Science / Medicine : Big Quakes Found to Be Preceded by Smaller Jolts

Scientists who have studied the historical record before major earthquakes in California over the last 150 years have concluded that large earthquakes may be preceded by smaller quakes in the same seismic zone during the one or two decades before the main event.

Lynn R. Sykes and Stephen C. Jaume of Columbia University’s Lamont-Doherty Geological Observatory found that moderate earthquakes occurred with increasing frequency during the two decades before the San Francisco Bay Area quakes in 1968, 1906 and 1989 and a Southern California quake in 1948. The areas remained seismically quiet for years after the larger quakes.

The scientists concluded that the smaller quakes preceding the larger jolts “are casually related to those four main shocks,” and thus it would be reasonable to assume that major quakes in California should follow years of increasing seismic activity. The scientists published their study last week in the journal Nature.

They argue that the record suggests that there is a low probability of large quakes on the San Andreas Fault north of San Francisco or the northern half of the Hayward Fault in the next few decades, but the implications are less clear for other areas of California.

However, in the same issue of the journal, Allan G. Lindh, chief of seismology at the U.S. Geological Survey’s Menlo Park facility, warned against drawing any sweeping conclusions from the study.