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Topography, Fault Characteristics, and
Earthquake History Along the San
Andreas System
• When viewed from an
airplane, almost any part
of the San Andreas Fault
appears on the ground as a
sharp linear scar that cuts
across all features of the
landscape, displacing
ridges, valleys, and
streams faster than they
can be smoothed by
erosion and deposition.
The San Andreas as it crosses the
Carrizo Plain in central California
• At the northern end of the
fault is the Mendocino
Triple Junction. This
boundary is the junction
between the Pacific,
North American, and
Gorda Plates, officially
placed at the isolated
village of Petrolia in
Humboldt County, and
epicenter of 3 large
earthquakes in 1992. On
April 25 and 26, 1992,
three earthquakes with
magnitudes between 6.0
and 7.0 shook the region.
This is one of the most
seismically active parts of
the state.
• From the Mendocino triple junction south to
Point Arena, the San Andreas Fault Zone lies
offshore. At Point Arena, the fault emerges from
the Pacific and can be traced to Fort Ross, where
it again runs offshore.
San Andreas Fault
Zone
Point Arena
Point Reyes Peninsula
• Further south, Tomales Bay is one of the
San Andreas Faults’ most striking
landforms, separating the Point Reyes
Peninsula from the mainland along a
narrow slash that extends south of Tomales
Bay to Bolinas Bay. From Bolinas Bay the
fault lies offshore, west of San Francisco,
until it intersects the San Francisco
Peninsula and remains on land for the rest
of its length.
Another
view of the
Tomales Bay,
the North
American
Plate is on
the right,
Pacific Plate
on the left.
Bolinas Lagoon, the San Andreas runs
beneath the lake, the North American Plate
is on the right, Pacific Plate is on the left.
•As the San Andreas
system crosses the San
Francisco Bay Area,
movement is shared
among 4 major rightlateral fault zones, which
in addition to the San
Andreas includes the
Rodgers Creek, Hayward,
Calaveras Faults.
Rodgers Creek Fault
Hayward Fault
San Andreas Fault
Calaveras Fault
1906 San Francisco Earthquake
Golden Gate Avenue in San Francisco,
1906 following the earthquake but before
the fire that destroyed many of the
buildings
• San Francisco in 1906 was a
prosperous city of 400,000
inhabitants, and a major center
for world trade and banking.
But at 5:12 AM on April 18, a
great earthquake brought
disaster to San Francisco and
surrounding areas. Centered on
the San Andreas Fault west of
the city, the earthquake
registered about ML 8.2, and
was felt as far away as Los
Angeles and central Nevada.
Damage in Marina District, San Francisco
Damage
to historic
buildings
in Santa
Cruz
• In addition, a slight bend
in the San Andreas in the
Santa Cruz Mountains
area appears to have
caused the fault to
generate numerous faults
throughout the region.
One of these, the Sargent
Fault, is believed
responsible for the ML 7.1
(MW of 7.0) 1989 Loma
Prieta Earthquake.
Central California: The Creeping
Segment and the Parkfield Area
• Between San Juan Bautista and Parkfield, the San
Andreas system behaves very differently from its
neighboring segments. This central segment
experiences numerous small earthquakes, usually
with magnitudes less than 4.0. The Calaveras and
Hayward Faults east of San Francisco Bay show a
similar pattern.
Left: The San
Andreas Fault as it
cuts just to the south
of the town of
Parkfield.
Right: Creeping
along the San
Andreas Fault has
caused offset across
this road.
Distorted
Fence
across the
San
Andreas
Fault,,
Melendy
Ranch
Curb offset
in 1974
(above) and
in 1993
(below)
Hayward
• These faults display a
relatively constant, slow
displacement called
fault creep. The
continuous offset
displaces sidewalks,
curbs, and other cultural
features along the
faults.
Offset of culvert near Almaden Cienega Winery near Hollister
The Carrizo Plain and the Fort
Tejon Earthquake
• Along this relatively straight,
relatively simple segment of the fault,
one can view the offset streams,
compressional ridges, linear valleys,
and sag ponds that characterize a
transform fault on land.
