or near known faults. Future, more detailed information on fault
locations may indicate that further evaluation of some existing structures or facilities is warranted.
Structures which could be considered for evaluation include hospitals, rest homes, churches, large
commercial buildings, important industrial structures, schools, residential or commercial buildings
over two stories in height and critical utility facilities.
2.2.7 Conclusion
Available geologic information indicates that the potential for the occurrence of surface
displacement along one or more of the major east- west trending faults within the County and within
the life of existing structures is high compared to the potential hazard Statewide. Major
development along most of the east- west faults within the County should be carefully considered
until such time as adequate information is available to conclude that such faults are not active or
potentially active.
Experience has shown that when sudden surface displacement occurs along faults, structures
located over those faults are almost totally destroyed. Although the hazard is considered real
within the County, the effect of the hazard is low compared to the likelihood of greater losses that
could occur as a result of strong ground shaking.
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In the event of surface displacement along a fault transecting one of the urbanized or industrialized
areas of the County, loss of life and property damage could occur both in the unincorporated and
incorporated areas.
Much of the existing land development occurred many years ago, before the full potential danger of
concealed or obscure faults was recognized and, therefore, little subsurface investigation of
geologic conditions was conducted. In general, little is known of the recency of past movement
along most of the faults within the County, or whether any related fault branches may be present.
Several recent investigations for private development in the vicinity of some of the faults have
indicated no fault disturbance of near- surface earth materials. Nonetheless, future investigations
could however reveal that some segments and branches, or extensions of faults within the zones
are active.
The information in this section has been updated since the original information was compiled. This
section reflects current knowledge, as of 2002, particularly with respect to precise fault locations
and potential for activity. This section will, however, be updated periodically as part of future
updates of the Hazards Appendix. Additionally, fault maps will be updated periodically as new data
from future investigations is obtained, future earthquakes occur, and the California Geological
Survey issues revised maps. The policies of the State Earthquake Fault Zoning Act, in conjunction
with available State and local fault information, are considered an adequate method to reduce fault
rupture hazards.
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M A R I C O P A   H W Y
Not to Scale
Ventura County General Plan
Hazards Appendix - Figure 2.2.1a
Source: Ventura County Data and Dibblee Fault Maps
January 2002
Positively Identified
EARTHQUAKE FAULTS
North Half
Camarillo
1 0 1   F W Y
1 1 8   F W Y
H W Y   1 5 0
1 2 6   F W Y
O J A I   F W Y
H W Y   1 5 0
1 2 6   F W Y
1 0 1   F W Y
1 0 1   F W Y
1 1 8   ( L O S   A N G E L E S   A V E . )
H W Y   1
H W Y   3 3
H W Y       2 3
Thousand Oaks
Simi Valley
Oxnard
Moorpark
San
Buenaventura
Ojai
Santa Paula
P o r t  
H u e n e m e
Fillmore
Not to Scale
Ventura County General Plan
Hazards Appendix - Figure 2.2.1b
Source: Ventura County Data and Dibblee Fault Maps
January 2002
EARTHQUAKE FAULTS
South Half
Positively Identified
Figure 2.2.2
Resources Affected By Faults And Fault Zones
FAULT SINGLE-FAMILY
SCHOOLS FIRE
STATIONS SEWER WATER GAS ELEC-TRICITY
MAJOR
ROADS OTHER
Mission Ridge- Arroyo
Parida- Santa Ana
Fault System
( 1) n n n
Red Mountain Fault n n n n n n n
Lion Mountain Fault n ( 2) n n n n n n
San Cayetano Fault n ( 3) n n n n n n n
Fault East of
Moorpark
n n n n n
Fault North of
Moorpark
n
Sycamore Canyon
Fault
n n n n n
Fault West of
Sycamore Canyon
n n n
Santa Susana Fault n n
Simi Fault n n n n n
Boney Mountain
Fault
n
Bailey Fault n n n n
Fault Northeast of.
California State
University, Channel
Islands
( 1) n ( 5)
Santa Rosa Fault n n n n n
Springville Fault ( 4) n n n n n
Oak Ridge Fault n n n n
Oak Ridge Extension n n
Camarillo Fault n n
( 1) Villanova Prep. School
( 2) Oak View and Sunset Schools
( 3) Summit and Piru Schools
( 4) Camarillo Heights Elem. School
( 5) California State University, Channel Islands
Note: This table deals only with resources in unincorporated territory
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M A R I C O P A   H W Y
Not to Scale
Ventura County General Plan
Hazards Appendix - Figure 2.2.3a
Source: Alquist- Priolo Earthquake Fault Zoning Act
January 2000
Earthquake Fault Zone
EARTHQUAKE FAULT
HAZARD ZONES
North Half
Camarillo
1 0 1   F W Y
1 1 8   F W Y
H W Y   1 5 0
1 2 6   F W Y
O J A I   F W Y
H W Y   1 5 0
1 2 6   F W Y
1 0 1   F W Y
1 0 1   F W Y
1 1 8   ( L O S   A N G E L E S   A V E . )
H W Y   1
H W Y   3 3
H W Y       2 3
Thousand Oaks
Simi Valley
Oxnard
Moorpark
San
Buenaventura
Ojai
Santa Paula
P o r t  
H u e n e m e
Fillmore
Source: Alquist- Priolo Earthquake Fault Zoning Act
January 2000
EARTHQUAKE FAULT
HAZARD ZONES
South Half
Earthquake Fault Zone Not to Scale
Ventura County General Plan
Hazards Appendix - Figure 2.2.3b
2.3 Ground Shaking
2.3.1 General
" Ground shaking" is the physical movement of the land surface due to seismic waves caused by
earthquakes. When a fault breaks, the accumulated strain energy is released as seismic waves
that travel outward in all directions from the earthquake focus ( the point of first release of tectonic
stress located below the earth's surface, causing ground shaking). Seismic waves travel through
the ground like the ripples from a pebble dropped into a pond. The intensity of the ground shaking
depends on the quake magnitude ( size of pebble) and the distance from the fault ( ripples get
smaller as they radiate outward). Seismograms ( records of earthquake motion) indicate that
earthquakes create several kinds of motions, or seismic waves. These waves exhibit different
types and directions of movement. Each type of wave can affect buildings differently depending on
many diverse variables. This is due to the Earth’s crust not being homogeneous like water, but
rather a complex mixture of rocks and sediments of varying types that respond to the shaking in
different ways. The combined effect of these waves makes up the ground- shaking component of
an earthquake. In a single earthquake, the shaking at one site could be 10 times stronger than the
shaking at a neighboring site even though the distance to the ruptured fault is the same.
