Focal Mechanisms of LFEs in Parkfield by the amplitude inversion using synthetic waveforms

Various types of earthquakes including ordinary earthquakes and slow earthquakes are observed at the Parkfield section of the San Andreas Fault. Since Shelly et al. (2009) detected low frequency earthquakes (LFEs) at Parkfield, various characteristics have been studied for the 88 LFE families. Thomas et al. (2012) reported the variation in the sensitivities to the tidal stress among LFE families. The stress variation across the San Andreas Fault is also found from the focal mechanisms of ordinary earthquakes; i.e., there are reverse fault earthquakes at several km away from the fault, such as Coalinga earthquake and San Simeon earthquake, while most of the earthquakes are characterized by right-lateral strike slip, which is consistent with plate motion.

In this study, we estimate the focal mechanisms of 88 LFE families in the catalog of Shelly (2017). We first stack seismograms of over one million events in the catalog to improve the signal-to-noise (S/N) ratio. Then, we evaluate the absolute amplitudes of original seismograms using the stacked waveforms. The waveform amplitudes are inverted for the focal mechanisms for each LFE family using synthetic waveforms. Specifically, we first estimate maximum absolute amplitude of three-component waveforms for both stacked and synthetic waveforms, then estimate site effect and focal mechanism by solving an inverse problem. Synthetic waveforms are given by the wavenumber integration method by Zhu and Rivera (2002). As a regional structure, we use 1D structure of velocity and attenuation near Parkfield extracted from Eberhart-Phillips (2016).

As a result, while most of the focal mechanisms are strike-slip consistent with the plate motion at the San Andreas Fault, those for families near central Parkfield and the family off the surface trace contain large dip-slip components. These variations are consistent with the tidal sensitivities of LFEs in Parkfield (Thomas et al., 2012). Our study reveals the regional existence of dip-slip focal mechanisms even along a mature strike-slip fault system.