Fig. 2: Time series of Δv/v (relative changes in seismic velocity), precipitation, and hydraulic head.

a The comparison between Δv/v (bold blue line) and annual cumulative precipitation (light blue bars). Δv/v is measured in 0.2–2.0 Hz and averaged over all spatial grids in the study area. The annual cumulative precipitation is measured at the meteorological station ANA (Fig. 1c). The gray line is obtained by low-pass filtering the blue line to show the long-term trend of Δv/v. The shaded areas (in gray) denote humid periods. b A local-scale comparison between Δv/v (bold blue line) and the hydraulic head (yellow line with circles). Δv/v is averaged over spatial grids within the Santa Ana Basin. Head data are measured at a dedicated groundwater monitoring well GGM-1/MP1 (Fig. 1c).