Implications of coseismic groundwater level changes observed at multiple-well monitoring stations

Abstract

Earthquake-related groundwater level changes have been recorded by a dense network of monitoring well stations in Taiwan. At most multiple-well stations, the direction and magnitude of coseismic water level changes vary in wells of different depths. Comprehensive water level data recorded in 209 wells in the vicinity of the seismogenic fault during the 1999 M L 7.3 earthquake demonstrate a preliminary 3-D distribution of coseismic changes. The largest coseismic rise at a station observed typically in a confined gravel aquifer indicates the association of the magnitude of coseismic change with characteristics, rather than depth, of the aquifer. The variation of coseismic changes in the vertical direction implies possible inconsistency between the observed water level changes and the volumetric strains calculated from simple dislocation models. At a given hypocentral distance, the coseismic change in the footwall of the ruptured segment of the fault was much greater than that of the unruptured segment. This phenomenon suggests that fault displacement plays an important role in the generation of coseismic changes. While hypocentral distance correlates well with coseismic rise or fall in the vicinity of the ruptured seismogenic fault, poor correlation is found for coseismic change further from the earthquake epicentre. © 2007 The Authors Journal compilation © 2007 RAS.