Two separate indexes, or scales, are commonly used in describing seismic or earthquake events.
The qualitative rating of the degree of earthquake shaking based upon feeling and visual
observation is indicated by an intensity scale ( Mercalli). The size or energy release of earthquakes
is measured by a magnitude scale ( Richter).
Measurement of the radiated energy released by an earthquake was originally proposed by C. F.
Richter in 1932. This method assigns a number to the calculated energy release ( magnitude), and
can rank earthquakes and compare them to one another. By this method, an earthquake is rated
independently of the place of observation. The Richter Scale is logarithmic, and is not limited,
either at top or bottom. Each magnitude step on the scale represents an increase of ten times in
measured wave amplitude of the earthquake, and 30 times the amount of energy released as
seismic waves. A magnitude 2 is about the smallest earthquake that can be felt by human beings.
The index used to measure earthquake intensity ( as opposed to magnitude) is the modified
Mercalli Intensity Scale, with intensity scales ranging from I for earthquakes barely perceptible by
human beings to XII for “ the ultimate catastrophe." The scale is a description of the physical
effects of earthquakes. There is only a rough correlation between the magnitude of an earthquake
and the intensity.
The intensity of ground shaking during an earthquake depends in large part on various
characteristics of local geologic conditions ( i. e., the thickness and physical properties of the
materials comprising the upper several hundred feet beneath the area). By combining
observations from past earthquakes with computer- based predictions, geologist ( seismologists)
found that the two most important characteristics are the softness of the ground and the total
thickness of sediments below a particular site. Seismic waves travel faster through hard rock than
through soft rock and sediments. As the waves pass from harder to softer materials and slow
down, they must increase in amplitude to carry the same amount of energy. Thus the shaking
tends to be stronger at sites with softer surface materials. In general, the greatest amplitudes and
longest durations of ground shaking usually occur on thick, unconsolidated alluvial sediments.
Other variations in earthquake shaking depend on the specific details of the earthquake, such as
orientation of the fault, irregularities of the rupturing fault surface, and the scattering of waves as
they bounce off of subsurface changes. Other factors that may contribute significantly to the
damage potential of structures include: magnitude of the earthquake, distance and direction from
the epicenter and causative fault, duration of shaking, the structural integrity of buildings before the
earthquake, and many others. Each earthquake provides additional data to review and help
improve the understanding of seismic hazards.
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2.3.2 Location Of The Hazard
The ground- shaking hazard exists throughout Ventura County, as well as, all of California. Certain
areas may have increased ground shaking due to local geologic conditions, as well as, the location
and orientation of the earthquake fault. The approximate peak horizontal acceleration for uniform
soft rock site conditions is shown on Figure 2.3a and 2.3b. These maps provide the anticipated
ground acceleration for a site with a 10 percent probability of exceedance in 50 years. This data is
based on the California Geological Survey Open File Report 96- 08, and the ground acceleration is
a percentage of gravity ( g).
The highest amplification of ground shaking occurs in areas with the greatest potential for long
period wave shaking. Basically, this is the San Andreas Fault zone in the northern part of the
County and the Oakridge Fault zone in the southern part of the County.
The areas with the greatest amplification of short period shaking are along the base of the hills, in
minor river valleys and in the broken bedrock along fault lines such as the San Cayetano, Oak
Ridge and Simi- Santa Rosa Faults. Slight to moderate amplification of short period oscillations
may occur on terrace deposits or soft bedrock, however, certain locations may experience higher
than normal ground shaking due to boundary effects or wave propagations. These materials are
found in young hill areas such as South Mountain, Oak Ridge, Sulphur Mountain, and the north
coastal hill lands and the Piru area in the south half of the County. In the north half of the County,
these are along the margins of the valley areas such as Hungry and Lockwood Valleys and north of
Cuyama.
In addition to the forces causing horizontal movement, such as those that predominant along the
San Andreas Fault, Ventura County and portions of adjacent areas are subject to compressional
forces acting in north- south directions. These forces tend to compress or shorten the distance from
the San Andreas Fault south to the coast. These compressional forces caused the San Fernando
Earthquake of 1971, resulting in the thrusting of the southern margin of the San Gabriel Mountains
several feet southward over the north margin of the San Fernando Valley. These forces also
resulted in the 1994 Northridge Earthquake. Several faults in Ventura County have been formed
by and are related to these same forces. These fault systems are described in the Fault Rupture
section.
Southern Ventura County
The south half of the County is considered that portion southerly of the east- west projection of
Nordoff Ridge located immediately north of Ojai Valley. Even though the historic record indicates
that no strong earthquakes or surface displacement have occurred along the faults within the south
half, the likelihood of the occurrence of one or more of such events within the next 50 to 100 years
is not remote. The San Fernando Earthquake of 1971 occurred along a fault having little historic
record of activity. Several of the faults within the south half of the County, such as Santa Susana
and San Cayetano, are subject to similar tectonic forces as those that caused the San Fernando
Earthquake. Crustal deformation ( shortening) resulting in earthquakes will continue into the
indefinite future. It is probable that earthquakes of magnitude 6 or larger will occur in the south half
of the County area, in the nearby offshore areas, and along the San Andreas in the northern
portion of the County.
According to the " Geology and Mineral Resources Study of Southern Ventura County" ( 1972)
prepared by the State Division of Mines and Geology in cooperation with the Ventura County
Department of Public Works, the earthquake history of the south half of the county is dominated by
small to moderate shocks. No earthquake greater than magnitude 4.7 has been recorded in
Ventura County, or the immediate offshore area, since 1934, when adequate instrumental records
became available. These relatively minor shocks have caused local damage but no recorded loss
of life. A review of the earlier less accurate record from 1769 to 1934 suggests a similar history for
the south half, although there were significant earthquakes in 1812, 1857, 1925, 1971, and 1994
that caused structural damage in specific areas of the south half of the County.
