The overdue quake: Unusual activity along the san andreas hints at a long‐expected tremor

Abstract

B y rights, California earthquake scientists should be feeling mighty contrite these days. As their first and, to date, only foray into official quake prediction, researchers at the U.S. Geological Survey issued a forecast in 1985 for the tiny town of Parkfield, Calif., located on the San Andreas fault halfway between San Francisco and Los Angeles. According to the prediction, a magnitude 6.0 earthquake would rattle Parkfield sometime before 1993. At estimated odds of 20 to 1, it was as sure a bet as seismologists had ever seen. The USGS and the state of California decided to spend $1 million each to wire the Parkfield area in hopes of detecting precursors of the predicted tremor. But the San Andreas stood everybody up: 1993 rolled by without the expected quake. With each subsequent New Year's Day, the hopes of researchers deflated as they struggled to keep their instruments running and to maintain funding for their experiments. Even today, the calendar continues to turn, and the Parkfield earthquake still has not struck. Yet despite the fabulously wrong prediction, despite the millions of dollars and years of precious research time spent waiting, and despite the humiliation and loss of public trust, Parkfield researchers are displaying renewed excitement. The source of their inspiration is the San Andreas fault, which has started showing intriguing signs of activity Last year, it became obvious that measurements of the ground were picking up unusual stirrings at Parkfield. What's more, four medium-sized earthquakes had struck the San Andreas fault near the town, two of them within the critical zone thought to be the nucleation site for the next big quake. Scientists are divided on how to interpret the recent changes. Some wonder whether they are catching signs of the fault preparing for a major shock-one of the key goals of the Parkfield experiment. If we get the earthquake now, it will be really interesting, because everybody will wonder whether it had anything to do with these changes that we're seeing, says Evelyn Roeloffs, a geophysicist with the USGS in Vancouver, Wash., who managed the Parkfield prediction experiment from 1990 to 1991.