Tidal modulation of seismicity in the Southern California

How the seismogenic fault systems response to periodic stress fluctuations can improve our understanding of earthquake physic process (e.g., earthquake nucleation). Tidal stress provides a good opportunity to explore seismicity response to known persistent periodic loading. The lack of significant correlation between continental earthquakes with tides might be due to insufficient earthquake events or due to the longer earthquake nucleation time relative to tides. Recent a relatively complete Southern California earthquake catalog (Quake Template Matching catalog) from 2018 to 2017 might improve the ability to capture the tidal triggering features. We first generate declustered catalogs by using Reasenberg Declustering, Nearest Neighbors declustering, and Epidemic-Type Aftershock Sequence (ETAS) declustering. The catalog declustered by ETAS shows least clustering effect. We related earthquake occurrence time to tidal stress by the tidal phase which is defined -180o to 180o through two stress troughs. The significance of the periodic distribution of tidal phase can indicate how significant the tidal modulation is. We apply this tidal modulation calculation to the whole catalog and the catalogs of regions divided into spatial bins of 0.2 degree × 0.2 degree. Our results show that: (1) one day periodic signal is most significant for the earthquake with magnitude < 0.5. This is probably due to the anthropogenic actives, because the observed peak seismic activity is consistent with quiescence of human activities and this signal can be observed every year; (2) The seismic sensitivity to tidal modulation is spatial varied. We find that the seismic activity near San Gorgonio Mountain is the only region show a slight tidal modulation.