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Northern Ventura County
The most important faults in the vicinity of the northern County area are the San Andreas, Big Pine,
San Gabriel, and Frazier Mountain Thrust, all of which converge at the northeast corner of Ventura
County. All of these faults, except perhaps the Frazier Mountain Thrust, are considered to be
active, i. e., are potential focal points for the occurrence of earthquakes and displacement of the
ground surface. Other mapped and unknown faults within the north half may also prove to be
active by future displacement or detailed investigations. The earthquakes of November 1852 were
accompanied by about 30 miles of surface faulting in Lockwood Valley. The exact location of the
surface breaks is unknown, but geologic evidence and reports indicate that it may have been along
the Big Pine Fault, a major left- lateral fault with some oblique slip ( subject to both horizontal and
vertical displacement).
Several other faults found in the Lockwood Valley area have had recent movement identified by
virtue of their cutting of terrace deposits and offset of other faults. These faults range from several
hundred to a few thousand feet in length. Some of them indicate the region has recently
undergone, and is probably still undergoing compression along north- south directions.
Geologic and survey evidence indicate that stress is building up along the San Andreas Fault to the
north. It is just a question of time until the fault in this area again displaces; the resulting
earthquake will probably be severe. Prediction of when displacement will occur is not possible at
this time; however, it is likely that it will occur within 100 years and possibly much sooner.
Earthquakes and strong to severe ground shaking originating along faults within the north half is
highly possible, but again, prediction of when this will happen is not possible. The historic record
shows that the north half has experienced several severe shocks originating along faults both
within and immediately outside of the county.
2.3.3 Conclusion
Individual site investigation to provide detailed estimates of ground shaking sufficient for design
purposes are presently performed by two methods: a Deterministic Seismic Hazard Analysis
( DSHA), and a Probabilistic Seismic Hazard Analysis ( PSHA).
The DSHA analysis considers a specific scenario earthquake ( with a magnitude and location) and
the ground motion is computed for the particular site based applicable attenuation equations. The
deterministic assessment of the causative earthquake is specific in terms of its magnitude and
distance to the site. There still is a large potential range of ground motions that could occur due to
the various attenuation relations utilized. Most deterministic approaches consider the median ( 50th
percentile) or median plus- one standard deviation ( 84th percentile) for design.
The PSHA approach considers multiple potential earthquakes, that is, all of the magnitudes and
locations believed to be applicable to the potential sources are included in the analysis. The PSHA
also considers the rate of earthquake occurrence, and the probabilities of earthquake magnitudes,
locations, and rupture dimensions. The PSHA approach yield a description of how likely it is the
different levels of ground motion will be exceeded at the site within a given period of time.
There are considerable misunderstandings of the relationship of deterministic to probabilistic
analyses. DSHA are most often thought to provide a “ Worst Case” ground motion, although the
magnitude of any particular earthquake is correlated to the length or rupture area of the causative
fault.
Ground motion provisions of the Uniform Building Code ( UBC), which forms the basis for most
building design in Ventura County, are based loosely on probabilistically derived accelerations that
have a 10 percent probability of exceedance in 50 years. This equates to a return period of 475
years. Ground Motion Maps are in the process of being created for the County of Ventura by the
California Geologic Survey ( CGS). The maps show ground motion as a maximum horizontal
acceleration ( MHA) having a 10 percent probability of being exceeded in a 50- year period in
keeping with the UBC hazard level. The color images of seismic hazard zone maps, and the text of
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associated evaluation are accessible at the CGS ( formerly CDMG) web site address:
http:// www. consrv. ca. gov/ dmg/ shezp/ map/ data. htm.
Mitigation of the potential ground- shaking hazard is generally by the implementation of the UBC in
the design and construction of structures.
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M A R I C O P A   H W Y
Not to Scale
Ventura County General Plan
Hazards Appendix - Figure 2.3a
Source: California Geologic Survey, January 2002
GROUNDSHAKING
North Half
1.05g
0.95g
0.85g
0.75g
0.65g
0.55g
0.45g
0.35g
P r o b a b i l i s t i c   s e i s m i c   h a z a r d   m a p   f o r   p e a k   h o r i z o n t a l   a c c e l e r a t i o n   o n   f i r m - r o c k   s i t e   c o n d i t i o n s
a n d   f o r   1 0 %   p r o b a b i l i t y   o f   e x c e e d a n c e   i n   5 0   y e a r s .   C o l o r s   i n d i c a t e   p e a k   a c c e l e r a t i o n   i n   %   g   u n i t s .
Camarillo
1 0 1   F W Y
1 1 8   F W Y
H W Y   1 5 0
1 2 6   F W Y
O J A I   F W Y
H W Y   1 5 0
1 2 6   F W Y
1 0 1   F W Y
1 0 1   F W Y
1 1 8   ( L O S   A N G E L E S   A V E . )
H W Y   1
H W Y   3 3
H W Y       2 3
Thousand Oaks
Simi Valley
Oxnard
Moorpark
San
Buenaventura
Ojai
Santa Paula
P o r t  
H u e n e m e
Fillmore
Not to Scale
Ventura County General Plan
Hazards Appendix - Figure 2.3b
Source: California Geologic Survey, January 2002
GROUNDSHAKING
South Half
P r o b a b i l i s t i c   s e i s m i c   h a z a r d   m a p   f o r   p e a k   h o r i z o n t a l   a c c e l e r a t i o n   o n   f i r m - r o c k   s i t e   c o n d i t i o n s
a n d   f o r   1 0 %   p r o b a b i l i t y   o f   e x c e e d a n c e   i n   5 0   y e a r s .   C o l o r s   i n d i c a t e   p e a k   a c c e l e r a t i o n   i n   %   g   u n i t s .
1.05g
0.95g
0.85g
0.75g
0.65g
0.55g
0.45g
0.35g
2.4 Liquefaction
2.4.1 General
During earthquakes or shortly after, shaking of the ground may cause a loss of strength or stiffness
that result in the settlement of buildings, formation of landslides, failure of earth dams, and other
hazards. The process that leads to the loss of strength is a widespread term called “ liquefaction.”
Recent examples of liquefaction damage are the 1989 Loma Prieta, 1994 Northridge, 1995 Kobe,
and the 1999 Turkey Earthquakes. Liquefaction areas resulting from the 1994 Northridge
Earthquake within Ventura County occurred near the mouth of the Santa Clara River in
Oxnard/ Ventura, in Simi Valley, and along the Santa Clara River between Fillmore and Newhall
( Barrows et al., 1995). Past liquefaction- related events with unusual patterns of ground shaking
and localized damage in alluvial areas have occurred within Ventura County as interpreted from
historical reports by Weber, Jr., and Kiessling, 1976. Please note that liquefaction is generally
thought of in reference to earthquakes, however, other methods of groundshaking such as blasts or
explosions may result in local areas of liquefaction.
