Quakes Keep the Element of Surprise

As the Central Coast temblor this week demonstrated, earthquakes in California occur unpredictably and in unexpected places.

The magnitude 6.5 San Simeon earthquake took place without any warning that was recognized at the time -- as was the case with Northridge in 1994, Loma Prieta in 1989 and Tehachapi in 1952 -- and like those large temblors, it happened on a fault system that was unknown or barely known. Scientists initially were not even sure on which fault the Central Coast quake occurred.

Even in the case of the magnitude 7.3 Landers quake of 1992, where there had been a 6.1-magnitude quake nearby two months before and then an intense series of minor temblors, scientists did not know it was coming.

Most significant California quakes do not occur on well-known, sharply delineated faults such as the San Andreas, Hayward or Newport-Inglewood.

The most famous fault of them all, the San Andreas, has had only two noteworthy quakes in the last 150 years, the 1857 Fort Tejon quake in Southern California and the 1906 San Francisco quake. Initial reports in 1989 put the Loma Prieta quake on the San Andreas, but it was later determined to have occurred on a small adjacent fault.

There are about 300 faults in Los Angeles County alone that are capable of generating a temblor in the damaging 6-magnitude range, and scarcely a year passes without new ones being identified, according to seismologist Lucy Jones of the U.S. Geological Survey. Some of the most damaging Los Angeles-area quakes, such as the Northridge earthquake, have been on buried thrust faults that left no surface indication of their presence.

Scientists say even California locales where quakes are comparatively rare, such as Sacramento or San Diego, will have some temblors over time.

For that reason, seismic safety officials caution homeowners that no matter where they live in the state they should ensure that their properties are as close to earthquake-safe as possible. These officials suggest bolting houses to foundations, using plywood to build lateral resistance and avoiding unreinforced masonry.

In general, it is known that California is quake prone. As a result, government agencies and scientists have been trying to quantify the potential damages.

The Federal Emergency Management Agency, for instance, calculated in 1999 that 74% of all earthquake damage in the United States on an annualized basis would occur in California, an average over the long term of $3.26 billion a year.

What remains unknown is when or where damaging earthquakes will occur. Many scientists who have researched the possibilities of quake predictions conclude that earthquakes inherently are random, chaotic events. But researchers continue to try to observe the process leading to a quake.

NASA is attempting some of this work with the intricate observation by satellite of movements in the Earth’s crust, or through such experiments as the boreholes that are being dug deep into the San Andreas fault at Parkfield.

Still, as Caltech earthquake engineering professor Tom Heaton said recently, “More and more, there is a consensus that not only don’t we have quake prediction now, but we are unlikely to have it in the foreseeable future.”

Seismologist Jones said it is sometimes argued that the potential for devastating quakes in California is bigger in the Bay Area than in Los Angeles, because huge faults such as the San Andreas and the Hayward pass directly through big cities, while the San Andreas is at least 40 miles from Los Angeles.

“But, here, we have those 300 faults capable of generating at least a magnitude 6,” she said. “Here, there’s a greater uncertainty exactly where the next quake will be.”