Distribution of Earthquake-triggered Groundwater-level Changes in the Vicinity of Seismogenic Fault

Groundwater-level data have been recorded by a dense network of multiple-well monitoring stations in Taiwan. Changes in groundwater-level have often been triggered by earthquakes due to the movement between the Philippine Sea plate and Eurasian plate. Oscillatory changes could be observed by high-frequency monitoring wells during earthquakes. These changes are caused by passing seismic waves, especially surface waves. Sustained groundwater-level rises and falls recorded during earthquakes are attributed to in-situ co-seismic strain due to the fault movement. Thus, the distribution of co-seismic groundwater-level changes may provide a framework for crustal deformation in the shallow crust induced by earthquakes. Sustained changes were recorded in 210 monitoring wells during the 2010 Mw6.5 Jiasian earthquake. The spatial distribution of co-seismic changes indicates that water-level falls appeared in the wells near the thrust seismogenic fault, while water-level rises predominated elsewhere. Similar pattern of distribution was found during the1999 Mw 7.6, Chi-Chi earthquake when co-seismic changes were observed in 276 wells. This phenomenon reveals co-seismic crustal compression was widespread in the island, while crustal extension occurred near the thrust fault during these earthquakes. Sustained co-seismic changes are seldom observed at wells in the sediments of a basin or a valley, suggesting the concentration of co-seismic strain on the nearby hills. Generally the observed spatial distribution of co-seismic rises and falls in the vicinity of seismogenic fault is not consistent with the strains calculated from simple dislocation models, possibly caused by topography, geologic conditions, and focal mechanism. More high-frequency groundwater-level data are needed for a better understanding of crustal deformation during earthquakes.