Earthquakes Happen
San Andreas Fault Zone between the North American and Pacific Plates
- We know why earthquakes occur in the Bay Area: Two of the earth's largest tectonic plates meet in the vicinity of the San Francisco Bay Area. When boundary faults break, the North American and Pacific Plates roll suddenly and erratically past each other and quakes occur.
- We know that large and damaging earthquakes are certain to occur in the future: At least eight faults in the Bay Area are capable of producing earthquakes of magnitude 6.7 or larger. Such quakes can cause substantial damage to buildings, roads, bridges, and utilities, resulting in injury or death to people in the area.
Between 1850 and 1900, damaging earthquakes (magnitude 6 or greater) occurred in the San Francisco Bay region on average every four years. This pattern changed in 1906.
- On April 18, 1906, the San Andreas Fault ruptured violently over a length of 300 miles. This magnitude 7.8 earthquake relieved stresses on faults throughout the Bay region.
- Because fault stresses were reduced, the rate of large quakes in the San Francisco Bay Area dropped abruptly after the 1906 earthquake.
- For three-quarters of a century following the 1906 quake, the Bay Area experienced a "Golden Age" in which quake activity was minimal. Commerce thrived. Urban areas and population expanded rapidly throughout the area.
- Although the level of seismic activity in the area has not yet reached that of the late 1800s, experts agree that stresses on Bay Area faults have been building up.
- As a result, the Bay Area can expect more frequent and stronger earthquakes in the future.
Scientists are now telling us that there is a 62 percent probability that at least one earthquake of magnitude 6.7 or greater will occur on a known or unknown San Francisco Bay Area 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. (For more information, visit http://quake.usgs.gov/research/seismology and http://pubs.usgs.gov/fs/2003/fs039-03/ An example of a damaging quake on a previously unknown fault occurred on September 3, 2000, in Yountville (Napa County). This magnitude 5.2 quake struck at 1:36 a.m., 10 miles northwest of Napa. It injured 25 people, caused at least $10 million in damage, and forced 70 people to seek shelter at Red Cross facilities. To find out more about the potential danger where you live, you may visit http://gmw.consrv.ca.gov/shmp/SHMPrealdis.htm
Members of the Structural Engineers Association of Northern California recently helped write and produce a booklet about earthquakes, what happens during and after them and what people who live in earthquake country need to know and do. Some of the material is excerpted here. For a full view of the booklet go to http://earthquake.usgs.gov/hazards/pdrlinks.
Most Earthquake Damage Is Caused by Shaking
Damage from earthquakes is mainly the result of the shaking. Although shaking causes most earthquake damage, other damaging effects of quakes can be just as devastating. For example, in the 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.
Fires:
Earthquakes in urban areas are often followed by destructive fires because
gas lines break, electrical shorts ignite fires, damaged water tanks and broken
pipes limit water for firefighting, and 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).
Infrastructure
Collapse: 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. The photo shows a car that crashed when
a section of the eastern span of the San Francisco-Oakland Bay Bridge collapsed
in the 1989 Loma Prieta quake. (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.
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.
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. The photo shows a home that
was destroyed when the hillside beneath it gave way following the 1994 Northridge
earthquake (FEMA photo).
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 Loma Prieta
earthquake (USGS photo).
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 1906 earthquake (USGS photo).
Tsunamis :
Great earthquakes that occur 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. The photo shows Hazel's Fish Stand, a Half Moon Bay
bait show that was ruined when it was hit by debris in the tsunami generated
by the 1964(magnitude 8) Alaska earthquake (photo copyright by MS & SB
Collection).
For more detailed information about the causes of earthquakes and the probability of future earthquakes go to http://earthquake.usgs.gov/. Here you will find a useful list of websites that supplement the information in the booklet Putting Down Roots in Earthquake Country that SEAONC helped produce with other scientists and experts. For a copy of the booklet, email Ken Miles, SEAONC Administrator: kmiles@hp-assoc.com.
