Tidal Triggering of Earthquakes near Parkfield, California Indicates a High Coefficient of Friction on the Shallow San Andreas Fault

Abstract:

Investigators have searched for evidence of tidal triggering of earthquakes for decades because triggering behavior provides potentially valuable information about the stress conditions on faults and how earthquake nucleate. Earth tides are caused by the gravitational pull of the sun and moon which induce periodic stresses with two important components related to the rotation of the Earth relative to the sun and moon (semi-diurnal 12 and ~12.4 hours) and the orbit of the moon around the Earth (fortnightly, ~14.7 days). Semi-diurnal tidal stresses have previously been shown to trigger low frequency earthquakes (LFEs) and non-volcanic tremors (NVTs) on the San Andreas Fault. LFEs and NVTs occur within the brittle-ductile transition of the Earth’s crust, deeper than where regular earthquakes occur. Here we show that tidal stresses trigger earthquakes in the brittle upper crust, which is caused by and detectable due to interactions between the semi-diurnal and fortnightly stress cycles. The triggering of LFEs and NVTs are correlated with tidal shear stresses indicating a low coefficient of friction on the deep San Andreas Fault. In contrast, we find that regular earthquakes are correlated with tidal normal stresses indicating a high coefficient of friction in the shallow San Andreas Fault. That earthquakes are triggered during peak normal (extensional) tidal stresses suggests that pore pressures are below lithostatic pressure in the upper crust. These findings suggests tidal triggering can be applied to infer crustal stress state and pore pressure conditions, two properties of faults that are difficult to measure but are important for understanding earthquake physics and seismic hazards.