In the global aftershock zone that followed the major April 2012 Indian Ocean earthquake, seismologists noticed an unusual pattern -- a dynamic stress shadow, or period of seismic1 silence when some faults（断层） near failure were temporarily rendered incapable2 of a large rupture3. The magnitude 8.6 earthquake, a strike-slip event at intraoceanic tectonic plates, caused global seismic rates of M4.5 to spike4 for several days, even at distances tens of thousands of kilometers from the mainshock site. But beginning two weeks after the mainshock, the rate of M6.5 seismic activity subsequently dropped to zero for the next 95 days.Why did this rare period of quiet occurIn a paper published today in the Bulletin of the Seismological Society of America , Fred Pollitz of the U.S. Geological Survey and co-authors suggests that the Indian Ocean earthquake caused short-term dynamic stressing of a global system of faults. Across the planet, there are faults that are close to failure and ready to rupture. It may be, suggests Pollitz and his colleagues, that a large quake encourages short-term triggering of these close-to-failure faults but also relieves some of the stress that has built up along these faults. Large magnitude events would not occur until tectonic movement loads stress back on to the faults at the ready-to-fail levels they reached before the main shock.Using a statistical5 model of global seismicity, Pollitz and his colleagues show that a transient（短暂的） seismic perturbation（扰乱，不安） of the size of the April 2012 global aftershock would inhibit6 rupture in 88 percent of their possible M6.5 earthquake fault sources over the next 95 days, regardless of how close they were to failure beforehand.This surprising finding, say the authors, challenges the previously7 held notion that dynamic stresses can only increase earthquake rates rather than inhibit them. But there are still mysteries about this process; for example, the global rate of M4.5 and M5.5 shocks did not decrease along with the larger shocks.
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1 seismic a.地震的，地震强度的 参考例句： Earthquakes produce two types of seismic waves.地震产生两种地震波。 The latest seismic activity was also felt in northern Kenya.肯尼亚北部也感觉到了近期的地震活动。
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