Wallace Creek,
offset by
motion along
the San Andreas
Aerial view of the San Andreas
as it crosses the Carrizo Plain
The 1857 Fort Tejon Earthquake
•
San Andreas Fault
Epicenter
1857 rupture
Garlock Fault
Rupture area (in red) of 1857 Ft. Tejon earthquake
On January 9, 1857, an
enormous earthquake
ruptured the San Andreas
Fault from Parkfield through
the Big Bend segment and
southeast at least to
Wrightwood, a total of at
least 360 kilometers. Fort
Tejon, a military outpost at
the southernmost end of the
Carrizo Plain was one of few
population centers near the
epicenter. There the ground
shook for 1 to 3 minutes.
The earthquake produced as
much as 9 meters of offset in
the Carrizo Plain and 3 to 4
meters in the Mojave Desert.
The Big Bend: The San Andreas Fault System
in the Transverse Ranges and the San Gabriel
and San Bernardino Mountains
• From its junction with the
Garlock Fault, the San
Andreas Fault makes a
marked bend to the
southeast for about 120 km.
This segment is
appropriately referred to as
the Big Bend. Because of
the significant component
of compression in the
region, rocks are actively
being squeezed and
uplifted.
• As a result of the
compression, spectacular
mountain ranges have been
thrown up along the margins
of the fault, such as the San
Gabriel, San Bernardino,
and San Jacinto ranges. The
steep mountain slopes have
shed enormous quantities of
debris that are spread across
the range fronts in large
alluvial fans. The fans,
increasingly occupied by
high-density housing and
commercial developments,
are in many places cut by
reverse faults caused by
compression in this area.
San Andreas Fault
San Jacinto Fault
Newport-Inglewood Fault
Whittier-Elsinore Fault
• The right-lateral motion
along the San Andreas
system is taken up by a
number of different
number of fault strands
in the Big Bend segment
including the San
Jacinto, WhittierElsinore, and NewportInglewood Faults. The
San Andreas itself
defines the eastern end of
the Big Bend segment,
and at present is the most
active fault strand.
The 1933 Long Beach Earthquake
• The Long Beach earthquake
centered on the NewportInglewood Fault struck March 10,
1933. Although it was a moderate
event with a moment magnitude of
6.2, the earthquake killed 120
people, injured hundreds, and
caused over $40 million in
damage. Poor construction
accounted for tragic failures of
school structures in the Long
Beach area (see left). Luckily, the
schools were almost empty when
the quake hit at 5:54 PM.
Following the earthquake, the
California legislature passed the
Field Act which requires state
approval and inspection of both
plans and construction of school
buildings.
The 1971 San Fernando Earthquake
• Early on the morning of February 9,
1971, a moderate earthquake (M 6.4)
shook the densely populated San
Fernando Valley. Although lasting
only 15 seconds, the earthquake
killed 58 people, destroyed 2
hospitals and a modern freeway
overpass, and caused over $500
million in damage. Had the shaking
lasted a mere 5 seconds longer the
tragedy would have been far worse.
The Van Norman Dam, built in 1915
overlooking the San Fernando
Valley, broke during the shaking.
Partly because the reservoir was only
half full, the dam held, and the
80,000 residents below were spared
from a disastrous flood.
Collapse of the Olive View Hospital, San
Fernando
• Due to this earthquake, three
important acts were passed by
the California legislature, the
Alquist-Priolo Earthquake
Zoning Act, the Dam Safety
Act and the Hospital Safety
Act. The Alquist-Priolo Act
prohibits the construction of
most human-occupied
structures within 50 feet of an
active fault. The Dam Safety
Act requires evaluation of the
safety of existing dams in
California and the Hospital
Safety act called for the
strengthening of construction
standards for hospitals.
Near collapse of the Van Norman Dam, San Fernando
Pallett Creek and Earthquake
Frequency
The San Andreas Fault as it cuts through and
offsets coal seams at Pallett Creek
• Along the bank of Pallett Creek,
about 55 km northeast of Los
Angeles, is a marsh that has
been cut by a strand of the San
Andreas Fault. Deposits here
record amazing evidence of past
earthquakes on the San Andreas
Fault in southern California. At
least 12 earthquakes have
broken the sediments at Pallett
Creek in the past 1700 years at
this site, giving geologists the
best information about
earthquake history along the
San Andreas.
