Wenchuan earthquake enhanced crustal permeability in Taiwan

Surface wavetrain from the 2008 M7.9 Wenchuan earthquake generated water-level changes in wells in Taiwan, some 1900 km away from the epicenter. The water level changes, recorded every second, are compared side-by-side with broadband recordings of the surface wavetrain (courtesy of Drs. Wen-Tzong Liang and Rong-Yuh Chen) to decipher the effect of the seismic waves on far field groundwater. Water level in one well oscillated with an amplitude <2 cm on the arrival of the wavetrain and then decline gradually and monotonically over a period of ~15 minutes after the passing of the wavetrain, with a total decease of ~14 cm. The oscillations of the water level reflect a pore-pressure response in the aquifer to the passing wavetrain; but the later monotonic decline of water level that occurred after the passing of the wavetrain cannot be a direct response to seismic waves, but may have been triggered by the small pore- pressure oscillations in the aquifer. We hypothesize that the surface wavetrain may have unclogged some pre-existing fractures in the shallow crust, enhanced the crustal permeability and connected the aquifer to a low-pressure reservoir. The study suggest that great earthquakes can enhance shallow crust permeability over a large region several thousands km from the epicenter.