Coseismic Groundwater Temperature Response Associated with the Wenchuan Earthquake.

Various physical, geophysical, geochemical, electrical and hydrological parameters are measured on the surface and in shallow/deep boreholes throughout mainland China to obtain early warning signals of impending earthquakes. Numerous wells are equipped with water level and temperature sensors for continuous observations of the water level and groundwater temperature. An analysis of water temperature data from boreholes equipped with water temperature sensors reveals that nearly half of the boreholes show coseismic response associated with the Wenchuan earthquake (Mw 7.9, 12 May, 2008). The coseismic response of the groundwater temperature cannot be differentiated from the groundwater flow or movement when the earthquake occurred, but there is no fixed relationship between the temperature variation and the water flow. At the same time, we observed that the rock temperature in dry wells can record the seismic events and even the pre-seismic abnormal information. The spatial distribution of the coseismic groundwater temperature response is random and irregular, which does not support the dislocation model of seismic faults at the regional or larger scale. Changes in the groundwater temperature are closely related to the borehole temperature gradient, lithology profile and geological environment of the borehole and depths of the aquifers. The mechanism of the coseismic groundwater temperature response can be explained by an enhanced permeability induced by an earthquake. The groundwater temperature increases if the temperature sensor in the borehole is located near the deep-circulating aquifer and decreases if the sensor is near the shallow-circulating aquifer when seismic waves arrive. The groundwater temperature may be slightly affected or even unchanged if the temperature sensor is far from the aquifer during the propagation of seismic waves. However, it was hard to conclude the changes of rock temperature observed in six dry wells. [ABSTRACT FROM AUTHOR]