Changes of groundwater level due to the 1999 Chi-Chi earthquake in the Choshui River alluvial fan in Taiwan

Changes of groundwater levels induced by the M-L 7.3 Chi-Chi earthquake on 21 September 1999 were recorded at 157 out of 179 monitoring wells in the C hoshui River alluvial fan. Of those, 67 observed large groundwater-level ch anges, ranging from 1.0 to 11.1 m. These 157 wells are clustered at 64 stat ions located approximately 2 to 50 km west of the north-south-trending Chel ungpu fault. Both oscillatory and steplike changes of water level were obse rved on the analog records at the time of earthquake, while only steplike c hanges were observed on the hourly digital records. Coseismic changes of gr oundwater level were recorded not only in the confined aquifers but also in the partially confined aquifers and the unconfined aquifers. The recovery of water-level changes took minutes to months, depending primarily on hydro geologic conditions of the confining layers. The sign and magnitude of cose ismic water-level change at a well varied with its distance from the fault. The distribution of coseismic water-level changes induced by the Chi-Chi e arthquake indicates that water-level rise predominated in most of the footw all area, whereas water-level fall prevailed in a narrow zone adjacent to t he fault trace.