Two Tests to Investigate the Causes of Aftershocks

地震发生的基本物理机制，即使在最基本的层面上，也难以证明其关键机制是静态还是动态应力传递。其他可能由初始应力变化引起的过程，如微压裂、余震、孔隙流体流动或粘弹性松弛，其重要性也尚不清楚。这个项目的工作集中在最基本的问题上，即静应力还是动应力对触发近场到中场余震最重要。以前在这方面的研究发现，静应力和动应力的变化都可以解释余震的方位分布。本研究的重点是检查应力阴影的产生是否可以证实，这是一种由静态触发而不是动态触发模型预测的现象。初步结果表明，应力阴影可能并不存在，支持动态触发。对于不同震级的主震，余震密度随离主震断层距离的衰减也同样受到触发机制类型的影响。研究发现，余震发生的距离可能达到或超过7到10个断层长度。这也支持动态触发，这在这些距离上要强得多。确定余震是由静态应力还是动态应力引发的，对于与地震危害估计有关的一些问题很重要，包括估计不同震级的地震可能引发余震的距离，确定主震是否可以预期会抑制区域地震活动（例如产生应力阴影），而确定余震模式的方向性是否总是可以期望遵循主震破裂的方向性。通过对余震触发过程的调查，研究人员还希望深入了解地震是如何形成核的。

The basic physical mechanism by which earthquake triggering occurs has remained difficult to prove even on the most basic level of whether the key mechanism is static or dynamic stress transfer. The importance of other processes that might be set off by the initial stress change, such as microfracturing, afterslip, pore fluid flow, or viscoelastic relaxation, are also unknown.The work in this project focuses on the most basic question of whether static or dynamic stresses are most important for the triggering of near to mid-field aftershocks. Previous work in this area has found that both static and dynamic stress changes may explain the azimuthal distribution of aftershocks. This study focus on examining whether the creation of stress shadows, a phenomena that is predicted by the static triggering but not by the dynamic triggering model, can be substantiated. Preliminary results indicate that stress shadows may not actually exist, supporting dynamic triggering. The decay of aftershock density with distance from the mainshock fault for mainshocks of different magnitudes should likewise be influenced by the type of triggering mechanism. The study is finding that aftershocks may occur at distances up to or exceeding seven to ten fault lengths. This also supports dynamic triggering, which is substantially stronger at these distances.Determining whether aftershocks are triggered by static or dynamic stresses is important for a number of issues related to earthquake hazard estimation, including estimation of the distances to which earthquakes of different magnitudes might be expected to trigger aftershocks, determination of whether mainshocks can ever be expected to depress regional seismic activity (e.g. create stress shadows), and determination of whether the directionality of an aftershock pattern can always be expected to follow the directionality of the mainshock rupture. Through investigation of the aftershock triggering process the investigators also hope to gain insight into how earthquakes nucleate.