Detecting Aseismic Stressing Transients using Stochastic Seismicity Models

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Attempts to understand the physical mechanisms controlling the timing of earthquake occurrence often focus on linking observations of changes in earthquake productivity to events that alter the stress state within the crust. Common examples include the aftershock sequences triggered by large crustal earthquakes and seismic swarms triggered by the movement of magma beneath volcanoes. These studies rely on the idea that there is a direct link between what we can easily observe, namely the occurrence of small earthquakes, and what we typically cannot, namely the space-time variations in the stress-state of the crust and the underlying physical phenomena that cause stress to vary rapidly in time. We are developing a new technique that can invert seismicity catalogs (i.e. lists of earthquake times, locations and magnitudes) for the space-time history of the stressing-rate in the catalog region. This technique will have three primary applications. First, it will hopefully be a very sensitive detector of crustal transients, including fluid flow, that cannot be detected with surface geodetic data. Second, it will allow us to test a constitutive law for earthquake generation that has been derived from laboratory rock mechanics data. Previous studies have demonstrated that this law appears to work at the order of magnitude level, but our approach will allow for much more widespread and sensitive tests. Thirdly, there are numerous fundamental questions about the physical processes controlling the timing and triggering of earthquakes that require detailed information about the space-time variations in stress and stressing-rate on faults. If the technique is successful, it will provide new constraints on our understanding of earthquake nucleation and the forces within the crust that cause large earthquake swarms.The project will foster a collaboration with Japanese scientists and provide training for a female graduate student, who will also travel to Japan and participate in the foreign collaboration.