Degrees of Danger

It will not come as a surprise to many Californians that some parts of the state have a far greater risk of powerful earthquakes than others.

But the 1999 seismic shaking hazard maps developed by the California Division of Mines and Geology, in cooperation with the U.S. Geological Survey, are still striking because they demonstrate how comparatively narrow the bands of maximum danger are.

The San Andreas fault is the principal zone of concern--with the exception of a 60-mile segment southeast of Monterey Bay in Central California, where the fault is creeping slowly rather than being subject to infrequent, large ruptures.

But there are other worrisome areas, including the Santa Susana Mountain range in Ventura County and part of Los Angeles County, along the San Jacinto fault trending southeast from San Bernardino, and along the peripheries of San Francisco and Humboldt bays.

Earthquake scientists such as Caltech’s Egill Hauksson emphasize that the maps have limitations. They certainly are not meant to signify that a destructive earthquake could never occur in the areas shown in blue.

One of the smaller maps, for instance, shows areas in California that have sustained severe damage in quakes since 1800. It puts both San Diego and most of the Central Valley in the gray, less hazardous zone. This, however, does not mean that in the long sweep of geological time, San Diego and Sacramento would never be hit by powerful quakes.

But the interval of powerful quakes in less hazardous zones might be in the thousands of years compared to hundreds of years or less in the more hazardous areas.

In each colored band of the large map, the Division of Mines and Geology has estimated the maximum intensity of shaking during a 475-year period. For example, the red and pink areas would reach a peak acceleration of close to 100% of the force of gravity at least once during that period. But in the bluest areas, the peak acceleration would reach only 10% of the force of gravity.

From this, officials extrapolate roughly that there is a 10% chance of this shaking occurring in each 50-year period.

Sylmar, Northridge Quakes Perhaps Linked

But these are averages. It is conceivable that a particular area could be severely shaken more than once every 475 years, or even more than once every 50. Only 23 years elapsed, for example, between the Sylmar-San Fernando and Northridge quakes in the San Fernando Valley. It is now thought that the first of these, in 1971, caused stresses nearby that may have brought on the second quake, in 1994.

Study of recording instruments in the Northridge earthquake showed that there can be wide variability, within very small areas, of shaking intensities. Some parts of the San Fernando Valley in the 1994 quake shook at nearly twice the force of gravity, but that was generally confined to very small areas.

Also, the Division of Mines and Geology cautions that the main map does not include “hazards from ground deformation such as liquefaction, landslides or surface fault ruptures.” These can be important factors determining damage.

Yellow, pink and red areas on the main map have a 10% probability in 50 years of shaking that exceeds 50% of the force of gravity.

But the state’s building codes are predicated on designing buildings to withstand 40% of the force of gravity without collapse or loss of life.

This could mean that the building codes are inadequate in the areas considered to be riskiest.

But Thomas Heaton, a professor of earthquake engineering at Caltech, cautions that it is probably a mistake to engage in comparisons of this kind.

“The 40%-of-gravity figure doesn’t tell you how a building would actually do if an earthquake at such a level occurred,” he said.

One of the smaller maps, showing which areas of California have been damaged by earthquakes, reflects quakes that have reached a Mercalli scale intensity of at least VII near the epicenter. Mercalli levels, which are always indicated in Roman numerals, differ from those of conventional magnitude scales in that they can be calculated at any point that feels the quake, not just the epicenter. There are countless Mercalli readings for any quake, but only one magnitude.

A level of VII is considered strong. At such an intensity, people find it difficult to stand, hanging objects quiver, furniture is broken and there is damage to older masonry structures. Bodies of water are roiled by waves and irrigation ditches are damaged.

In large quakes Mercalli numbers can climb to anywhere from VIII to XII for quakes capable of collapsing structures and even throwing objects into the air.

Shaking scale

Historic earthquakes

Earthquake damage 1800-1998 of at least VII on Mercalli scale

Source: California Division of Mines and Geology