SPATIAL DISTRIBUTION OF FLUID PRESSURE CHANGE DUE TO PLATE COLLISION

While high formation fluid pressure may trigger fault movement, fluid pressure can also be changed by fault movement. Groundwater level changes during earthquakes due to plate motion have been observed in many places around the world; however, distribution of these changes in the vicinity of an earthquake fault remains poorly understood. Here we use records of 211 monitoring wells in the southwestern Taiwan to report the spatial distribution of fluid pressure change during an Mw 7.6 earthquake induced by the collision between the Phillippine Plate and the Eurasian Plate. All coseismic groundwater level falls were observed in wells near the earthquake fault, either ruptured or unruptured. Coseismic groundwater level rises were observed primarily in wells away from the fault. The magnitude of coseismic rise and fall tends to decrease with hypocentral distance from the well; however, it fluctuates over a wide range with depth at most multiple-well stations. The largest coseismic rise at each multiple-well station in the central coastal plain was typically observed in a regional confined aquifer. Such a coincidence suggests that coseismic change is associated with characteristics, rather than depth, of the aquifer. A hydromechanical model which couples fluid pressure with displacement is proposed to describe the generation and distribution of coseismic change. The recovery of coseismic changes considerably increased the hydraulic gradient in the subsurface after the earthquake, inducing a transient flow in both the lateral and the vertical directions.