The potential for liquefaction to occur depends on both the susceptibility of a soil to liquefy and the
opportunity for ground motions ( shaking) to exceed a specified threshold level. Simply stated,
“ liquefaction" is a process by which loose, water- saturated granular materials behave for a short
time as a fluid rather than as a solid mass. Liquefaction can occur at any level in the ground, but
usually occurs within the first 50– 80 feet. Depending upon specific soil conditions, such as density,
uniformity of grain size, confining pressure and saturation of the soil materials, a certain intensity of
groundshaking is required to trigger liquefaction. Ground shaking intensity depends on the
magnitude, distance and direction from the site, depth, and type of earthquake, the soil and
bedrock conditions beneath the site, and the topography of the site and vicinity. The duration of
the shaking and/ or the repeatable intensity of the ground motion is also important, as it takes a
certain number of cycles of ground shaking for sufficient pore pressure to build- up and liquefaction
to occur.
The liquefaction phenomenon is typically associated with medium to fine- grained sands in a fairly
loose to medium- dense condition. If the material is finer- grained ( clays) rather than fine sand or
silt, it is generally not prone to liquefaction. The size fraction that is below 0.005 mm and makes up
greater than 30 percent of the material within any specific layers is considered not to be prone to
liquefaction. This inhibits liquefaction, since the bonding of the grains to one another prevents the
loss of contact between them. Therefore, most silty clays and clays may not liquefy.
Knowledge concerning liquefaction and its effects has come from mainly three efforts. These are:
1. Field observations during and following earthquakes;
2. Laboratory experiments on saturated soil samples and models of foundations and earth
structure;
3. Theoretical studies.
Fine- grained soils in a saturated state are so widespread in their distribution, and earthquake
vibrations extend over such large areas, that liquefaction phenomena have been a part of every
large earthquake that has been closely studied. In some earthquakes it has been a major factor in
the damage and destruction caused. Liquefaction phenomena played a part in massive soil
movements in Alaska in 1964. Also in 1964, in Niigata, Japan, liquefaction caused major disruption
of services and utilities as well as giving rise to substantial building settlements and displacements.
In the San Fernando Earthquake of 1971, liquefaction of material in the San Fernando Dam caused
a landslide of the upstream portion of the dam structure. Slumping and displacement of other
slopes and embankments in this earthquake have also been attributed to liquefaction ( Bolt, et al.,
Geological Hazards, 1977).
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2.4.2 General Effects Of The Hazard
There are two types of liquefaction. The first type is where surface or near- surface liquefaction of
soils occurs. Structures with foundations located within such a liquefaction zone lose support
under part or all of their foundations, which causes them to tilt or settle into the ground surface. If a
building is not designed to take this amount of stress, the entire building may collapse. A partially
liquefied layer can also flow out from under the weight of the foundation with similar settling effects.
In addition, the liquefied layer may exceed the design capacity of retaining walls and result in
failure of the wall. Near surface manifestations of liquefaction include sand boils, lateral spread
failures, loss of bearing capacity and ground settlement, buoyant rise of buried structures, and
failure of retaining walls. Differential settlement may affect almost any structure and the ground
surface.
Following the 1994 Northridge earthquake, portions of southeastern Simi Valley experienced
liquefaction evidenced by sand boils, sand craters, and/ or fissures. These features were observed
in areas with very shallow ground water (< 10 feet in depth) and areas situated in fill material
overlying the predevelopment course of the Arroyo Simi ( CDMG Special Publication 116, 1995).
The second type of liquefaction occurs when the liquefiable soil layer is below the surface.
Structures with foundations above the liquefiable zone may be subject to increased ground
oscillations. Liquefaction beneath a firm soil may result in a decoupling of the upper soil layers
causing fissures to form and different impacts between the soil blocks ( settlement and tilting, etc.),
as well as, between the liquefied area and the adjacent non- liquefied area ( reference). The higher
susceptible areas for damage occur at the boundary between these zones.
All engineered structures including roadways, bridges, dams, single family housing and utility lines
( water, gas, sewer) as well as, oil and gas pipeline and production, processing and storage
facilities are subject to the potential damage resulting from liquefaction.
If the subsurface liquefaction occurs on a slope, the liquefied layer can act as a lubricated plane for
the layer( s) above it to respond to gravity and move downhill. This type of liquefaction is one
common cause of earthquake- induced landslides. Structures built within and across the edges of
the slide are torn apart in much the same manner as if they were located on a fault; a good
example of this occurred in the 1971 San Fernando Earthquake, where an area of almost 163
acres moved down a 2.5% slope. Movement down a slope with such a low gradient had not
previously been recorded, but such effects must be considered in future earthquakes.
Liquefaction occurred in Calleguas Creek, Mugu Lagoon and the lower Santa Clara River during
the February 21, 1973, Point Mugu Earthquake. The effects were mainly the development of minor
ephemeral features such as shallow cracks and sand boils, but as Morton and Campbell point out
in their report ( see Bibliography), if the shaking had been more severe, such effects might well
have been widespread and could have resulted in significant agricultural crop losses. The effects
on structures could also have been significant. Eyewitness reports of the effects of the 1857 Fort
Tejon Earthquake ( magnitude ~ 8.0) on the San Andreas Fault suggest liquefaction occurred along
the Santa Clara River, along with other damage.
Liquefaction often causes settlement of the soil. In Niigata, Japan, after the 1964 earthquake,
settlement of over 3 feet was common. In Alaska, the ground around one wellhead settled 4.5 feet;
there were also numerous bridge foundation settlings. Liquefaction can also destroy or disrupt
much of the infrastructure ( i. e., gas lines, water, sewer, roads, etc.) in an area. Pipelines could be
broken either by being floated to the surface or by landslide displacement. Bridge abutments could
suffer differential settlement, cutting off roads. The settlement of large areas of land could drop
some areas below sea level and produce a new shoreline, or at least require reconstruction to re-establish
continuity of roads, etc. ( see Subsidence Hazard). Roadways may be disrupted by
uneven surface.
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2.4.3 General Inventory Of The Hazard
A liquefaction threat may exist in the entire hazard zone identified on Figures 2.4a and 2.4b. This
map is a compilation of the quadrangle maps prepared by State of California Geologic Survey
and/ or Division of Mines and Geology that include the areas of potential liquefaction. Liquefaction
may have occurred in these areas and can be expected to occur whenever an earthquake of
sufficient intensity occurs.
