Seismic Velocity Recovery in the Subsurface: Transient Damage and Groundwater Drainage Following the 2015 Gorkha Earthquake, Nepal

Luc Illien , Christoph Sens-Schönfelder , Christoff Andermann , Odin Marc (1) , Kristen L. Cook , Lok B. Adhikari , Niels Hovius

1 GET - Géosciences Environnement Toulouse

Shallow earthquakes frequently disturb the hydrological and mechanical state of the subsurface, with consequences for hazard and water management. Transient post-seismic hydrological behavior has been widely reported, suggesting that the recovery of material properties (relaxation) following ground shaking may impact groundwater fluctuations. However, the monitoring of seismic velocity variations associated with earthquake damage and hydrological variations are often done assuming that both effects are independent. In a field site prone to highly variable hydrological conditions, we disentangle the different forcing of the relative seismic velocity variations δv retrieved from a small dense seismic array in Nepal in the aftermath of the 2015 M_w 7.8 Gorkha earthquake. We successfully model transient damage effects by introducing a universal relaxation function that contains a unique maximum relaxation timescale for the main shock and the aftershocks, independent of the ground shaking levels. Next, we remove the modeled velocity from the raw data and test whether the corresponding residuals agree with a background hydrological behavior we inferred from a previously calibrated groundwater model. The fitting of the δv data with this model is improved when we introduce transient hydrological properties in the phase immediately following the main shock. This transient behavior, interpreted as an enhanced permeability in the shallow subsurface, lasts for ∼6 months and is shorter than the damage relaxation (∼1 yr). Thus, we demonstrate the capability of seismic interferometry to deconvolve transient hydrological properties after earthquakes from non-linear mechanical recovery.

Keywords: earthquake damage; earthquake hydrology; relaxation; Gorkha earthquake; seismic monitoring; ambient noise

Domains: Sciences of the Universe [physics]; Earth Sciences

Luc Illien, Christoph Sens-Schönfelder, Christoff Andermann, Odin Marc, Kristen L. Cook, et al.. Seismic Velocity Recovery in the Subsurface: Transient Damage and Groundwater Drainage Following the 2015 Gorkha Earthquake, Nepal. Journal of Geophysical Research : Solid Earth, 2022, 127, ⟨10.1029/2021JB023402⟩. ⟨insu-03619990⟩