Mapping Groundwater Fluctuations in the Coastal Los Angeles Basin by Seismic Interferometry

Changes in crustal seismic velocity (dv/v) can be monitored continuously in time by interferometry of seismic ambient noise. This approach has been successfully employed to study temporal perturbations in stress fields, rock fractures, and fluids. Here we go one step further with this monitoring technique, by not only detecting the temporal changes but also imaging the inhomogeneous spatial distributions of dv/v. We implement the spatial imaging by leveraging travel-time shifts at successive lag times and solving inverse problems based on coda-wave sensitivity kernels. We then use these space-time observations of dv/v to investigate the groundwater fluctuations in the Coastal Los Angeles (LA) Basin during 2000-2020. Imaging of dv/v demonstrates the spatial patterns of groundwater variations: Seasonal changes are most pronounced within confined zones in Santa Ana Basin and LA Central Basin, whereas the long-term changes extend to a broader area including the unconfined forebay. We further compare dv/v observations with InSAR measurements and find strongly consistent spatial patterns. Compared with surface deformation measurements, dv/v additionally help to characterize the depths of several aquifers in the study area. This real-data application substantiates the validity of our dv/v imaging protocol, and shows the promise of using spatio-temporal dv/v observations to monitor surficial hydrological processes.