Large areas of the County have a surface layer of unconsolidated sand deeper than 40 feet.
Therefore, the primary variable factor for liquefaction in the County is the depth of the water table.
The highest historic water level is used as the basis for all liquefaction analysis. This is reasonable
in urbanized areas where the water table is usually rising due to a number of factors, including
curtailment of agricultural pumping, importation of increased amounts of water, reduced
evaporation due to paving, heavy irrigation from watering of yards, percolation of sewage, etc.
Long- term changes may also result from the cumulative effects of the above factors, as well as
extended periods of above or below normal rainfall. The threat posed by this hazard also varies
depending upon the seasonal water level in some areas.
Structures proposed for human occupancy within the zone of potential liquefaction will require a
geotechnical investigation to determine the liquefaction potential, potential effects of liquefactions,
and to provide mitigation recommendations.
In terms of loss of lives or injury occurring from liquefaction hazards, the predominant threat exists
in the Oxnard Plain, Santa Rosa and Pleasant Valley, the Ventura and Santa Clara River flood
plains and portions of Ojai, Thousand Oaks, Simi Valley, and Newbury Park. In addition, those
communities located in the Ventura River flood plain having concentrations of people, especially in
single- family homes, may be affected. A number of schools could be affected by liquefaction,
although they are in the moderate hazard zone, including DeAnza Junior High and all of the
schools of Rio School District, and Rio Mesa High School. The Briggs Road Industrial Park is
located in the high hazard zone, as are the nearby industries in the County areas of Saticoy. Other
areas that could be affected include: ( 1) Ormond Beach Generating Plant and most of the high
voltage transmission lines on the Oxnard Plain and crossing the Santa Clara and Ventura Rivers;
( 2) most of the oil facilities along Ventura Avenue, and ( 3) the entire Naval Base Ventura County,
Point Mugu. There are also numerous pipelines and other underground utilities that could be
affected on the Oxnard Plain and near the rivers.
Simi Valley appears to have a high liquefaction potential in the southern part of both the east and
west basins. Most of the remainder of the Calleguas Creek areas appear to have adequate
drainage to avoid the hazard, except for the lower Arroyo Conejo. Higher groundwater elevations
may be present in the lower Arroyo Conejo due to the discharge from the City of Thousand Oaks
Hill Canyon Wastewater Treatment Plant. This plant may contribute to higher groundwater levels
in the western Santa Rosa Valley area and along Calleguas Creek further to the west. Thousand
Oaks may have problems in the low- lying valley areas, including Hidden Valley, because of their
alluvial nature.
The information used to define the hazard zones on Figures 2.4a and 2.4b was the best available
at the time the maps were prepared. Also, the boundary lines represent a transition zone that may
fluctuates seasonally, with changes in water supply.
2.4.4 Nature Of The Information
Data on the water surface level was taken from the extensive well records maintained by the
Ground Water Section of the Ventura County Watershed Protection and Water Resources
Department. These well records include up to 50 years of actual measurements at approximately
one- month intervals. However, certain areas did not have usable well records. So special reports
were used or actual field data was collected. As a first approximation, the Quaternary geology of
Ventura County was mapped in detail by William Lettis and Associates ( William Lettis, 2001). This
maps defines the areas of the County that have younger sediments. The State of California utilized
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these maps, as well as, well records for material type and density, highest groundwater elevation,
to produce the Seismic Hazard Maps for Liquefaction. The Lettis maps also include a range of low
to very high liquefaction potential areas, however, the use of this data is limited to surface mapping
and age of deposits. The State of California Seismic Hazard Maps should be utilized for all
determinations for liquefaction potential.
The groundwater levels in alluvial areas were arrived at by taking the highest figure measured from
the records of the Ground Water Section of the County Flood Control and Water Resources
Department and the Leaking Underground Fuel Tank ( LUFT) Section of the Environmental Health
Division. The estimated effects of liquefaction may vary greatly within a given zone during a given
earthquake. Any specific conclusions should be reached on the basis of detailed site- specific soils
and geologic studies. The California Geological Survey Special Publication 117 provides
recommended guidelines for the evaluation of the potential for liquefaction.
2.4.5 Conclusion
Liquefaction was a damaging hazard in Simi Valley during the 1994 Northridge Earthquake and it
remains the biggest seismic threat in the County.
The hazard exists wherever there are certain soils, particularly loose sands that are constantly or
seasonally saturated with water. This might include most of the river valleys and the low- lying
plains areas that have poor drainage. Since subsurface soil properties are not precisely known, it
is necessary to assume that all alluvial areas having high groundwater may be subject to
liquefaction during strong ground shaking. If general surface liquefaction were to occur, most
structures in the hazard zone could be affected to a greater or lesser degree.
There is little that can feasibly be done to reduce the regional liquefaction hazard. Important or
critical structures can utilize special designs to alleviate the effects of the hazard, except possibly in
areas subject to landsliding. Land use controls are the only other methods available to reduce the
threat to life and property.
Special attention should be given to the liquefaction potential in evaluating the adequacy of existing
critical or essential facilities in the high hazard areas, since the threat may be quite severe,
especially to larger buildings.
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M A R I C O P A   H W Y
A r e a   n o t   m a p p e d
a s   o f   p u b l i c a t i o n   d a t e
Source: William Lettis & Associates, Inc.
Liquefaction Areas
Not to Scale
Ventura County General Plan
Hazards Appendix - Figure 2.4a
LIQUEFACTION AREAS
North Half
Camarillo
1 0 1   F W Y
1 1 8   F W Y
H W Y   1 5 0
1 2 6   F W Y
O J A I   F W Y
H W Y   1 5 0
1 2 6   F W Y
1 0 1   F W Y
1 0 1   F W Y
1 1 8   ( L O S   A N G E L E S   A V E . )
H W Y   1
H W Y   3 3
H W Y       2 3
A r e a   n o t   m a p p e d
a s   o f   p u b l i c a t i o n   d a t e .
