Why Should I Care?—The Bay Area Is Your Home

All Bay Area Residents Live on an Active Plate Boundary Where Earthquakes Are Frequent Events!

Don't be fooled!—Myth number 1
The idea or myth of California sliding into the Pacific Ocean in an earthquake and creating new beachfront property to the east appeals to those having a bit of fun at the Golden State's expense. Although part of the State west of the San Andreas Fault system is very slowly moving northward and in millions of years could become an island, earthquakes caused by this horizontal motion of the Earth’s tectonic plates will not make California disappear into the sea, like fabled Atlantis.

Plate Motions Load the Faults

Plate motion load the faults

Deep beneath California, the Pacific and North American Plates relentlessly grind past one another, straining or "loading" faults in the Earth’s rigid crust above. The horizontal ("strike slip") movement between these plates along the San Andreas Fault Zone is about 1.7 inches per year (40 mm/yr), about as fast as your fingernails grow. At this rate, Los Angeles will be west of San Francisco in about 12 million years.

January 1700 M 9

January 1700 M 9

In this computer simulation, tsunami waves are radiating outward after a magnitude (M) 9 earthquake that occurred on the Cascadia Subduction Zone offshore of northern California, Oregon, and Washington on January 26, 1700. This view shows the waves 4 hours after the quake. Colors indicate wave heights—red is highest. Along parts of the coast of the Pacific Northwest, 30-foot-high (9 m) waves rushed inland. Within 20 hours the tsunami did damage throughout the Pacific, and it is well documented in written records from Japan. More information on tsunami waves. "Orphan tsunami."

January 1857 M 7.9

January 1857 m 7.9

The magnitude (M) 7.9 Fort Tejon earthquake in 1857 caused a horizontal shift of about 30 feet (9 m) on this stretch of the southern San Andreas Fault (red arrows indicate this "strike-slip" movement). Although the quake ruptured nearly 200 miles (300 km) of the fault, it did little damage because southern California’s population was small at that time. (USGS photo)

April 1906 M 7.8

April 1906 m 7.8

In the Great 1906 magnitude (M) 7.8 earthquake, nearly 300 miles (480 km) of the San Andreas Fault ruptured, producing strong shaking along all of coastal northern California. Shaking was most intense in Santa Rosa and San Francisco. In San Francisco, broken water mains kept firefighters from battling the fires that swept through the city and contributed to the devastation shown in this photograph (looking south) taken from a tethered balloon 5 weeks after the earthquake.

More information on 1906 Earthquake

History Shows That Damaging Earthquakes Have Occurred Throughout the Bay Area...

In the last half of the 1800s, damaging earthquakes (magnitude 6 or greater) occurred in the San Francisco Bay region on average every 4 years. However, this changed in 1906:

  • On April 18, 1906, the San Andreas Fault ruptured violently over a length of 300 miles, causing damage from San Juan Bautista north as far as Eureka. This magnitude 7.8 earthquakethe "Great San Francisco earthquake"relieved stresses on faults throughout the bay region.
  • Because fault stresses were reduced, the rate of large quakes in the San Francisco Bay region dropped abruptly after the 1906 earthquake.
  • The three-quarters of a century following the 1906 quake was a golden age for the bay region, in which urban areas and population expanded rapidly during a time of minimal quake activity.
  • Although the level of seismic activity has not yet reached that of the late 1800s, since 1906 stresses on Bay Area faults have been building up once again. The area can expect more frequent and stronger earthquakes in the future.

Fault Rupture Lengths for Historical Quakes in the Bay Area

The Past and Future of Significant Bay Area Quakes

1969—Damage in Santa Rosa1906—Damage in Healdsburg
1868—Damage in Hayward1865—Damage in San Francisco

(Historical photos of shaking damage courtesy National Information Service for Earthquake Engineering (NISEE), University of California, Berkeley.)

The threat of future quakes extends across the Bay Area...

San Francisco Bay Region Earthquake probability

There is a 62% probability that at least one earthquake of magnitude 6.7 or greater will occur on a known or unknown San Francisco Bay region fault before 2032. After a century of study by geologists, many faults have been mapped in the region, but not all faults are apparent at the surface—some quakes occur on previously unknown faults.

An example of a damaging quake on a previously unknown fault is the September 3, 2000, Yountville (Napa) earthquake. This magnitude 5.2 quake struck at 1:36 a.m., 10 miles northwest of Napa, rudely awakening many residents of the Bay Area. It injured 25 people, caused at least $10 million in damage, and forced 70 people to seek shelter at Red Cross facilities.

Most Earthquake Damage Is Caused by Shaking...

Damage in earthquakes is mainly from shaking. The intensity of shaking that a structure will experience during an earthquake is a function of three main factors:

  1. The magnitude of the earthquakethe larger the quake, the stronger the shaking.
  2. The distance from the fault that rupturedthe nearer you are to the fault, the greater the shaking.
  3. The type of ground materials beneath the structuresoft soils amplify the shaking; hard bedrock does not.

Buildings in the Marina District of San Francisco were badly damaged in the 1989 Loma Prieta earthquake. Soft story buildings, typically with parking on ground floor, like the one pictured here, are common throughout the Bay Area and are particularly at risk when exposed to strong shaking. (USGS photo)

Expected Levels of Shaking from Future Earthquakes

More information on expected level of shaking

  • On this map, bands of highest expected shaking generally follow major faults.
  • Shaking levels are also influenced by the type of materials underlying an areasoft soils tend to amplify and prolong shaking, even at great distances from a quake.
  • The worst soft soils in the Bay Area are the loose clays and filled areas bordering San Francisco Bay and the Sacramento-San Joaquin Delta.
  • Deep soils in valleys shake more than bedrock in the hillsmost urban development is in the valleys.
  • Intense shaking can damage even strong, modern buildings and their contents.

