The shallow layers of the Earth host the interplay of many physical processes, some being driven by atmospheric forcing (precipitation, temperature, loading) whereas others have their origin at depth, for instance ground shaking due to seismic activity. These forces cause the subsurface to continuously change its mechanical properties, therefore impacting our societies that settle, rely and evolve on the shallowest layers of the Earth. This portion of the subsurface where life and rocks meet is known as the critical zone. One way to investigate the ever changing time-dependent properties of this layer is through the use of seismic interferometry. This technique is based on cross-correlations of the ambient seismic noise wavefield recorded by seismometers and allows for the continuous monitoring of seismic wave velocities. This project aims to monitor and constrain seismic velocity changes that are caused by hydrological changes and ground shaking-induced damage after earthquakes to better mitigate postseismic hazards and predict water availability. We analyze seismic datasets in a range of field settings that spans several climatic conditions (wet to arid climate) in various seismic-prone areas. We constrain our results using simple physical models and independent datasets. The first part of the project is conducted in the Nepal Himalayas where we estimate seismic velocity changes to reveal detailed insights on groundwater dynamics during three monsoon seasons. We also investigate the effect of damage caused of the 2015 Gorkha earthquake on ground properties. Motivated by this work, we are working on the time- dependent evolution of subsurface rock properties following earthquakes.

• Niels Hovius
• Christoph Sens-Schönfelder

• Illien, L., Andermann, C., Sens-Schönfelder, C., Cook, K., Baidya, K. P., Adhikari, L. B., Hovius, N. (2021): Subsurface moisture regulates Himalayan groundwater storage and discharge. - AGU Advances, 2, 2, e2021AV000398. https://doi.org/10.1029/2021AV000398
• Illien, L., Sens-Schönfelder, C., Andermann, C., Marc, O., Cook, K., Adhikari, L. B., Hovius, N. (2022): Seismic velocity recovery in the subsurface: transient damage and groundwater drainage following the 2015 Gorkha earthquake, Nepal. - Journal of Geophysical Research: Solid Earth, 127, 2, e2021JB023402. https://doi.org/10.1029/2021JB023402