Disruption of Groundwater System by Earthquakes

Abstract:

Earthquakes can enhance permeability of groundwater systems at great distances. Here we use the tidal response of the water level in a deep well (~4 km) in the near field of the 2008 Mw 7.9 Wenchuan earthquake, western China, to investigate how large earthquakes may disrupt groundwater systems in the near field. The well is located near the eastern margin of the Qinghai-Tibet plateau and 21 km from the ruptured fault of the Wenchuan earthquake, and is the only well we have found that is in the near field of a large earthquake and showed clear tidal signals before and after the earthquake. The well is open to a Triassic medium-grained sandstone aquifer confined by thick layers of shales on both sides. We show from tidal analysis of the water level in the well that large earthquakes can change not only the permeability but also the poroelastic properties of a groundwater system in an unexpected way. Using the lithologic well logs and experimental data for rock physical properties we interpret this change to reflect the breaching of aquitards of the groundwater system to depths of several km. Such disruption may connect previously isolated groundwater domains to impact groundwater supply, safety of underground waste repository, and hydrocarbon production. The method demonstrated here holds promise for monitoring the breaching of aquitards by hydraulic fracturing of shales during hydrocarbon production and deep injection of waste water.