Thousand Oaks
Simi Valley
Oxnard
Moorpark
San
Buenaventura
Ojai
Santa Paula
P o r t  
H u e n e m e
Fillmore
Source: California Department of Conservation
Division of Mines & Geology - 2003
LIQUEFACTION AREAS
South Half
Liquefaction Areas Not to Scale
Ventura County General Plan
Hazards Appendix - Figure 2.4b
2.5 Seiche
2.5.1 Nature Of The Hazard
A seiche can be considered very similar to a tsunami with the difference being that the water waves
are generated in a closed or restricted body of water such as a lake or within a harbor. The most
common seiche experienced by County residents in most swimming pools occurred during the
1994 Northridge earthquake. The shaking of an earthquake ( or other vibration) can result in large
and destructive oscillations that produce waves tens of feet above normal lake ( water) level. In
harbors ( such as Ventura Harbor, Mandalay Bay and the Park of Hueneme) and closed or
restricted bays, these waves can destroy harbor and shore facilities. Indirectly, tsunamis, by
causing a rapid change in sea level or more commonly by the wave itself, can set up smaller
internal oscillations in bays and harbors. These seiches are very similar to tsunamis, but the
waves are usually smaller and of lower energy. The trigger mechanism for seiche waves is similar
to tsunamis wave generation.
The extent of most seiches is small, usually no more than ten to twenty feet above water level, and
the duration is short, usually only a few minutes. However, a landslide can displace a wave that
could travel hundreds of feet up the opposite shore of a body of water. Also, tsunami- caused
seiches can last for many hours due to the possible rejuvenation of the seiche by each passing
tsunami crest; however, each seiche would last only a few minutes and be of decreasing severity.
2.5.2 History Of The Hazard
There is no record of a seiche occurring in Ventura County. Nevertheless, the worldwide history of
the phenomenon illustrates the damage that seiches can do, and that seismic disturbances at great
distances from this County could have an effect here. The Lisbon Earthquake of 1755 caused
seiches in canals and lakes as far away as Holland, Switzerland, Sweden and Scotland, while on
the Firth of Forth in Scotland, the water rose quickly eight inches or more soon after the time of the
earthquake, and boats rocked at their moorings for three or four minutes ( Bolt, et al., Geologic
Hazards, 1977).
In Italy, in 1963, a landslide into Vaiont Reservoir caused a seiche that traveled up 800 feet on the
opposite bank of the lake and swept over both abutments of the dam ( the world's highest thin- arch
concrete dam) to a height of 328 feet. The water completely destroyed the town of Longarone
below the dam, killing almost 3,000 people.
The 1964 Alaskan Earthquake led to the agitation of wells as far away as the coast of the Gulf of
Mexico. Seiche surges along the Louisiana and Texas coasts commenced between 30 and 40
minutes after the earthquake origin, or about the time the surface waves were passing through the
area. Minor damage was widespread, with parting of barge moorings in the Mississippi River ( Bolt,
et al., Op. cit).
2.5.3 Conclusion
It appears that the actual threat that is posed by seiches in Ventura County is small, in that it is
probably the most remote of the hazards studied, although it may not be the least severe.
There is no way to alleviate the effects of possible seiches except by prohibiting construction within
the hazard area. The project geologist and geotechnical engineers evaluate mitigation of potential
seiche effects during the preliminary design for structures located near known seiche areas.
Typically, where practical, the structure is moved to a slightly higher elevation to reduce the
damage potential and amount. Due to the indefinite nature of the triggering mechanisms, it seems
doubtful that enough information will ever be known for general prediction of the hazard or
predicting accurate seiche uprush limits for planning purposes.
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2.6 Tsunami
2.6.1 Local History
Tsunamis are geologic hazards that can be the result of both ground shaking forces and forces
other than ground shaking. Tsunami hazards remain the same regardless of whether caused by
an earthquake event or not associated with an earthquake.
A tsunami ( pronounced “ soo- nahm'ee”) is a series of waves generated by an undersea disturbance
such as an earthquake. From the area of the disturbance, the waves will travel outward in all
directions, much like the ripples caused by throwing a rock into a pond. The time between wave
crests may be from 5 to 90 minutes, and the wave speed in the open ocean will average 450 miles
per hour.
Tsunamis reaching heights of more than 100 feet have been recorded. As the waves approach the
shallow coastal waters, they appear normal and the speed decreases. Then as the tsunami nears
the coastline, it may grow to great height and smash into the shore, causing much destruction.
1. Tsunamis are caused by an underwater disturbance - usually an undersea earthquake.
Landslides, explosions, volcanic eruptions, and even impact of cosmic bodies ( meteorites)
can also generate a tsunami.
2. Tsunamis can originate hundreds or even thousands of miles away from coastal areas.
Local geography may intensify the effect of a tsunami. Areas at greatest risk are less than
50 feet above sea level and within one mile of the shoreline.
3. People who are near the seashore during a strong earthquake should listen to a radio for a
tsunami warning and be ready to evacuate at once to higher ground.
4. Rapid changes in the water level are an indication of an approaching tsunami.
5. Tsunamis arrive as a series of successive “ crests” ( high water levels) and “ troughs” ( low
water levels). These successive crests and troughs can occur anywhere from 5 to 90
minutes apart. They usually occur 10 to 45 minutes apart.
The worst recorded tsunami to hit California was in 1812. An earthquake occurred in the Santa
Barbara Channel, and the resulting waves are reported by some disputed sources to have been up
to 50 feet above sea level at Gaviota ( Richter, Pg 113). The waves were probably at least 15 feet
above sea level at Ventura.
In Crescent City, widespread damage and some loss of life occurred in 1964 as a result of a
tsunami caused by the Alaska Earthquake. This tsunami caused more than $ 84 million in damage
in Alaska and a total of 123 fatalities. The tsunami from that earthquake also caused
approximately $ 35,000 damage to the marinas in Ventura County. This damage was mainly to the
marinas’ channel banks and was caused by the rapid change in sea level.
The historic record indicates that there is a small probability of occurrence of a major tsunami in
Ventura County. Statistically it has been over 170 years since the last major tsunami, but many
smaller, unrecorded tsunamis may have occurred.
Most deaths during a tsunami are a result of drowning. Associated risks include flooding, polluted
water supplies, and damaged gas lines.
2.6.2 Location Of The Hazard
All of the coastal and near coastal river areas in Ventura County are susceptible to tsunamis. A
tsunami from the north Pacific could move down the Santa Barbara Channel and affect the
northerly coastal areas; one from the South Pacific or from South America could strike the County
coastal areas from the south to southwest; or a tsunami generated along one of the faults within
the Santa Barbara Channel could affect much of the County coastal areas. The Channel Islands
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provide some protection, however, they do not provide adequate protection for the County coastal
area.
