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断层强度是孕震带的一个基本性质,它随时间的变化特性可以增加或减小断层破坏的可能性并最终触发地震。虽然断层强度变化可以用来解释诸如地震活动的远程触发(HillandPrejean,2007)等多种现象,但至今还没有切实的方法对这个重要性质进行现场监测。文中我们认为,通过对圣安德烈斯断层帕克菲尔德地区近20年(1987～2008)的观测,揭示了一种监测断层强度的方法。研究中我们已经识别出了2个实例,每个实例中断层强度的长期变化最有可能被强震事件远程触发;这两个震例分别是2004年的苏门答腊—安达曼9·1级地震和1992年初的兰德斯7·3级地震。在这两个震例中,断层强度变化都有两方面的表现:其一是地震散射体特性随时间的变化,这或许反映了应力导致的流体运移;其二是与断层强度变化最为一致的重复地震序列特征的系统的时变性。在1992年兰德斯地震的实例中,很可能是断层强度在一段时间内的降低触发了1993年帕克菲尔德地区的无震瞬态滑动(Langbein et al,1999;Gao et al,2003;Niu et al,2003;Murray and Segall,2005)并伴生了4次M>4级地震的丛集。由遥远的2004年苏门答腊-安达曼地震产生的断层强度变化显得尤为重要,因为这有可能显示出巨大地震会对地球的断层系统强度带来全球性影响。由于强震引起的扰动会使全球很多断层趋于破坏,从而导致全球地震活动出现时间上的丛集性。2004年苏门答腊-安达曼地震后几年内发生的超乎寻常的多次M≥8级地震似乎可以支持这种假说。
Fault strength is a basic property of the seismogenic zone. Its characteristics over time can increase or decrease the possibility of fault damage and eventually trigger the earthquake. Although changes in the intensity of the faults can be used to explain phenomena such as remote triggering of seismic activity (HillandPrejean, 2007), so far there is no practical way to monitor this important property on site. In our opinion, a method of monitoring the strength of a fault is revealed through observations of the San Andreas Fault Parkfield over the past two decades (1987-2008). We have identified two examples in the study, and the long-term changes of fault intensity in each instance are most likely to be triggered remotely by strong earthquakes. These two earthquakes are respectively the 2004 Sumatra-Andaman Earthquake of magnitude 9.1 and Lands 7.3 earthquake of early 1992. In both cases, the variation of fault strength has two aspects: one is the change of seismic scatterers over time, which may reflect the fluid migration caused by stress; the other is the most consistent with the change of fault intensity Time-history of the characteristics of repeated earthquake sequence. In the case of the 1992 Lander earthquake, it is likely that a decrease in fault strength over time has triggered unsteady transients in the Parkfield area in 1993 (Langbein et al., 1999; Gao et al., 2003; Niu et al. al, 2003; Murray and Segall, 2005) and accompanied by 4 clusters of M> 4 earthquakes. Changes in the intensity of the fault produced by the distant 2004 Sumatra-Andaman earthquake are particularly important because it is possible to show that a huge earthquake can have a global impact on the Earth’s fault system strength. Because of the disturbance caused by strong earthquakes, many faults in the world tend to be destroyed, which leads to the temporal clustering of earthquakes around the world. The extraordinary M ≥ 8 earthquake that occurred several years after the Sumatra-Andaman Earthquake in 2004 seems to support this hypothesis.