Marine Denolle, Tim Clements

his study investigates changes in seismic velocities in the period 1999-2021 using about 700 permanent and temporary broadband seismic stations in the state of California. We compute single-station cross-correlations of the ambient seismic noise and use the coda-wave interferometry to measure the changes in seismic velocities ({\it dv/v}) using a stretching technique. We focus on the 2-4Hz frequency band and the upper 500 m of the near-surface sensitivity. We discuss {\it dv/v} within the context of nonlinear elasticity. We fit models of thermoelastic strains, various hydrological models that diffuse rain water, and slow-dynamics healing models for post-seismic response of earthquakes. In general, we find that both thermoelastic strains and hydrological strains have similar amplitude of impacts on {\it dv/v}. We find that the diffusion of rainwater using a drained response in a poroelastic medium explains most of the data. The best fit hydraulic diffusivity are high in the mountains and low in the basin. We find that the largest drop in seismic velocity occurs during the 2004-2005 wet winter, and that the 2011-2016 Drought is characterized by a multi year marked increase in {\it dv/v}. We interpret site-specific variations with land subsidence or inflation detected by remote sensing. We also find decade-long post-seismic response of two major earthquakes and bound the time scale of relaxation processes to a few years. Together, we see long-term changes in seismic velocities are showing positive trend over two decades that we can interpret as long term lowering of the groundwater table.