Two Tests to Investigate the Causes of Aftershocks

地震触发发生的基本物理机制仍然很难证明，即使在最基本的水平上，关键机制是静态还是动态应力传递。 初始应力变化可能引发的其他过程的重要性，如微破裂、后滑、孔隙流体流动或粘弹性松弛，也是未知的。本项目的工作集中在最基本的问题上，即静态或动态应力对近中场余震的触发是否最重要。 以往的研究发现，静应力和动应力的变化都可以解释余震的方位分布。 本研究的重点是探讨是否可以证实压力阴影的产生，这种现象是由静态触发模型预测的，而不是由动态触发模型预测的。 初步结果表明，应力阴影可能实际上并不存在，支持动态触发。 对于不同震级的主震，余震密度随距主震断层距离的衰减同样受触发机制类型的影响。 这项研究发现，余震可能发生在距离达到或超过七到十个断层长度的地方。 这也支持动态触发，在这些距离上，动态触发明显更强。确定余震是由静态还是动态应力触发对于与地震危险性估计相关的许多问题是重要的，包括估计不同震级的地震可能预期触发余震的距离，确定主震是否会抑制区域地震活动（例如，创建应力阴影），以及确定余震模式的方向性是否总是可以预期跟随主震破裂的方向性。 通过对余震触发过程的研究，研究人员还希望深入了解地震是如何成核的。

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.