Tidal response variation and recovery following the Wenchuan earthquake from water level data of multiple wells in the nearfield

• We examine water level data in the nearfield region of the Wenchuan earthquake.
• Most wells having resolvable tidal responses show permeability enhancement.
• Permeability enhancement is a distinct process from static poroelastic strain.

An important dataset to emerge from the Wenchuan earthquake Fault Scientific Drilling project is direct measurement of the permeability evolution of a fault zone. In order to provide context for this new observation, we examined the evolution of tidal responses in the nearfield region (within ~ 1.5 fault lengths) at the time of the mainshock. Previous work has shown that seismic waves can increase permeability in the farfield, but their effects in the nearfield are more difficult to discern. Close to an earthquake, hydrogeological responses are generally a combination of static and dynamic stresses. In this work, we examine the well water level data in the region of the large Mw 7.9 Wenchuan earthquake and use the phase shift of tidal responses as a proxy for the permeability variations over time. We then compare the results with the coseismic water level pattern in order to separate out the dynamic and static effects. The coseismic water level pattern for observed steps coincident with the Wenchuan mainshock mainly tracks the expected static stress field. However, most of the wells that have resolvable tidal responses show permeability enhancement after this large earthquake regardless of whether the coseismic response for the well water level is increasing or decreasing, indicating permeability enhancement is a distinct process from static poroelastic strain.