什么群啊作者: billyisfragile! 时间: 2019-10-8 13:55
Lindh and Baker found that six earthquakes took place at intervals of approximately twenty-two years along a fifteen-mile section of the San Andreas fault, near the small town of Parkfield. According to their theory, the next quake of at least magnitude 6.0 near Parkfield should have struck by 1988; by the end of 1992 the probability for its occurrence should have reached 95 percent. Previous earthquakes in Parkfield were reported to have been preceded by many anomalous precursors. To detect precursors, more than twenty observational networks have been installed near Parkfield. As of this writing, however, the long-awaited Parkfield earthquake still has not been felt. The negative result of the Parkfield experiment has cast strong doubt on the idea that earthquakes are periodic. Evidence against earthquake cycles has also come from a relatively new field, call paleoseismology. Large earthquakes give rise to logical features such as fault scarps, landslides and soil liquefaction that can be exhumed and radiocarbondated hundreds or even thousands of years after the event. In 1989 the paleoseismologist Kerry E. Sieh of Caltech and his colleagues published a chronology of earthquakes at Pallett Creek, which lies along the San Andreas fault about thirty-five miles northeast of Los Angelas. They determined that the mean interval between ten earthquakes that took place in the past two millenniums was 132 years. But individual intervals ranged from forty-four to 332 years.
Lindh remains unfazed. "I think earthquake prediction is like working on an AIDS vaccine," he says. "You are not allowed to be pessimistic or optimistic." Robert Wesson, too, maintains that predictions can help mitigate hazards. But other seismologists take a hard line. Robert Geller vigirogously opposes any kind of forecast, insisting that neither short-term nor long-term predictions have a sound scientific basic. "The danger in basing government policy on such unsound predictions," he says "is that there is a tendency to assume that particular regions are especially dangerous. That has the effect of concentrating earthquake disaster mitigation efforts, rather than spreading them out over an entire region of similar geological and seismic type."