Tsunamis can also proceed up rivers for many miles if the gradient of the river is shallow. The
effects of the waves are most noticeable on man- made features, but the waves can also change
river channels and modify coastal landforms and these effects are noticeable for many years. The
watercourses for the Ventura and Santa Clara Rivers and Calleguas Creek could be altered by a
tsunami, and their biosystems temporarily damaged. There are likely to be similar effects on Mugu
Lagoon.
The uncertainty of local effects makes the definition of the hazard zone difficult. The hazard zone
defined by this study includes all areas of the County up to 30 feet ( 10 meters) ( see Figure 2.6)
above sea level and within one mile of the mean high tide line. Areas at greatest risk are less than
50 feet above sea level and within one mile of the shoreline ( FEMA, July 21, 1998). The 50- foot
FEMA height may include debris and structure effects on the wave uprush. Areas of exception are:
east of Point Mugu and north of the Ventura River where the zone includes all areas up to 30 feet
above sea level and up to 50 feet above sea level, respectively. Most of the land between the
beach and the cliffs on both the North and South coasts is included within the hazard zone. he
hazard zone from the Santa Clara River to Point Mugu extends inland approximately one mile.
The recommended areas of evacuation in the event of a tsunami are all areas below the
aforementioned elevations or within a mile of shore ( whichever is of the greatest inland extent), and
two miles inland on the Santa Clara River, Ventura River, and Calleguas Creek. The reason for
extension of the zone two miles upstream from the mouths of these watercourses is that tsunami
can generate a wall of water called a " bore", and the breaking waterfront can cause great damage.
There are a number of small communities within the county whose residents could be affected by a
major tsunami. These include, on the north coast ( west to east), Rincon, Del Mar ( Bates Point), La
Conchita ( Punta), Mussel Shoals ( Punta Gorda), Seacliff, Faria and Solimar ( Deulah). The areas
of Hollywood- by- the- Sea and Silver Strand, as well as scattered multiple dwellings on the south
coast, could suffer damage. During the summer months, many people camp at several parks
within the hazard zone. These include: Hobson, Faria, Emma Wood, McGrath State Beach and
Point Mugu State Park.
All or portions of the cities of Port Hueneme and Oxnard, the Naval Base Ventura County, Point
Mugu, and the Port Hueneme Seabee base are within a tsunami hazard area. More specifically,
the Ventura Police Department on Ventura Avenue and the Fire Station on Santa Clara Street are
also in the hazard zone, as are Pierpont School and sewage treatment plants. In a major tsunami,
the Holiday Inn, parking structure and County of Ventura Fairgrounds could be damaged. The San
Buenaventura State Park could be disrupted and be flooded in places, and the railroad and both
highway bridges could be damaged.
2.6.3 Alleviation Of The Hazard
The threat to human life can be nearly eliminated by an effective warning system when advance
notice is available. The County territory as well as the Cities of Oxnard and Port Hueneme have an
efficient warning system in effect, which can alert the entire affected population, if enough warning
time is available.
As part of an international cooperative effort to save lives and protect property, the National
Oceanic and Atmospheric Administration's ( NOAA) National Weather Service operates two
tsunami- warning centers. The Alaska Tsunami Warning Center ATWC) in Palmer, Alaska, serves
as the regional Tsunami Warning Center for Alaska, British Columbia, Washington, Oregon, and
California.
The Pacific Tsunami Warning Center in Ewa Beach, Hawaii, serves as the regional Tsunami
Warning Center for Hawaii and as a national/ international warning center for tsunamis that pose a
Pacific- wide threat. This international warning effort became a formal arrangement in 1965 when
PTWC assumed the international warning responsibilities of the Pacific Tsunami Warning System
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( PTWS). The PTWS is composed of 26 international Member States that are organized as the
International Coordination Group for the Tsunami Warning System in the Pacific.
Tsunami Watch and Warning Determination
The objective of the PTWS is to detect, locate, and determine the magnitude of potentially
tsunamigenic earthquakes occurring in the Pacific Basin or its immediate margins. Seismic
stations operated by PTWC, ATWC, the U. S. Geological Survey’s National Earthquake Information
Center and international sources provide earthquake information. If the location and magnitude of
an earthquake meet the known criteria for generation of a tsunami, a tsunami warning is issued to
warn of an imminent tsunami hazard. The warning includes predicted tsunami arrival times at
selected coastal communities within the geographic area defined by the maximum distance the
tsunami could travel in a few hours. A tsunami watch with additional predicted tsunami arrival
times is issued for a geographic area defined by the distance the tsunami could travel in a
subsequent time period. If a significant tsunami is detected by sea- level monitoring
instrumentation, the tsunami warning is extended to the entire Pacific Basin. Sea- level ( or tidal)
information is provided by NOAA's National Ocean Service, PTWC, ATWC, university monitoring
networks and other participating nations of the PTWS. The International Tsunami Information
Center, part of the Intergovernmental Oceanographic Commission, monitors and evaluates the
performance and effectiveness of the Pacific Tsunami Warning System. This effort encourages the
most effective data collection, data analysis, tsunami impact assessment and warning
dissemination to all TWS participants.
Tsunami Warning Dissemination
Tsunami watch, warning, and information bulletins are disseminated to appropriate emergency
officials and the general public by a variety of communication methods.
• Tsunami watch, warning and information bulletins issued by PTWC and ATWC are
disseminated to local, state, national and international users as well as the media. These
users, in turn, disseminate the tsunami information to the public, generally over commercial
radio and television channels.
• The NOAA Weather Radio System, based on a large number of VHF transmitter sites,
provides direct broadcast of tsunami information to the public.
• The US Coast Guard also broadcasts urgent marine warnings and related tsunami
information to coastal users equipped with medium frequency ( MF) and very high
frequency ( VHF) marine radios.
• Local authorities and emergency managers are responsible for formulating and executing
evacuation plans for areas under a tsunami warning. The public should stay- tuned to the
local media for evacuation orders should a tsunami warning be issued. And, the public
should not return to low- lying areas until the tsunami threat has passed and the local
authorities announce the “ all clear”.
The above material was modified from the National Tsunami Hazard Mitigation Program at website:
http:// www. pmel. noaa. gov/ tsunami- hazard/.
