Tidal and hydrological seismicity modulations reveal pore fluid diffusion during earthquake nucleation

The occurrence of seismic events can be modulated by external periodic stress perturbations, such as daily tidal stress and annual hydrological stress. Such periodic modulations are crucial for understanding earthquake triggering, yet their underlying physical mechanisms are not fully understood. Here, we find that ordinary earthquakes (OEs) and low-frequency earthquakes (LFEs) on the Central San Andreas Fault (CSAF) are more sensitive to the long-period hydrological and the short-period tidal loadings, respectively. These different frequency-dependent modulations suggest pore fluid diffusion during the noninstantaneous earthquake nucleation and confirm different nucleation times of OEs and LFEs. We constrain the depth-varying physical properties of the CSAF and reveal that fluid content distribution and loading conditions fundamentally control slow-to-fast fault slip behaviors. Our study provides an alternative perspective to understand earthquake nucleation by using the information in periodic seismicity modulations, which can be applicable to subduction zones where similar slip behavior transitions occur.