The following is a brief description of some of the active
and potentially active faults present within the Southern California area. The
list below is not a complete catalog of all faults within the area, but is
intended to provide an overview of some of the more recognized faults.
San Andreas Fault
The San Andreas Fault forms a major tectonic boundary between the Pacific Plate
and the North American Plate and has been the source for many large, devastating
earthquakes. Due to the length of the fault surface, the San Andreas Fault is
capable of producing a magnitude 8.0 or larger earthquake. The San Andreas Fault
displays strike-slip movement with an average slip rate of about 5mm per year.
The San Andreas Fault is zoned active based upon the Alquist-Priolo Special
Studies Act with a moderately high risk of producing the next large scale
earthquake to effect the Southern California area.
San Fernando Fault Zone
The San Fernando Fault Zone consists of five major, en echelon segments, all
displaying reverse-left-oblique movement. The five segments consist of the
Reservoir Segment, the Mission Wells Segment, the Sylmar Segment, the Tujunga
Segment and the Lake View Segment. The fault system is approximately 17 km in
length and 5 km in width (Ziony and Yerkes, 1985). The San Fernando Fault Zone
forms the frontal system of the San Gabriel Mountains. This fault system was
most recently active during the 1971 San Fernando Earthquake which produced a
Richter magnitude 6.4 earthquake. Surface breaks were mapped along fault system
during the 1971 earthquake which demonstrated left lateral and normal components
of fault movement. The fault system is zoned active by the Alquist-Priolo
Special Studies Act and capable of producing a magnitude 6.5 earthquake.
Mission Hills Fault
The Mission Hills “Fault” actually consists of three individual fault traces
exposed between Sepulveda Pass and the Van Norman Reservoir. It has been
postulated that the faults comprise a Mission Hills thrust which dips 40 to 50
degrees to the north and forms the southwestern extension of the San Fernando
fault. The fault is not zoned active by the State of California, however recent
studies suggest Holocene movement.
Santa Susana Fault Zone
The Santa Susana Fault Zone consists of several individual fault segments which
extend from the Santa Susana Mountains across the San Fernando Pass and into the
San Gabriel Mountains for a distance of 28 to 38 kilometers. The majority of the
fault system has most likely been inactive since the middle Pleistocene, however
there is still some debate as to whether or not any movement occurred along the
fault system during the 1971 San Fernando Earthquake. Sympathetic movement of
the western portion of the fault system is postulated to have occurred during
the 1971 Earthquake. The western portion of the fault has been designated an
Alquist-Priolo Special Studies zone fault and is zoned active. The fault has
been assigned a slip rate of 3 to 4.5 mm/year.
Newport-Inglewood Fault Zone
The Newport-Inglewood Fault can be structurally divided into two distinct
mechanical segments, based upon work performed by James F. Dolan and Kerry Sieh
with the Caltech Seismological Laboratory. The southern segment of the fault
extends from the northern end of the 1993 Long Beach earthquake rupture at
Signal Hill, southward parallel to the coast to near San Diego, where it comes
onshore as the Rose Canyon fault. This portion of the fault exhibits
right-lateral strike-slip movement. The northern segment of the
Newport-Inglewood Fault Zone is separated from the southern section by Signal
Hill, trends through the Los Angeles Basin, and consists of a discontinuous, en
echelon series of several fault traces. The northern segment of the fault is
considered to have lower seismic activity than the southern segment.
Malibu Coast Fault
The Malibu Coast Fault consists of several onshore and offshore en-echelon fault
segments trending east-west along the southern margin of the western Santa
Monica Mountains. Where the fault is encountered onshore, reverse motion can be
observed with dip angles ranging from 45 to 80 degrees to the north. A slip rate
of 0.04 to 0.09 mm/year has been assigned to the fault. The Malibu Coast Fault
is known to offset Pleistocene deposits of 11,000 to 2 million years in age. The
fault is considered to be “potentially active” as defined in the Alquist-Priolo
Special Studies Zones Act of 1972. Recent evidence by private consultants
indicates that sections of the fault may in fact be active. Excavations about a
mile west of the Malibu pier have encountered offset sediments thought to date
from 4,000 to 6,000 years old.
