Spatial distribution of groundwater level changes induced by the 2006 Hengchun earthquake doublet

Water-level changes were observed in 107 wells at 67 monitoring stations in the southern coastal plain of Taiwan during the 2006 M_w 7.1 Hengchun earthquake doublet. Two consecutive coseismic changes induced by the earthquake doublet can be observed from high-frequency data. Observations from multiple-well stations indicate that the magnitude and direction of coseismic change may vary in wells of different depths. Coseismic rises were dominant on the southeast side of the costal plain; whereas, coseismic falls prevailed on the northwest side. In the transition zone, rises appeared in shallow wells whilst falls were evident in deep wells. As coseismic groundwater level changes can reflect the tectonic strain field, tectonic extension likely dominates the deep subsurface in the transition area, and possibly in the entire southern coastal plain. The coseismic rises in water level showed a tendency to decrease with distance from the hypocenter, but no clear trend was found for the coseismic falls.