本来想说考完好好回忆……结果完全失忆了……我慢慢想…… 作者: Lyndonyang 时间: 2019-1-6 20:53
In 1971 researchers hoping to predictearthquakes in the short term by identifying precursory phenomena (those thatoccur a few days before large quakes but not otherwise) turned their attentionto changes in seismic waves that had been detected prior to earthquakes. An explanation for such changes was offeredby “dilatancy theory,” based on a well-known phenomenon observed in rocks in the laboratory: as stress builds,microfractures in rock close, decreasing the rock’s volume. But as stress continues to increase, the rockbegins to crack and expand in volume, allowing groundwater to seep in, weakeningthe rock. According to this theory, such effectscould lead to several precursory phenomena in thefield, including a change in the velocity of seismic waves, and an increase insmall, nearby tremors.
Researchers initially reported success in identifying these possibleprecursors, but subsequent analyses of theirdata proved disheartening. Seismic waves with unusual velocities were recordedbefore some earthquakes, but while thehistorical record confirms that most large earthquakes are preceded by minor tremors, these foreshocks indicate nothing aboutthe magnitude of an impending quake and are indistinguishable from other minortremors that occur without large earthquakes.
In the 1980s, some researchers turned theirefforts from short-term to long-term prediction. Noting that earthquakes tend to occur repeatedly in certainregions, Lindh and Baker attempted to identify patterns of recurrence, orearthquake cycles, on which to base predictions.In a study of earthquake-prone sites along the San Andreas Fault, they determined that quakes occurred at intervals ofapproximately 22 years near one site and concludedthat there was a 95 percent probability of an earthquake in that area by 1992.The earthquake did not occur within the time frame predicted, however.
Evidence against the kind of regular earthquakecycles that Lindh and Baker tried to establish has come from a relatively new field,paleoseismology. Paleoseismologists haveunearthed and dated geological features such as fault scarps that were causedby earthquakes thousands of years ago. They have determined that the average intervalbetween ten earthquakes that took place at one site along the San Andreas Faultin the past two millennia was 132 years, butindividual intervals ranged greatly, from 44 to 332 years.
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In 1971 researchers hoping to predictearthquakes i...