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业已证实,发生在地壳中的地震能够触发后续地震,但对于更深的地震这种触发机制是否存在还未见文献说明。浅断层相互作用的模型表明,静态(永久)应力变化能够触发附近的地震,一般在距原发地震几个断层长度的范围内(King et al,1994;Harris,1998;Stein,1993)。而由地震波携带的动态(瞬态)应力既可以触发附近的地震,也可以触发远震(Hill et al,1993;Harris and Day,1993;Belardinelli et al,1999;Kilb et al,2000;Gomberg et al,1997)。本文中,我们对2002年8月19日发生在汤加的深源地震序列做了深入的分析,并给出静态触发作用和动态触发作用的证据。一次深度为598km的7.6级地震发生后7min,在相距300km的以前无震区发生了一次7.7级的地震(深度664km)。我们发现在第一个主震后,附近的余震往往集中在主震引发的静态应力升高的地区。但是第二次主震和其他的被触发的地震发生在距离第一次地震很远的地方,在那里静态应力的增加可以忽略不计,因此这表明是动态应力触发机制在起作用。被触发地震的发震时间同主震的主要地震波到时并不对应,并且这种动态触发的地震通常发生在地震区下面或附近的无震区。我们认为这些地震是在接近临界状态的地区由瞬变效应触发的,而在这些地区,如果没有外部影响,地震起始非常困难。
It has been confirmed that earthquakes that occur in the earth’s crust can trigger subsequent earthquakes, but there is no literature on the existence of a trigger mechanism for deeper earthquakes. The model of shallow fault interaction shows that static (permanent) stress changes can trigger nearby earthquakes, usually within a few faults of the original earthquake (King et al 1994; Harris 1998; Stein 1993). The dynamic (transient) stresses carried by the seismic waves can either trigger nearby earthquakes or trigger tele-earthquakes (Hill et al, 1993; Harris and Day, 1993; Belardinelli et al, 1999; Kilb et al, 2000; Gomberg et al, 1997). In this paper, we conduct an in-depth analysis of deep-seated seismic sequences that occurred in Tonga on August 19, 2002 and give evidence of static and dynamic triggering. After a magnitude 7.6 earthquake with a depth of 598 km, a magnitude 7.7 earthquake (depth 664 km) occurred in a former earthquake-free zone 300 km apart after 7 minutes of occurrence. We find that after the first mainshock, nearby aftershocks tend to be concentrated in areas where the static stress induced by the mainshock rises. However, the second mainshock and other triggered earthquakes occurred far away from the first earthquakes, where the increase in static stress was negligible, thus indicating that the dynamic stress triggering mechanism is at work. The triggering time of the triggered earthquake does not correspond to the arrival of the main seismic wave of the main shock, and such dynamic triggering usually takes place in the earthquake-free zone below or near the seismic zone. We believe these earthquakes are triggered by transient effects in areas near critical conditions where earthquakes start very difficult without external influences.