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.

Point Arena

San Andreas Fault Zone

• 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.

Bolinas Lagoon, the San Andreas runs beneath the lake, the North American Plate is on the right, Pacific Plate is on the left.

Point Reyes Peninsula

Another view of the Tomales Bay, the North American Plate is on the right, Pacific Plate on the left.

•As the San Andreas system crosses the San Francisco Bay Area, movement is shared among 4 major right-lateral fault zones, which in addition to the San Andreas includes the Rodgers Creek, Hayward, Calaveras Faults.

Rodgers Creek Fault

Hayward Fault

Calaveras Fault

San Andreas Fault

1906 San Francisco Earthquake

• 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.

Golden Gate Avenue in San Francisco, 1906 following the earthquake but before the fire that destroyed many of the buildings

• 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.

Damage in Marina District, San Francisco

Damage to historic buildings in Santa Cruz

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.

• 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

Curb offset in 1974 (above) and in 1993 (below) Hayward

Distorted Fence across the San Andreas Fault,, Melendy Ranch

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.

Aerial view of the San Andreas as it crosses the Carrizo Plain

Wallace Creek, offset by motion along the San Andreas

The 1857 Fort 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.

San Andreas Fault

Rupture area (in red) of 1857 Ft. Tejon earthquake

Epicenter

1857 rupture

Garlock Fault

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.

• 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, Whittier-Elsinore, and Newport-Inglewood Faults. The San Andreas itself defines the eastern end of the Big Bend segment, and at present is the most active fault strand.

San Andreas Fault

San Jacinto Fault

Newport-Inglewood Fault

Whittier-Elsinore Fault

The 1933 Long Beach Earthquake• The Long Beach earthquake

centered on the Newport-Inglewood 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.

• 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.

Collapse of the Olive View Hospital, San Fernando

Near collapse of the Van Norman Dam, San Fernando

Pallett Creek and Earthquake Frequency

• 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 San Andreas Fault as it cuts through and

offsets coal seams at Pallett Creek

• 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.

The Southern San Andreas, the Salton Trough and the Imperial Valley

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.

• 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 pull-apart 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.

The San Andreas Fault (in red) as it cuts across the Mecca Hills adjacent to the Salton Sea

Salton Sea

• Clues within these volcanic rocks suggest that mid-ocean 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.

Volcanic rocks at the southern end of the Salton Sea. Evidence of magma close to the surface.

Space shuttle image of Salton Sea and Gulf of California.

The Eastern California Shear Zone

• 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 right-lateral 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.

CRF

EF

JVFHVF

LLF

BFSan Andreas

• 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.

Offset roads by Landers Fault

Tower destroyed by fault motion

Fault rupture through Landers

• 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.This rather large boulder was dislodged during the Big

Bear Earthquake and landed on Highway 138 blocking traffic.

Roof damage to restaurant in Big Bear City

• 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

Above: Offset tire tracks along Bullion Fault rupture

Right: Inside the fault