Santa Monica Fault
The onshore extension of the Malibu Coast Fault has been designated as the Santa
Monica Fault. The fault is buried under unconsolidated alluvial sediments and is
not exposed anywhere at the ground surface. For this reason, the precise
location of the fault is not known. The Santa Monica Fault is an
oblique/left-lateral fault which exhibits pronounced near-surface strain which
has caused development of a series of near-vertical, left-lateral strike slip
faults and a near-surface blind thrust. Work performed in 1992 and 1993 by James
F. Dolan and Kerry Sieh for the Caltech Seismological Laboratory consisted of
fault trenching, geomorphic mapping and paleoseismologic analysis. The results
of their studies indicate that at least three surface-rupturing earthquakes have
occurred on one of the fault strands during the past 12,000 to 15,000 years. The
predicted recurrence interval is approximately 4,000 to 5,000 years. While the
researchers consider the Santa Monica Fault to be active, the fault has not yet
been zoned active by the State of California.
Where the Santa Monica Fault joins with the Malibu Fault, this zone has been
referred to as the Potrero Canyon Fault. Where exposed, the Potrero Canyon Fault
is a north-dipping reverse fault which is part of the frontal fault system of
the Transverse Ranges. The fault has been studied extensively by John T. McGill
and described in the publication, “Geologic Maps of the Pacific Palisades Area,
Los Angeles, California, 1989". The fault is exposed in the mouth of Potrero
Canyon, extending onshore with a trend of about N75E. The fault can be observed
to displace upper Pleistocene marine and nonmarine alluvial terrace deposits.
Movement along the fault has contributed to the uplift of the overlying alluvial
terrace deposits. The rate of uplift has been estimated at 1.2 feet per 1,000
years. The Potrero Canyon Fault is considered to be potentially active because
of the geologically recent (late Pleistocene) displacement on the fault and the
historic seismic activity along the southern frontal fault system which included
the Point Mugu earthquake (M 6.0) of February 21, 1973. East of the Pacific
Palisades area, McGill has reported topographic features which suggests
deformation since formation of the late Pleistocene alluvial plane. Topographic
and geomorphic features suggest that the fault extends to within about a mile of
Beverly Hills. The Potrero Canyon Fault has not been zoned active by the State
of California.
Raymond Fault Zone
The Raymond Fault Zone is a northeast trending reverse-left-oblique slip fault
located north of downtown Los Angeles. The fault has a surface expression of
about 22 km and a width of 0.4 km. The slip rate of the fault has been
determined to be 0.22 mm/year (Ziony and Yerkes, 1985). The zone comprises one
to eight strands which diverge from the Sierra Madre Fault Zone in the area of
Monrovia, and trends to South Pasadena. The fault lies entirely within the
greater Los Angeles area, where it forms the geologic boundary between the
Raymond Basin and the San Gabriel Valley. It trends generally east-west through
an urbanized area. Geomorphic features indicate vertical and probable
left-lateral movement during the Holocene. Trench studies and radiometric dating
of exposed sediments indicate five major seismic events in the last 36,000 to
155,000 years, and an additional three events which cannot be dated precisely in
the last 29,000 years (Crook, et al, 1987). The fault features a nearly
continuous fault scarp between Monrovia Canyon and Arroyo Secco, as well as a
scarp feature in alluvium on the south side of the fault in the South Pasadena
area.
Verdugo Fault
The Verdugo Fault forms a prominent break at the base of the Verdugo Mountains
and youthful geomorphic conditions suggest recent seismic activity. Surface
breaks in alluviated slopes suggest that the fault zone is approximately one
kilometer wide. The fault is not zoned active by the State of California.
However, recent investigations suggest that seismic activity on the fault is
possible. During the 1971 Sylmar Earthquake, surface ruptures occurred
subparallel to and outbound of the main fault trace.
Simi-Santa Rosa Fault
The Simi-Santa Rosa Fault System consists of parallel and subparallel segments
which form a frontal fault zone along the Simi Hills within the cities of
Camarillo, Moorpark, Simi Valley and portions of Ventura County. The fault
system can be separated into several major segments consisting of the
Springville Fault Zone, the Camarillo Fault Zone, the Santa Rosa Fault Zone and
the Simi Fault Zone. The system of faults extends approximately 30 miles and
displays north-dipping reverse movement. Recent studies suggest a slip rate of
0.8 to 2.0 mm/year. Numerous private investigations have been recently performed
along the fault system as a condition for residential and commercial
construction. The recent investigations have revealed that segments of the fault
zone display late Quaternary movement. Geomorphic evidence also suggests late
Quaternary movement along the fault. Carbon dating has been performed along
segments of the fault which indicate movement at least as recent as 26,000 years
ago. The State of California however found insufficient evidence for Holocene
movement to zone the fault system active, as required by the Alquist-Priolo Act.
