M_s8.0汶川地震断层的应力状态以及对余震危险性的影响

The state of stress of the M_s8.0 Wenchuan earthquake faulting and its implication to the aftershock hazard

利用中国地震局在汶川地震前后对四川盆地以及龙门山断裂进行的水压致裂绝对应力测量数据、近断层强震记录、以及由美国USGS公布的包括地震矩和地震波能量等在内的远场震源参数解,从简单断层模型出发,应用地震能量分配原理和库仑摩擦准则,初步估算和判断了2008年Ms8.0汶川地震断层破裂过程和震源参数,以及滑移弱化模型下应力变化(静态应力降的大小)对断层内部余震强度的影响.结果表明,对主震而言,地震辐射效率和地震效率上限值分别约为37%和14%,平均滑移弱化距离D_c约为0.5 m,破裂能约为1.29×10^(16)J,松弛功约为1.13×10^(16)J.主断层内所贮存的黏弹性松弛功的大小与主震的地震波辐射能相当,假设主震在应变能变化中的松弛功全部用于一个余震的触发,且主断层上的余震发生与主震一样遵从相同的断层破裂机制:即能量分配准则,那么汶川地震余震释放的全部能量相当于一个M_s6.9级的地震.由此可推断汶川地震主断层上不会发生M_s6.9级以上的余震,而主震后更多较小震级的余震可能持续几十年甚至上百年.

With an integration of in-situ borehole strain-meter data recorded at the Central Longmenshan Fault Zone, seismological observations, including the near-fault strong motion records, and fault slip inversion given by worldwide earthquake research groups, we have tried to investigate earthquake faulting processes and source parameters related to the MsS. 0 Wenchuan faulting processes based on the fault mechanic theory. In order to put different data sets together for analyzing and interpretation purposes, Coulomb failure criterion and earthquake energy partition principle have been used in the determination of the strength and state of stress on the fault by involving the slip-weakening frictional model. Strong motion records from Qingping station, the nearest location to the main fault （about 5 km）, has been used to quantitatively estimate the averaged dynamic stress drop during fault rupture. The primary results show that,for the Wenchuan main event, the seismic radiation efficiency and seismic efficiency are about 37% and 14%, respectively, the critical slip distance Dc is about 0. 5 m, the fracture work and relaxation work are about 1. 29 × 1016 J and 1. 13 × 1016 J, respectively. The relaxation work remained inside the fault zone is about the same order as the total radiated seismic energy from the main shock. We present a simple model to predict the size and time length of aftershock sequences based on the elastostatic mechanism and regional deformation model. We propose that, if the faulting mechanism of the aftershocks inside the main fault zone obeys the same energy partition process of the Wenchuan main shock, the largest aftershock in which the radiated energy could by release by remained visco-elastic relaxation work inside the fault zone should be less than the energy released by a Ms6.9 event or much more smaller events will occur later on, and the aftershock sequences within the slowly deforming region are predicting to be significantly longer than decade typically observed at rapidly loaded plate boundaries. This infers that the aftershock sequences after the Wenchuan main shock will continue for many decades based on the current aftershock activity patterns.