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复发地震大致使同一断层地段发生破裂,且具有几乎相等的波形,这就使其成为测量地壳内波传播的时间变化方面有用的手段。由于在复发地震序列(多重震)中震源和传播途径对所有地震是共同的,因而它们的波形差别可能是由介质特征的变化所引起。在加利福尼亚1989年洛马普列塔地震和1984年莫尔根山地震的余震区,我们识别出多于20次的多重震,每次多重震含有5~40次的复发地震。相对主震前事件,主震后的事件显示了早期S波的震相延迟可多达0.2S。平均路径的同震波速的延迟量对P波来说减少约1.5%,而S波则减少了3.5%。由于多数多重震为余震且遵循大森定律,在主震后紧接的期间我们有十分好的时间变化取样。我们发现,主震后速度减小的幅度在时间上呈对数衰减。在某些情况下,它恢复至主震前的数值,而在其他情况下则没有这种现象。莫尔根山主震亦可得到类似的结果。因为S波波速的相对变化大于P波波速的相对变化,因而可以认为,充满液体的裂隙的张开或连接是根本的原因。波速变化的量级表明,波速变化的震源区存在低有效压力。我们的结果说明,变化主要位于近台站处且发生于浅层,但我们也不能排除变化发生于较深处的可能性。若变化发生于浅层,我们就有可能测定主震强地面运动时非线性特征的残留效应。若变化发生于深处,就说明在孕震深度的孔隙压力很高,这在地震过程中似乎能起关键作用。
Recurring earthquakes cause roughly the same fault zone to rupture and have almost equal waveforms, making it a useful tool for measuring time variations of wave propagation in the earth’s crust. Since the sources and propagation paths are common to all earthquakes in a recurrent earthquake sequence (multiple earthquakes), the differences in their waveforms may be caused by changes in the characteristics of the medium. In the aftershocks of the 1989 Loma Prieta Earthquake in California and the Morganshan Earthquake in 1984, we identified more than 20 multiple earthquakes with 5 to 40 recurring earthquakes each time. Relative to the pre-main event, the events after the main shock show that the phase delay of the early S-wave can be up to 0.2S. The average path delay for coseismic velocities decreases by about 1.5% for P-waves and 3.5% for S-waves. Since most multiple earthquakes are aftershocks and follow Omorom’s law, we have a very good time-varying sampling immediately after the main shock. We find that the magnitude of the decrease in velocity after the main shock decays logarithmically in time. In some cases, it returned to the value before the mainshock, while in others it did not. Morganshan main shock can also be a similar result. Because the relative change in the S-wave velocity is greater than the relative change in the P-wave velocity, it is believed that the fluid-filled fracture opens or connects at all. The magnitude of the change in wave velocity shows that there is low effective pressure in the source region where the wave velocity varies. Our results show that the changes are mainly located near the station and occur in the shallow layer, but we can not rule out the possibility that the changes occur deeper. If the changes occur in the shallow layer, it is possible to measure the residual effect of the nonlinear characteristics of the main earthquake with strong ground motions. If the changes occur deep, it shows that the pore pressure at the seismogenic depth is high, which seems to play a key role in the earthquake process.