Very-low frequency earthquakes on the San Andreas Fault that occur independently of low-frequency earthquakes

Abstract:

Fault slip at varying time scale has been detected in the brittle-ductile transition zone of multiple subduction zones, e.g., slow slip events (SSEs), low-frequency earthquakes (LFEs) and very-low frequency earthquakes (VLFEs). While observations of LFEs are common among both subduction thrust and transform faults, VLFEs have been primarily reported in subduction zone environments. Here we show some of the first known observations of VLFEs occurring on a plate boundary transform fault, the San Andreas Fault near Cholame, California.

We use data from permanent broadband stations and a temporary deployment of 13 broadband stations installed in 2010-2011 within approximately 70 km of Cholame, California. We first remove time periods when teleseismic events occur, as identified in the global Centroid Moment Tensor (CMT) and the Northern California Seismic Network ( NCSN) catalogs, and then filter waveforms between 0.02-0.05 Hz. We find multiple VLFEs with waveform amplitudes that are, in some cases, signicantly larger than amplitudes in the LFE band (2-8 Hz). The preliminary location calculations based on a grid search of travel-time variance reduction suggest that VLFEs occur nearly coincident with the mapped fault trace, and at similar depths to LFEs (~15-30 km). We perform a moment tensor inversion considering both near- and far-field wave terms, and obtain solutions consistent with dominant double-couple source mechanisms exhibiting dextral plate motion at depths of 20 km.