Time-dependent properties of the shallow subsurface: Groundwater and Earthquake damage dynamics from seismic interferometry

Abstract: The Earth’s surface is shaped by restless phenomena: landslides are the scars of intermittent earthquake damage while the flow of subsurface water molds the landscape. These time dependent behaviors fundamentally originate in the variation of the mechanical state of the subsurface. Since the advent of seismic ambient noise interferometry, the transient properties of the subsurface can be monitored at high-temporal resolution, probing directly the invisible. In this presentation, a new look and novel observations for two of the main iconic topics in passive monitoring are introduced: groundwater evolution and seismic damage. I first show seismic velocity changes retrieved in a typical steep catchment of the Nepal Himalayas as a proxy for groundwater storage. With complementary data, we constrain the hydrological cycle due to the monsoon precipitation and highlight the key controls for freshwater resources generation at our field site. Notably, we show that the onset of groundwater recharge is strongly modulated by the moisture contained in the near-surface materials. In the second part of the talk, I try to answer a fundamental question regarding subsurface damage after earthquakes. How long does it take for the velocity to recover to pre-earthquake values (aka the relaxation timescale) ? I show preliminary results obtained in Chile, which hints at a predictive dynamics of the subsurface, that is poorly dependent on ground shaking intensity.