The City of Camarillo in 1986 however was compelled to classify the fault as
active within the city limits and established a special studies zone of 400 feet
around known traces of the fault system. The maximum possible earthquake along
the fault system has been estimated between Richter Magnitude 6.5 and 7.5
Palos Verdes Fault Zone
The Palos Verdes Fault Zone consists of three segments, all displaying reverse
right oblique or reverse movement. Insufficient evidence currently exists to
zone the fault system active. The Palos Verdes Fault Zone is considered active
however based upon recent studies. The fault system has a cumulative length of
about 77 kilometers and an assumed slip rate of 0.02 to 0.7 mm/year.
San Cayetano Fault
The San Cayetano Fault forms a portion of the southern boundary of the Ventura
Basin from north of Ventura to Piru Creek. The fault displays gentle to moderate
dips to the north with a thrust sense of movement. The eastern portion of the
fault exhibits one of the highest slip rates in the region (>7.5mm/yr; Rockwell,
1988). The fault is considered active with a maximum probable earthquake of 7.3.
Elysian Park Thrust
The Elysian Park Thrust consists of a series of shallowly north and
northeast-dipping blind thrusts that extend from Orange County through downtown
Los Angeles and westward beneath the Santa Monica Mountains. The thrust system
is not exposed at the surface but is buried under the unconsolidated alluvial
sediments of the Los Angeles basin. Recent studies suggest that the fault
experiences an average slip rate of 1.5mm/year and is capable fo producing a
Magnitude 6.7 earthquake.
Oak Ridge Fault
The Oak Ridge Fault is a major south-dipping reverse fault which extends from
the eastern portion of Ventura County into the Santa Clarita Valley and forms a
portion of the southern boundary of the Ventura Basin. The fault is postulated
to join the Santa Susana Fault to the east. Large Quaternary displacements are
indicated in the area of Fillmore-Piru. Near Saticoy faulting in quaternary
sediments has produced a groundwater barrier. The eastern portion of the fault
is considered active, displaying Holocene movement. The fault is considered
capable of producing a magnitude 7.0 to 7.5 earthquake.
Hollywood Fault
The Hollywood Fault is a very steep (65 to 90 degree) north-dipping left-lateral
strike slip fault located along the southern foothill region of the Santa Monica
Mountains and forms the landward extension of the Malibu Coast/Santa Monica
Faults. Work performed by James F. Dolan and Kerry Sieh with the Caltech
Seismological Laboratory in 1992 just west of downtown Hollywood consisted of
analyzing continuously cored borings from the MetroRail Red Line as well as
observation of storm drain trenches on Fuller and Vista Avenues. Based upon the
observation of offset paleosoils younger than 17,000 years, it was concluded
that the Hollywood Fault is active and that at least one surface-rupturing
earthquake has occurred in the last 12,000 to 15,000 years. The work also
indicated that it has been at least 2,000 to 4,000 years since the last seismic
event, suggesting a 4,000 to 8,000 year recurrence interval. To date however the
Hollywood Fault has not been zoned active by the State of California.
Northridge Hills Fault
The Northridge Hills Fault transects the northern San Fernando Valley from
west-northwest of Chatsworth, in the vicinity of the Santa Susanna Pass, to the
east central valley where it is concealed beneath thick alluvial sediments. In
the northwest section of the valley, discontinuous, aligned topographic
expression defines the fault (Wentworth and Yerkes, 1971). The Northridge Hills
Fault may very well be associated with or part of the fault system related to
the uplift of the Santa Susanna Mountains (Barnhart and Slosson, 1973). The
western portion of the fault dips northward at approximately 80 degrees, with
the north side up relative to the south side. Farther east, evidence suggests
the fault is nearly vertical, and possibly southward-dipping.
Subsurface data from nearby petroleum exploration indicate that Miocene strata
has been vertically displaced between 500 and 1000 feet, with a corresponding
variance in thickness of the alluvium overlying the Miocene rock units (Barnhart
and Slosson, 1973). This shows that the fault experienced post-Miocene slippage
and seismic events with activity continuing during the Pleistocene, and possibly
extending into the Holocene. This fault is believed to be capable of producing
another 1971 San Fernando earthquake-type event (Barnhart and Slosson, 1973).