Two Tests to Investigate the Causes of Aftershocks

地震触发的基本物理机制仍然很难证明，即使在最基本的层面上，关键机制是静态的还是动态的应力传递。初始应力变化可能引发的其他过程的重要性也是未知的，例如微破裂、余滑、孔隙流体流动或粘弹性松弛。这个项目的工作集中在最基本的问题上，即静态应力还是动态应力对触发近场到中场余震最重要。前人在这方面的工作已经发现，静态和动态应力变化都可以解释余震的方位分布。这项研究的重点是检验应力阴影的产生，这是一种由静态触发预测的现象，而不是由动态触发模型预测的现象。初步结果表明，应力阴影可能并不实际存在，支持动态触发。不同震级主震的余震密度随距离主震断层的衰减也同样受触发机制类型的影响。这项研究发现，余震可能发生在超过七到十个断层长度的距离上。确定余震是由静态应力还是动态应力触发的，对于与地震危险性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触发余震的距离，确定主震是否可能抑制区域地震活动(例如产生应力阴影)，以及确定余震模式的方向性是否总是可以预期沿着主震破裂的方向性进行。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孕育成核的。

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.