Co-seismic water level changes in response to multiple large earthquakes at the LGH well in Sichuan, China

- We examined the water level data at the LGH well during earthquakes with seismic energy densities greater than 10-4 J/m3;
- Co-seismic water level declines in response to farfield earthquakes can be explained by the passage of seismic waves;
- Co-seismic water level increase in response to one nearfield case can be caused by the static stress and dynamic stress.

We examined the water level data at the LGH well in Sichuan, China, from December 2007 to July 2015 and their responses to multiple large earthquakes with seismic energy densities greater than 10− 4 J/m3. Co-seismic water level declines were observed in response to eleven earthquakes out of twelve in the farfield, and co-seismic water level increase was observed in one nearfield case. The water level declines in the farfield showed a linear relation with the common logarithm of the seismic energy densities, whereas the water level increase in the nearfield fell away from this relation, indicating that the farfield responses and the nearfield response were produced by distinct mechanisms. We used the phase shift of tidal responses as a proxy for permeability and found that permeability enhancements were observed both in the farfield and nearfield. The co-seismic water level declines in response to the distant earthquakes could be explained by permeability enhancements caused by the passage of seismic waves through the mobilization of colloidal particles; the co-seismic water level increase in response to the nearfield case could be caused both by the compression of the static stress and by the seismic waves.