Soft Soils Amplify Earthquake Shaking

The Cypress freeway structure in Oakland was built in the 1950s, before the use of modern seismic-safety standards. Part of the structure standing on soft mud (dashed red line) collapsed in the 1989 magnitude 6.9 Loma Prieta earthquake, whose epicenter was nearly 60 miles (100 km) to the south. Adjacent parts of the structure (solid red) that were built on firmer ground remained standing. Seismograms (upper right) show that the shaking was especially severe in the soft mud. (Photo by Lloyd S. Cluff, Pacific Gas & Electric)

The Exposure of the Bay Area’s Built Environment to Violent Earthquake Shaking—Many critical facilities in the Bay Area will likely experience damaging earthquake shaking in the next 30 years.

Source: Association of Bay Area Governments, 2005.

Earthquakes Also Cause Damage in Other Ways...

Although most earthquake damage is caused by shaking, other damaging effects of quakes can be just as devastating. For example, in the Great 1906 earthquake, the shaking damage in San Francisco was followed by fires that raged through the city almost uncontrolled, in part because water mains had broken in the quake. These and other destructive effects of quakes are discussed below.

Fires

Fires-Earthquakes in urban areas are often followed by destructive fires because (1) gas lines break, (2) electrical shorts ignite fires, (3) damaged water tanks and broken pipes limit water for firefighting, and (4) clogged roads and collapsed bridges prevent firefighter access. These factors can lead to fires spreading, causing extensive additional damage and burning entire neighborhoods. This photo shows fires in San Francisco’s Marina District following the 1989 magnitude 6.9 Loma Prieta earthquake (photo courtesy of CBS 5).


Damaged bridges, pipelines, powerlines, and roads

Damaged bridges, pipelines, powerlines, and roads-Earthquakes often damage roads, hindering rescue and recovery efforts and causing accidents. Water and sewer pipeline breaks result in water loss and can cause "sinkholes" that undermine roads and buildings. Damage to natural gas and electrical distribution systems can cause fires, as well as major service outages. This car crashed when a section of the eastern span of the San Francisco-Oakland Bay Bridge collapsed in the 1989 magnitude 6.9 Loma Prieta earthquake (Earthquake Engineering Research Institute photo).


Dam failures-Earthquake shaking can cause dams to fail, potentially causing catastrophic downstream flooding and reduced water supplies. In addition, many dams provide hydroelectric power, which could be critically needed following a quake. Cracks in the top of this dam were caused by the 1989 magnitude 6.9 Loma Prieta earthquake (USGS photo).


Dam failures

Hazardous material releases-Earthquake damage can cause releases of hazardous materials from refineries and other chemical storage and distribution systems, research and industrial laboratories, manufacturing plants, and railroad tank cars. Oil was released and caught fire when this storage facility was damaged by the 1999 magnitude 7.4 Izmit, Turkey, earthquake (photo by Kandilli Observatory and Earthquake Institute).


Landslides

Landslides-Earthquakes can trigger landslides that damage roads, buildings, pipelines, and other infrastructure. Steeply sloping areas underlain by loose or soft rock are most susceptible to earthquake-induced landslides. This home was destroyed when the hillside beneath it gave way following the 1994 magnitude 6.7 Northridge earthquake (FEMA photo).


Liquefaction

Liquefaction-Earthquake shaking can cause soils to behave like a liquid and lose their ability to support structures. Liquefaction often causes buried gas and water lines to break. The highest hazard is in low-lying areas where there are loose, sandy soils or poorly compacted artificial fill. This photo shows liquefaction-related damage in the Marina District of San Francisco following the 1989 magnitude 6.9 Loma Prieta earthquake (USGS photo).


Surface rupture

Surface rupture-Fault movements can break the ground surface, damaging buildings and other structures. This fence near Point Reyes was offset 8 feet ( 2.5 m) when the San Andreas Fault moved in the Great (magnitude 7.8) 1906 earthquake (USGS photo).


Tsunamis

Tsunamis-Great earthquakes occurring anywhere in the Pacific Ocean may displace the ocean floor, generating tsunamis that could affect the California coast. Some coastal communities are designating Tsunami Hazard Zones and planning evacuation routes. Although the tsunami hazard in most of the Bay Area is low, coastal areas are still at risk. For example, this bait shop (Hazel’s Fish Stand) in Half Moon Bay was ruined when it was hit by debris in the tsunami generated by the 1946 (magnitude 8) Alaska earthquake (photo copyright by MS & SB Collection).


Don't be fooled!—Myth number 2
"And the Earth opened"
A popular literary device is a fault that opens during an earthquake and then closes to swallow up an inconvenient character. Unfortunately for principled writers, such "carnivorous" faults exist only in novels and B-movies.
Map showing seismic hazard zones

Map Showing Seismic Hazard Zones—This map of part of the Oakland area shows regulatory zones defined by the California Geological Survey (CGS) for three seismic hazards—earthquake-induced landsliding, liquefaction, and surface rupture. These zones indicate where the hazard may exist. Sites of proposed construction (new or remodel) within a zone must be investigated for the hazard. These maps are also used in real-estate transactions—disclosure is required if a property is within any of these hazard zones. View CGS seismic hazard zone maps for the Bay Area.