JCP Reports - Hazard Information

Hazard Information - Earthquakes

By JCP Geologists

About 80% of Californians live in areas subject to damaging earthquakes. Most California residents have already accepted some risk related to earthquakes. However, few people know the factors that determine the magnitude of that risk. Earthquake risk depends on several factors. Considering these factors before buying a home in "earthquake country" may help alleviate earthquake fears.

What Causes Earthquakes?

California lies at the juncture of two of the continent-size plates that cover the Earth somewhat like a jigsaw puzzle. These plates are slowly moving and sliding past each other due to deep-seated tectonic forces. The pressure of moving these plates gradually builds at their junctures. In California, this juncture is the San Andreas Fault Zone. An earthquake occurs when the rocks deep within the earth suddenly release the built-up pressure as stored energy by "snapping" along the fault (fault rupture). The zone of rupture is termed a fault. In severe earthquakes, the fault can "rupture" all the way up to the surface and crack the ground. The stored energy is released as waves which we feel as earthquake shaking.

An earthquake occurs when energy is released along a fault. The energy builds-up gradually as rocks are "squeezed" against each other. If the rocks can't slide past each other gradually, then they slowly bend and deform until a breaking point is achieved. At this point, the rocks suddenly "snap" apart along the fault.

The potential for earthquake damage to a given home is a complex function of distance to the earthquake source, magnitude of the earthquake, the type and thickness of geologic materials underlying the site, the local topography, and how the home and its foundation are constructed.

Can we know where the next earthquake might occur? In general, faults which have historically caused earthquakes are the likely source of future quakes. As part of a hazard reduction program, California maintains an ongoing project to map faults. This mapping is accomplished through the Alquist-Priolo Earthquake Fault Zone Act.

Earthquake Risk

The actual risk at a site can be evaluated by defining the hazards present and considering the effects of economical mitigating measures. Because of the traumatic and inescapable nature of earthquakes, there is a very strong and natural tendency for people to take an emotional rather than a rational approach to the subject of earthquake risk. While we do not wish to downgrade the risks involved in areas of high seismic activity, we believe that the risks should be assessed in relation to other risks that the public accepts. In other words, judgments of earthquake risk should be approached in the same manner as judgments of other risks, where the factor of safety is weighed against the consequences of failure.

In discussing earthquake risk, two aspects of that risk need to be understood. The first aspect considers the risk of earthquakes occurring that will be felt at the property. The second aspect considers the risk of damage caused by earthquakes that may occur. Most everyone understands that the risk of earthquakes occurring in California is higher than most of the country. This is because most of the population and developed land are located near some of the most active faults in the country. Some of the better known active faults are the San Andreas, Hayward, and San Jacinto. Many other active and potentially active faults also exist in the region and produce large earthquakes such as the Northridge Quake on January 17, 1994.

At present it is not possible to predict when large magnitude earthquakes will occur except in very general terms. Techniques for accurately forecasting earthquakes are currently being researched; but, at present, no reliable methods are available.

Most earthquake damage on October 17, 1989 and January 17, 1994 resulted from poor construction practices, buildings not built according to current standards (older structures), or poor maintainence (no earthquake bolting, termite damage, etc.). A lesser magnitude earthquake in Armenia in 1988 killed over 25,000 and left hundreds of thousands homeless. Building codes in most of the U. S. outside of California are not nearly as stringent regarding seismic shaking as in this state. A major earthquake in the eastern U.S. may well cause more damage than that recently experienced in California. In conclusion, because of stringent and constantly updated building codes, the risk of damage from earthquakes in California is less than one may believe.

More stringent building codes have been in effect in the Bay area since the 1950's. The Loma Prieta Earthquake of October 17, 1989 provided an excellent test of the general level of earthquake resistant building in the Bay area. The fact is that outside of areas affected by liquefaction (Marina District, Cypress structure), soft sediments (Bay Mud), and those areas near the epicenter, there was little damage or loss of life and injury caused by this major earthquake.

The Northridge Earthquake in southern California also demonstrated that California's current building codes and practices are generally adequate to protect lives. However, more work needs to be done to upgade codes to reduce the kind of earthquake damage such quakes can produce.