Source-time functions of low-frequency earthquakes on the San Andreas fault

Abstract:

Low-frequency earthquakes (LFEs) are earthquakes that are depleted in high-frequency content relative to earthquakes of similar magnitude. Along the San Andreas Fault, as in many subduction zones, LFEs occur in rapid succession, forming tectonic tremor. Depletion of high frequencies can be attributed to either source or path effects, however studies of LFEs and nearby earthquakes with approximately the same path have shown that the LFE source is responsible for the differences in spectral content. In this study we use an empirical Green’s function approach to further investigate what physical properties of the LFE source cause high-frequency depletion. We use a dataset of approximately one million LFEs comprising 88 LFE families on the deep San Andreas Fault. For a subset of the shallowest LFE families, there are nearby earthquakes that locate within 3 km of the LFE hypocenter (a small fraction of the path length). We exploit the assumed difference in moment-duration scaling between LFEs and regular earthquakes and use these earthquakes as empirical Green’s functions. We determine relative LFE source time function using time domain deconvolution and increase the signal-to-noise ratio by jointly inverting multiple LFEs within individual families. Preliminary results show similar source-time functions derived at many different seismic stations, independent of source-station azimuth. The lack of directivity suggests the rupture velocity is a small fraction of the shear wave velocity. In many kinematic earthquake source models, the spectral corner frequency is proportional to the rupture velocity and the slip velocity. Hence, a low rupture velocity may explain why LFEs are depleted in high frequency content relative to traditional earthquakes.