Seismic Velocity Recovery in the Subsurface: Transient Damage and Groundwater Drainage Following the 2015 Gorkha Earthquake, Nepal
Sens‐Schönfelder, Christoph
Andermann, Christoff
Marc, Odin
Cook, Kristen L.
Adhikari, Lok B.
Hovius, Niels
DOI: https://doi.org/10.1029/2021JB023402
Persistent URL: http://resolver.sub.uni-goettingen.de/purl?gldocs-11858/9976
Andermann, Christoff; 1 Helmholtz Centre Potsdam German Research Centre for Geosciences (GFZ) Potsdam Germany
Marc, Odin; 3 Géosciences Environnement Toulouse UMR CNRS/IRD/UPS Observatoire Midi‐Pyrénées Toulouse France
Cook, Kristen L.; 1 Helmholtz Centre Potsdam German Research Centre for Geosciences (GFZ) Potsdam Germany
Adhikari, Lok B.; 4 Department of Mines and Geology Kathmandu Nepal
Hovius, Niels; 1 Helmholtz Centre Potsdam German Research Centre for Geosciences (GFZ) Potsdam Germany
Abstract
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 Mw 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.
Plain Language Summary: Earthquake ground shaking damage the rocks in the subsurface of the Earth, altering their strength and their permeability. After the main shock, the rock properties slowly return to their pre‐earthquake state, but the duration of this recovery is poorly constrained. One way to investigate these time‐dependent changes is through the monitoring of seismic velocity inferred from ambient ground vibration recorded at seismic stations. Here, we constrain the evolution of seismic velocity following the large 2015 Mw 7.8 Gorkha earthquake in Nepal, in a field site characterized by seasonal groundwater fluctuations. We find that the velocity recoveries after the main shock and the aftershocks can be modeled with the same recovery timescale, independently from the initial shaking intensity. This suggests that earthquakes of different sizes activate the same geological structures and mechanisms during the recovery phase. Thanks to the unique hydrological setting of our field site and a model that links seismic velocity and groundwater level, we also show that this change of rock properties after the main shock is accompanied by a transient change in hydrological properties, an observation inferred for the first time with seismic measurement.
Key Points:
We estimate a recovery time scale (<1 yr) in seismic velocity changes after the Gorkha earthquake using ambient noise correlations.
Velocity recoveries are modeled with relaxation functions characterized by a constant maximum relaxation timescale that is peak ground velocity‐independent.
We highlight a transient enhanced permeability from the velocity changes in the first ∼6 months following the main shock.
Subjects
earthquake damageearthquake hydrology
relaxation
Gorkha earthquake
seismic monitoring
ambient noise