Periodic Seismic Velocity Changes in Southern California Caused by Changes in Seasonal Stress

Seasonal changes in crustal stress have been observed in places worldwide, including in southern California, USA. These seasonal stress perturbations may be caused by some combination of hydrologic loading, tidal loading, pore pressure changes, or thermoelastic stress. Long-term monitoring of variations in seismic velocity traveling through the shallow crust has been shown to reflect changes in crustal stress from earthquakes and volcanoes, as well as environmental sources of stress such as changes in hydrology and temperature. Using 17 years of continuous seismic recordings at 12 seismic stations surrounding the Salton Sea in southern California, we show that seasonal changes in seismic velocity are (1) present in these timeseries and (2) can be related to changes in environmental stress. In order to address (1) we use a continuous wavelet transform (CWT) to decompose the seismic velocity change timeseries into their time-variable periodic components. Performing the CWT on the timeseries of seismic velocity change reveals that many timeseries contain a strong annual periodic component, similar to temporal behavior of hydrologic loading, deformation at nearby GNSS stations, and crustal temperature recorded at in-situ monitoring stations. In order to investigate the main cause of the annual changes in seismic velocity, we model changes in environmental crustal stress and strain from elastic unloading of terrestrial water storage from GRACE and lake level change recordings at the Salton Sea, pore pressure change at depth, and thermoelastic stress. By modeling the annual environmental stresses in southern California and testing the correlation with changes in seismic velocity at each station, we determine that annual velocity changes found at stations in the Salton Trough correlate higher to changes in thermoelastic stress. In contrast, stations outside the Salton Trough are mainly controlled by changes in hydrology. Through this investigation, we show that continuous seismic recordings can be a useful tool to monitor small changes in seasonal environmental stresses, providing an additional in-situ observational tool.