Daily and Seasonal Variations of Shallow Seismic Velocities in Southern California From Joint Analysis of H/V Ratios and Autocorrelations of Seismic Waveforms

Establishing a baseline of ongoing secular velocity variations at the subsurface can improve the accuracy of detecting and interpreting short-term velocity changes, and advance the understanding of observed seismic motions and the behavior of subsurface materials. Toward these goals, we develop and apply a deconvolved autocorrelation (DA) method to estimate regional daily and seasonal changes of seismic velocities in southern California. The DA method combines advantages of traditional autocorrelation and Horizontal-to-Vertical Spectral Ratio, and is used to analyze over 10 years of data recorded by 50 stations. The results indicate widespread daily and seasonal changes of up to 10% and 4%, respectively, in the top tens of meters of the crust. The thickness of the surface layer, distance from the coast, and topographic variations are important factors controlling the amplitudes of the resolved velocity variations. The results suggest that changes of soil moisture and thermoelastic strain are likely dominant factors affecting the daily and seasonal variations, respectively. The developed DA method can improve the accuracy and robustness of estimated changes of subsurface materials at other locations.

Automated summary

By establishing a baseline of ongoing secular velocity variations at the subsurface, the accuracy of detecting and interpreting short-term velocity changes can be improved, further advancing the understanding of observed seismic motions and the behavior of subsurface materials. In this study, a deconvolved autocorrelation method was developed and applied to estimate regional daily and seasonal changes of seismic velocities in southern California. The results showed widespread daily and seasonal changes in the top tens of meters of the crust, with amplitudes of up to 10% and 4%, respectively. Factors such as surface layer thickness, distance from the coast, and topographic variations were found to control the amplitudes of the resolved velocity variations. Changes in soil moisture and thermoelastic strain were identified as likely dominant factors affecting the daily and seasonal variations, respectively. The developed deconvolved autocorrelation method can enhance the accuracy and robustness of estimated changes of subsurface materials at other locations.