Two Tests to Investigate the Causes of Aftershocks

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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.