Typhoon-driven Near-surface Groundwater Dynamics Revealed by Ambient Noise in the Mountain Watershed

Near-surface groundwater dynamics (NSGD) reflect residence and recharge of the terrestrial water and control the water resource of the downstream area. Environmental seismology, using seismic velocity changes (dv/v), provides a non-invasive approach to observe NSGD. Here, we use ambient seismic noise at four seismic stations, hydraulic, and meteorological records from the Liwu watershed, Taiwan, following Typhoon Kompasu in 2021 to investigate NSGD in response to typhoon rainfall. We used the single-station cross-component (SC) method to construct daily correlation functions, and dv/v was computed by the stretching method in the frequency band of 4–8 Hz. Moreover, we simulated dv/v using a near-surface layer of limited storage capacity and adjusted hydraulic conductivity (Ks). Our results indicate that peak dv/v at the downstream station was about four times that at the divide, with recovery to pre-typhoon levels taking 29 days, compared to 8 days at the divide station. Simulated Ks shows that hydraulic conductivity values higher than borehole-derived estimates are required to best capture the observed dv/v responses, indicating the preferential flow may be via colluvium and fractured bedrock in the study site. In short, a five-day typhoon produced nearly one month of NSGD, demonstrating that near-surface groundwater may function as a temporary storage zone for deep groundwater. These results demonstrate that ambient seismic noise can resolve short-term subsurface water dynamics during extreme events, offering new constraints on water residence times and aquifer structure that are relevant for disaster management and biogeochemical studies in mountainous watersheds.