Spectra-semblance-based rupture directivity estimation: Application to earthquakes in California

Abstract:

We develop and apply a new approach to estimate rupture directivity of small to moderate earthquakes (M 2-5) using entire source spectra of events. We first extract waveforms around P-wave arrivals to calculate P-wave spectra. Stacked empirical Green’s functions based on data of smaller events are used to remove propagation and site effects, and derive observed source spectra of target earthquakes at different receivers. For each target earthquake, we correct the observed source spectra for possible directivity effects using a wide range of assumed rupture azimuths and horizontal rupture velocities, and estimate the common source spectra at all receivers. For each rupture-azimuth-velocity combination, the semblance (Marfurt et al., 1998) of the estimated common source spectra is calculated. The rupture azimuth and velocity of the target earthquake is determined by finding the best-fitting rupture-azimuth-velocity combination with the largest calculated semblance. The method is applied to over 10,000 earthquakes in southern California and over 5,000 earthquakes on the creeping section of San Andreas Fault, using data with a frequency range 5-40 Hz. The results are consistent with previous directivity studies of earthquakes on the San Jacinto fault zone (Meng et al., 2020) and the creeping section of San Andreas Fault (Lengliné and Got, 2011; Wang and Rubin, 2011). Since no assumptions on source model are required, this method can be applied to many more (and smaller) earthquakes than the above studies. We plan to apply the method systematically to all M2-5 events in California satisfying basic quality criteria. Updated results will be presented in the meeting.