Photo: Robert A. Eplett/OES CA
Earthquake Program
With offices in Northern and Southern California, the OES Earthquake Program provides planning and technical assistance on various earth-related areas of concern. Program staff members work with local and regional governments, businesses, hospitals, schools, human service agencies, neighborhood and community organizations, and individuals to address, among other things, preparedness planning, hazard mitigation, emergency response, business resumption planning, post-earthquake shelter and housing, and the complexities of disaster recovery. Formed in 1993 by combining the Bay Area Regional Earthquake Preparedness Project and the Southern Earthquake Preparedness Project, the Earthquake Program concentrates its efforts in the most seismically active areas of California, but it is involved in earthquake preparedness and damage reduction efforts across the state.
Earthquake Mitigation
Earthquake Hazard Mitigation Plans
Earthquake Hazard Mitigation Grants
Earthquake Mitigation Success Stories
Additional Earthquake Mitigation Resources
Earthquake Preparedness & Exercises
OES Earthquake Preparedness Tip Sheets
Earthquake Response & Recovery
Earthquake Information and Resources
Additional Earthquake Resources
Guidance & Regulations
Maps, GIS Data, Analysis Tools
Earthquake Frequently Asked Questions (FAQs)
A fault is a fracture in the crust along which one side has moved relative to the other side. Faults can be very small or hundreds of miles long. The earth's crust is composed of huge plates that are in slow but nearly constant motion. Part of California is on the Pacific Plate, and part is on the North American Plate. The San Andreas Fault, which runs from the Salton Sea in Imperial County to Cape Mendocino in Humboldt County, is the boundary between these plates. Sometimes one block of the crust moves up while the other moves down, sometimes they move horizontally in opposite directions (that's what's happening with the San Andreas Fault; Los Angeles is creeping closer to San Francisco). Some faults are well known and easy to spot, such as the San Andreas. Others are underground, with nothing on the surface revealing their presence (a blind thrust fault). The 1994 Northridge earthquake was caused by a blind thrust fault. (From http://www.consrv.ca.gov/index/Earthquakes/qh_earthquakes.htm)
 http://earthquake.usgs.gov/learning/glossary.php?term=fault
- What causes an earthquake?
Earthquakes occur when the two sides of a fault slip suddenly against each other. The Pacific and North American plates move past each other about 1.5 inches a year. The friction between the plates causes stress, which is released when the blocks of crust slip suddenly along a fault plane. That releases waves of energy that travel through the ground, causing the shaking you feel. (From http://www.consrv.ca.gov/index/Earthquakes/qh_earthquakes.htm)
- How common are faults in California?
There are hundreds of identified faults in California; about 200 are considered potentially hazardous based on their slip rates in recent geological time (the last 10,000 years). More than 70 percent of the state's population resides within 30 miles of a fault where high ground shaking could occur in the next 50 years. (From http://www.consrv.ca.gov/index/Earthquakes/qh_earthquakes.htm)
Earthquake ruptures usually begin far under the surface of the Earth. The point of origin miles down is called the hypocenter. The epicenter is the point on the surface directly above the hypocenter. (From http://www.consrv.ca.gov/index/Earthquakes/qh_earthquakes.htm)
 Epicenter & Hypocenter. (Modified from Charles Ammon, Penn State)
- What is surface faulting or surface rupture in an earthquake?
Surface rupture occurs when movement on a fault deep within the earth breaks through to the surface. NOT ALL earthquakes result in surface rupture. (From http://earthquake.usgs.gov/learning/faq.php?categoryID=1)
 Fault scarp produced by the M7.1 Hector Mine, Calfornia earthquake. (Photo by Ken Hudnut, U.S. Geological Survey)
- How often do earthquakes happen?
The National Earthquake Information Center (U.S.) reports 12,000-14,000 earthquakes a year around the world, or 35 a day. Throughout the world, there are one "great" (magnitude 8.0 or more), 18 "major" (7.0-7.9), 120 "large" (6.0-6.9) and 1,000 "moderate" (5.0-5.9) earthquakes in an average year. Each year, California generally gets two or three earthquakes large enough to cause moderate damage to structures (magnitude 5.5 and higher). (From http://www.consrv.ca.gov/index/Earthquakes/qh_earthquakes.htm)
- Can earthquakes be prevented?
While there's no way to stop an earthquake, there are ways to build safer buildings and structures and otherwise be prepared for them. Building codes in California are updated often as new information comes in. While earthquakes are a deadly threat, there have been few earthquake-related deaths in California relative to places with less stringent codes or enforcement, such as Turkey and China. (From http://www.consrv.ca.gov/index/Earthquakes/qh_earthquakes.htm)
- How are earthquakes measured?
There are several ways to measure an earthquake, but the most common is magnitude. Scientists no longer use the original Richter scale, but an updated version. Earthquakes should be referred to as "magnitude X'' rather than "an X on the Richter scale." A magnitude 6.0 earthquake releases 32 times more energy than a magnitude 5.0 and nearly 1,000 times more energy than a 4.0. But that doesn't mean the ground shakes a thousand times harder in a 6.0 than a 4.0, because the energy is released over a much larger area. (From http://www.consrv.ca.gov/index/Earthquakes/qh_earthquakes.htm)
- How much power does an earthquake pack?
A magnitude 6.0 quake releases approximately as much energy as 6,270 tons of TNT, an M 7.0 199,000 tons, an M 8.0 6.27 million tons and a M 9.0 99 million tons. Of course, all that energy is not focused in one particular spot, but spreads out in waves. (From http://www.consrv.ca.gov/index/Earthquakes/qh_earthquakes.htm)
- What factors influence what you feel in an earthquake?
There are three major factors: magnitude, your distance from the fault, and local soil conditions. Magnitude is discussed above. As for distance, the seismic waves that cause the shaking become less intense farther from the fault. Certain soil conditions amplify the shaking; generally, the looser the soil, the greater the amplification. Although most of San Francisco escaped serious damage in Loma Prieta, those with unconsolidated landfill or soft soils (such as the Marina District) suffered serious damage. The ground motion in such areas was 10 times stronger than at neighboring sites on rock. (From http://www.consrv.ca.gov/index/Earthquakes/qh_earthquakes.htm) |