Loma Prieta's unfinished business.

Loma Prieta's unfinished business

Astrologists must have loved it. On the anniversary of the great 1906 San Francisco earthquake, the San Andreas fault unleashed a series of moderate quakes near the town of Watsonville, Calif. The early-morning shakeup caught sleeping residents by surprise, but seismologists say they had anticipated that such quakes might occur although they could not predict when or where.

The April 18 tremors, the largest of which measured magnitude 5.4, actually qualify as aftershocks to the magnitude 7.1 Loma Prieta earthquake that broke the San Andreas fault in the Santa Cruz mountains last Oct. 17. They struck in a general zone defined by other Loma Prieta aftershocks, filling a gap along the southeastern end of the zone that had not broken during previous aftershocks.

From statistical studies, Paul A. Reasenberg of the U.S. Geological Survey (USGS) in Menlo Park, Calif., calculates that seven aftershocks of magnitude 5 or greater typically follow within a year after a quake the size of Loma Prieta. Only two struck within the first few days after the October main shock, so statistically speaking, scientists knew that more could come. On April 18, a third and fourth hit.

The northwest end of the Loma Prieta break -- near the city of Los Gatos -- also has potential for spawning an aftershock of magnitude 5 or larger, says USGS seismologist Lynn Dietz. But experts canht predict whether additional large aftershocks will ever hit. There could be eight more, or there might be none, says Allan Lindh of the USGS.