近断层应力变化快速计算方法及对强余震空间分布的指示意义:以M_w7.9汶川地震为例

构造地震一般由断层摩擦失稳所致.断层内部及周边所累积的剪切形变则通过同震滑动位移得到局部释放.因此,震后断层内部及近断层周边的静态剪切应力变化量的空间分布可通过断层面上的滑动位移分布计算得到.本文采用傅氏变换方法(FTM)计算单一有限断层同震滑移场所形成的静态剪切应力变化分布,近断层两侧的应力变化计算可由波数域内应力近似算法获得.结果表明,FTM快速有效、易于实现,有效地避免了常规应力计算中奇异值的出现.以2008年Mw7.9中国汶川大地震为例,采用前人所得有限断层滑动位移模型,得到了断层面和近断层周边准3D剪切应力分布解,并同主震后中强余震的空间分布特征作了比较.结果表明,大部分的中强余震震源位置处于剪切应力变化值为正的区域,由同震位移所产生的局部应力降峰值和均值大小同应力变化的正值大致相当,从而表明了快速且有效地计算断层内部及近断层附近的应力变化分布可以为主震后强余震发生的潜在区域提供指示意义.需要强调的是,应力变化空间分布特征的获取强烈地依赖于有限断层滑移模型解.有关滑动位移反演解的多解性对应力变化计算结果的影响,本文作了必要的讨论.

Fast calculation of near-fault static stress change and its implications for the hypo-central estimation of moderate to large aftershocks: A case study of the 2008 Wenchuan Mw7.9 earthquake

It is widely accepted that earthquake rupture occurs when the shear stress accumulated on the fault exceeds the fault frictional strength. The fault slip motion is accompanied with a partial shear strain release, and the shear stress change can be resolved from final slip distribution or co-seismic slip on the causative fault. We use the Fourier transform method (FTM) to fast calculate the shear stress changes on and near faults after the 2008 M_w7.9 Wenchuan, China, earthquake due to a heterogeneous slip distribution on the main fault and try to understand the relationship between stress change and aftershock distribution. For a single fault plane, the shear stress change on the main fault can be calculated efficiently by using the FTM, and the calculation of off-fault stress change is based on an approximate decay relation proposed by Helmstetter and Shaw. For a 2D anti-plane strain problem, we compare our numerical model with the analytical model given by Burridge and Halliday. The results show that the FTM algorithm is easy to be implemented and effectively avoid the singular values appearing in conventional stress calculation. Based on the finite fault slip model inverted from tele-seismic data for the Wenchuan event, we obtain the stress changes on and off the fault and create a 3D view of the aftershock distribution around the main fault together with the spatial stress change pattern resulted from heterogeneous fault slip. The result of 3D view of the aftershock distribution shows that the triggered events or aftershocks are mostly concentrated in the regions with a positive stress change, and the value of the maximum positive stress change caused by the co-seismic slip is roughly equal to the localized largest stress drop (stress change is negative). We calculate the seismicity rate against time following the Wenchuan main shock based on the temporal distribution of stress changes. The results my help us better understanding the aftershock duration and seismicity reduce caused by the stress drop. We emphasize that the fast and effective computations of stress changes on and off the fault immediately following a main shock can give us an insight into the source physics behind earthquake nucleation which provides us the plausible indicators of future potential areas of the moderate to strong aftershocks. In addition, the spatial distribution of stress change is strongly dependent on the finite fault slip model. So we discuss the influence of non-unique inversion of slip distribution on the stress change calculation.