Seismic rhythms: Earthquake response to tectonic, hydrological, and tidal forcing in California

Seismicity is primarily driven by tectonics, but stress variations of natural or human-made origin can induce detectable modulations, offering insights into earthquake physics. Here, we identify regions in California exhibiting significant seasonal modulations of seismicity rate linked to hydrological surface loading but no significant semidiurnal tidal modulation. The peak seismicity rate lags behind the peak stressing rate by half a month. Assuming instantaneous nucleation substantially overpredicts the response, whereas time-dependent nucleation governed by rate-and-state friction accurately captures both the amplitude and the time delay with friction-stress parameter aσ ~ 1 to 10 kilopascals (kPa) and characteristic relaxation time t_a ~ 0.05 to 1 year. The seismicity response to tidal and seasonal stress perturbations allows us to probe fault mechanical properties, providing a way to improve seismic hazard assessment.