The Physics of the Earthquake Source and Radiated Waves: Frequency Dependence, Spatial Distribution, and Directivity in Elastodynamic Models of Repeated Fault Ruptures

This proposal examines the extension of a new class of elastodynamic models, which have already been successful at simulating a wide range of earthquake behaviors, to the study of the radiated energy produced during earthquakes. Because radiated energy is the richest source of observational data, it will ultimately provide the most significant constraints on earthquake models. Previous work has examined how total radiated energy can be sensitive to the earthquake source physics, showing differences for slip weakening versus velocity weakening friction in elastodynamic fault models. This project proposes to look deeper into the radiated energy field to look for further signatures of the source physics. First, rather than looking only at the total radiated energy, the project will investigate the spectral content of the far field radiated energy, examining this as a function of moment, and for a range of frictions. Here we can expect to see further differences for slip versus velocity weakening friction. Next the research will examine the spatial dependence of the radiation, looking at how the radiated energy changes as one moves from the far field to the near field. Here, the project will examine the average spectral energy density for a fixed radius and angle relative to the event hypocenter, averaging over events of similar size. Interesting distance dependence effects are expected, particularly for distances of order a crustal depth away. Finally, the research will look for correlations in the motions on the fault with aspects of the radiated energy field. Specifically, the models show slip and propagation velocity variations as slip pulses pass through stress heterogeneities on the fault; how are these variations manifested in directivity effects and spectral content?