Tidal triggering of earthquakes suggests poroelastic behavior on the San Andreas Fault

• Published in: Earth and planetary science letters 2017-02, Vol.460, p.164-170
• Main Authors: Delorey, Andrew A., van der Elst, Nicholas J., Johnson, Paul A.
• Format: Article
• Language: English
• Subjects: California; Earth tides; earthquake triggering; pore pressure; seismology
• ISSN: 0012-821X; 1385-013X
• DOI: 10.1016/j.epsl.2016.12.014

Tidal triggering of earthquakes is hypothesized to provide quantitative information regarding the fault's stress state, poroelastic properties, and may be significant for our understanding of seismic hazard. To date, studies of regional or global earthquake catalogs have had only modest successes in identifying tidal triggering. We posit that the smallest events that may provide additional evidence of triggering go unidentified and thus we developed a technique to improve the identification of very small magnitude events. We identify events applying a method known as inter-station seismic coherence where we prioritize detection and discrimination over characterization. Here we show tidal triggering of earthquakes on the San Andreas Fault. We find the complex interaction of semi-diurnal and fortnightly tidal periods exposes both stress threshold and critical state behavior. Our findings reveal earthquake nucleation processes and pore pressure conditions – properties of faults that are difficult to measure, yet extremely important for characterizing earthquake physics and seismic hazards. •Tidally triggered earthquakes are observed on the San Andreas Fault.•Stress shadows influence seismicity.•Pore pressure is less than lithostatic in the upper crust of the San AndreasFault.•Seismicity is correlated with tidal normal stress.•Poroelastic conditions are different between the upper and lower crust.