These advisories and warnings are transmitted by National Oceanic and Atmospheric
Administration satellite to the California Office of Emergency Service ( OES). The Warning Control
Officer and Director of OES evaluate these warnings, and if necessary a statewide warning is
issued to the local sheriffs, along with the estimated time of arrival of the wave. Ultimately, the
Sheriff has the responsibility to decide whether to alert the coastal areas. If it is decided that an
evacuation is necessary, the Sheriff will call the Police Departments of Oxnard, Ventura and Port
Hueneme; the Highway Patrol; Fire Department; and the Director of Disaster Services. After this is
accomplished, appropriate jurisdictions and departments are alerted. It is the responsibility of each
jurisdiction to decide whether or not the effected population will be alerted. The alerting agencies
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can only warn people of the hazard; they cannot force evacuation. However, they can control re-entry
into a hazard area.
Unfortunately, neither the Seismic Sea Wave Warning System nor any other known means of
monitoring can provide sufficient warning time to all for evacuation of coastal areas should a
tsunami be generated along one of the faults within the Santa Barbara Channel. The arrival time
for such a wave at any point on the coast would only be a matter of minutes. The only warning
prior to arrival of a possible earthquake generated tsunami would be the ground shaking
experienced from the earthquake. Such shaking would be felt in advance of the tsunami's arrival
and, if heeded, could serve to alert people to move to higher ground.
2.6.4 Ongoing Research
Research on tsunami hazards is continuing on virtually all levels of government. UNESCO's
International Oceanographic Commission has established an International Tsunami Information
Center in Honolulu, to promote further research and exchange of information concerning tsunamis.
The National Ocean Survey ( NOS) and U. S. Coast and Geodetic Survey of the National Oceanic
and Atmospheric Administration are the primary investigators of tsunamis in the U. S. The U. S.
Geologic Survey is also assisting in the basic research of processes involved in the generation of
tsunamis. The California Geological Survey and the State Office of Emergency Services is
investigating the extent of hazard to California.
The University of Southern California ( USC) has a tsunamis research group that is actively involved
with all aspects of tsunami research; field surveys, numerical and analytical modeling, as well as
hazard mitigation and planning. The web site is: http:// www. usc. edu/ dept/ tsunamis/.
The use of the Internet has contributed to real time posting of potential tsunamis warnings. The
National Oceanic Atmospheric Administration, National Weather Service, West Coast and Alaska
Tsunamis Warning Center post recent events that may trigger tsunamis. The web site address is:
http:// wcatwc. arh. noaa. gov/ message. htm.
2.6.5 Conclusion
Because of the small possibility of a major tsunami occurring in Ventura County it is not reasonable
to prohibit all development near beaches, nor is it practical to recommend drastic measures to
protect existing coastline development. In addition, the warning systems and evacuation plans that
are in place are considered to provide adequate protection in the event of a major tsunami being
generated beyond the Santa Barbara Channel.
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Camarillo
1 0 1   F W Y
1 1 8   F W Y
H W Y   1 5 0
1 2 6   F W Y
O J A I   F W Y
H W Y   1 5 0
1 2 6   F W Y
1 0 1   F W Y
1 0 1   F W Y
1 1 8   ( L O S   A N G E L E S   A V E . )
H W Y   1
H W Y   3 3
H W Y       2 3
Thousand Oaks
Simi Valley
Oxnard
Moorpark
San
Buenaventura
Ojai
Santa Paula
P o r t  
H u e n e m e
Fillmore
Not to Scale
Ventura County General Plan
Hazards Appendix - Figure 2.6
Source: United States Geological Survey
Topographic Quadrangle Maps
TSUNAMI INUNDATION
HAZARD AREAS
( 10m Run- up Elevation)
Tsunami Inundation Hazard Areas
2.7 Landslides/ Mudslides
2.7.1 General
" Landslide" is a general term for the dislodging and fall of a mass of soil or rocks along a sloped
surface, or the dislodged mass itself. A " mudslide" is a flow of very wet rock and soil. Landsliding
can be considered a major hazard in any hillside area. Most destructive landslides have resulted
from the indiscriminate development of sloping ground or creation of cut and/ or fill slopes in areas
of unstable or inadequately stable geologic conditions. Many of these landslides could have been
prevented by recognition of potentially unstable geologic conditions through adequate investigation
and incorporation of design safeguards prior to grading or construction.
The hazard of landslides, however, is not always confined to areas commonly considered hilly or
mountainous. Under certain soil and ground moisture conditions landsliding can occur in areas of
nearly level ground. This was clearly demonstrated by landsliding triggered during the San
Fernando earthquake of 1971 which resulted in destructive lateral ground movement over large
areas with regional slopes of as little as 1 ½ percent. Conditions that could result in similar lateral
spreading or low- angle landsliding may exist in some areas of Ventura County.
The following simplified classification system is adopted from Geological Hazards ( Bolt, et al.,
1977):
Movement Material Behavior Type Rate Names
Fall Brittle Rock, Ice,
Cemented Soils
Rapid Rockfall, Icefall,
Soilfall
Slide Unstable Rock, Soil
Snow
Rapid to Slow Rotational slump
Planar block glide
Lateral spreading
Slab Avalanche
Flow Stable Rock Fragments,
Sand, Silt,
Clay, Snow
Rapid to Slow Rock Flow
Sand Run
Earth Flow
Mud flow
Avalanche
If the flow is taking place very slowly, the movement is referred to as creep, which is an extremely
common phenomenon, probably occurring on every hillside in the world. Creep is imperceptible in
the majority of these cases unless very precise measurements are performed.
In general, most landslides within the county are shallow, ranging up to perhaps 100 feet in depth
and limited in extent, generally less than 100 acres. Most are not presently in motion ( active), but
have moved down slope to positions of “ apparent” stability. The most notable landslide within
Ventura County is the La Conchita landslide that occurred in 1995. This landslide is a portion of an
older landslide and reactivated in March of 1995. Another landslide area in Ventura County is
along Ventura Avenue on the east side of the Ventura River.
Many of the existing landslides can be reactivated and downslope movement renewed after
exceptionally heavy rainfall periods or as a result of an earthquake or combination of events.
Landslides that occurred after the Northridge Earthquake in the north side of Simi Valley released
sufficient quantities of dust that contained another danger, valley fever.
Since the advent of grading equipment and the continuing development of valleys and plain areas,
hillside and coastal areas in Southern California have come under increasing