In-situ fault properties and earthquake nucleation revealed by tidal modulation of seismicity

Earthquakes are expected to be sensitive to solid Earth tides [1, 2], yet demonstrating this century old conjecture has been challenging because the signal is damped by the non-instantaneous earthquake nucleation process and can be revealed only with very large earthquake catalogs [3, 4]. Here we demonstrate a clear and statistically significant tidal modulation of seismicity at The Geysers geothermal field in California by constructing a high-resolution catalog of ~700,000 earthquakes using a deep-learning–based workflow [5-8]. Schuster spectrum analysis of this enhanced catalog reveals pronounced diurnal/semidiurnal peaks aligned with dominant constituents of solid Earth tides, as well as additional peaks at 24-hour and one year associated with geothermal operations. The modulation amplitude from diurnal/semidiurnal solid Earth tides is 3-4%. The tidal modulation amplitude decreases with depth and is enhanced for earthquakes with a higher non-double-couple source component, consistent with a low effective normal stress. The strong sensitivity of seismicity to normal stress perturbations implies a weak coupling between pore-pressure and applied stress and is compatible with a standard friction coefficient of ~0.5-0.6. The observed tidal signal is consistent with an earthquake nucleation process governed by rate-and-state friction [9], allowing us to derive further constraints on frictional properties of faults and the characteristic timescale of earthquake nucleation under tectonic and geothermal conditions [10-12].