The Southern San Andreas, the Salton
Trough and the Imperial Valley
• The Salton Trough, a
long desert valley that
contains both the
Imperial Valley and the
Salton Sea, is the most
striking feature of the
southern San Andreas
system. Bordered on
the east by the San
Andreas Fault and on
the west by the San
Jacinto Fault Zone, the
Salton Trough is about
390 kilometers long.
Map of the Salton Trough.
LANDSAT photo of the Salton
Trough region.
The San Andreas as it crosses just
north of the Salton Sea, near Palm
Springs.
The San Andreas as it crosses
through the Mecca Hills on
the eastern shore of the Salton
Sea.
Salton Sea
The San Andreas Fault (in red) as it
cuts across the Mecca Hills adjacent to
the Salton Sea
• The transform plate boundary in
the region of the Salton Trough
shows some features
characteristic of the divergent
boundary immediately to the
south in Baja California,
Mexico. The trough is a pullapart basin that has formed
during the past 4 million years
as the Peninsular Ranges have
pulled away from North
America. The spreading has not
yet caused a mid-ocean ridge to
appear, as it has further south.
However volcanic rocks on the
southeastern side of the Salton
Sea appeared about 16,000
years ago.
Space shuttle image of Salton Sea and Gulf of
California.
Volcanic rocks at the southern end of the Salton Sea.
Evidence of magma close to the surface.
• Clues within these volcanic
rocks suggest that midocean ridge magma is
interacting with the
continental crust beneath the
Salton Trough and that
rifting is extending
northward from the Gulf of
California. The high flow
of heat in the area due to
rising magma is enough to
produce several geothermal
reservoirs; some of which
have been tapped for
geothermal energy.
The Eastern California Shear
Zone
CRF
San Andreas
EF
HVF
JVF
LLF
BF
• A zone of faults known as the
Eastern California Shear Zone
(ECSZ) runs parallel to the San
Andreas Fault through the central
Mojave Desert. Many of the faults
are young normal faults, but rightlateral motion also occurs on several,
including the Johnson Valley (JVF),
Homestead Valley (HVF), Emerson
(EF), Camprock (CRF), Lavic Lake
(LLF) and Bullion Faults (BF). The
pattern of activity on these faults
indicates they could correctly be
considered part of the boundary
between the Pacific and North
American Plates.
Fault rupture
through
Landers
Tower destroyed by
fault motion
Offset roads by
Landers Fault
• Several moderate earthquakes have
ruptured the faults of the ECSZ,
including the 1975 Galway Lake (ML
5.2), the 1979 Homestead Valley (ML
5.6) and the April 1992 Joshua Tree
(ML 6.1) events. The Joshua Tree
earthquake was followed on June 28,
1992 by the Landers Earthquake ML
7.3 – the largest earthquake in
historical times in the Mojave Desert
and the largest to strike anywhere in
southern California since the Kern
County Earthquake of 1952.
Roof damage to restaurant in Big Bear City
This rather large boulder was dislodged during the Big
Bear Earthquake and landed on Highway 138 blocking
traffic.
• Three hours after the Landers
mainshock, a second
earthquake of ML 6.2 struck
near Big Bear Lake, about
twenty miles west of Landers.
The Big Bear Earthquake was
west of the ECSZ, and on a
left-lateral fault that trends
northeast. Because this
earthquake struck a more
populated area than did
Landers, it was more
destructive. Despite the great
amount of damage there was
no surface rupture in this
quake, nor was there a surface
fault that could be related to
the mainshock rupture.
•
Seven years later, on October 16, 1999, a ML 7.1
earthquake was accompanied by surface rupture in the
Marine Corps Training Center between Twentynine
Palms and Barstow. It was named the Hector Mine
Earthquake because there was no populated place
close enough to supply a name. The earthquake
ruptured both the Bullion Fault and the previously
unknown Lavic Lake Fault. Even though the
earthquake was northeast of the lineup of events
marking the ECSZ, the trend of the rupture was the
same: north-northwest.
Left: Aerial photo
of Lavic Lake Fault
rupture
Right: Inside the
fault
Above: Offset tire tracks along
Bullion Fault rupture