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Abstract:
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2001年MS8.1昆仑山口西地震和2008年MS8.0汶川地震发生在同一构造单元,但其余震序列无论在个数、空间分布,还是持续时间上都表现了显著的差别.余震通常由主震区域内背景场地震活动性受到的扰动所引起,这样的扰动则来自于主震造成的应力场状态的变化.本文从滑移速率和状态相依赖的摩擦定律(Rate- and State-Dependent Friction Law)出发,结合区域主震前后的地震活动性资料,定量地估算了这两个大地震后余震序列可能的持续时间,并对不同模型所得的结果进行了比较和对比.结果表明,汶川地震余震持续时间约为昆仑山口西地震余震持续时间的20倍,这是由于昆仑山口西地震和汶川地震余震序列的个数和持续时间不仅与地震成核过程的状态变化有关,还与作用在断层面上的正应力 σN 和剪应力加载速率 τ· 的大小有关.主震前后剪应力速率 τ· 的差别导致了在相同大小应力扰动ΔCFS之后的余震的活动性变化率的明显不同,导致了所触发的余震的个数和余震序列的持续时间的巨大差别.通过对昆仑山口西地震和汶川地震余震序列的时空分布特征和持续时间的定量化认识,可以为地震灾害定量评估提供合理和有益的物理参数.
The 2001 MS8.1 Kunlunshan earthquake and the 2008 MS8.0 Wenchuan earthquake occurred in the same tectonic unit. There are significant differences in spatial-temporal distribution, number of aftershocks and time duration for the aftershock sequence following these two main shocks. As we all know, aftershocks could be triggered by the regional seismicity change derived from the main shock, which was caused by the Coulomb stress perturbation.Based on the rate- and state-dependent friction law and the seismicity data before and after the mainshocks, we quantitatively estimated the possible aftershock time duration, and compared the results from different approaches. The results indicate that the aftershock time duration after the Wenchuan main shock is about 20 times of that after the Kunlunshan main shock. This can be explained by the relationships between aftershock time duration and earthquake nucleation history, normal stress σN and shear stress loading rate τ· on the fault. In fact, the obvious differences of shear stress loading rate τ· of these two main shocks result in an observable seismicity rate change after the same Coulomb Failure Stress perturbation ΔCFS and the distinction of number and time duration of aftershocks sequence. It is necessary to point out that, from our current study, the quantified understanding and interpretation of spatial-temporal distribution and time duration for the aftershock sequence can be used in the Probabilistic Seismic Hazard Analysis (PSHA) in order to avoid overestimating of the seismic hazard levels, and provide reasonable and helpful physical parameters in earthquake disaster mitigation.
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