活動斷層區地下水位及岩體應變之研究(I)：集集大地震發生前後地下水系統異常變化分析

Changes of ground water levels
induced by the ML7.3 Chi-Chi earthquake
on September 21, 1999 were recorded at 157
monitoring wells in the Choshui River
alluvial fan. Of those, 67 observed large
ground water level changes, ranging from
1.0 to 11.1m. These Wells are clustered at 64
stations located approximately 2 to 50 km
form the north-south trending Chelungpu
fault. Generally step-like changes of water
level were observed at the time of
earthquake on the hourly digital records,
while both oscillatory and step-like changes
were observed on the analog records.
Co-seismic changes of ground water level
were recorded not only in the confined
aquifer but also in the partially confined
aquifer and the unconfined aquifer. The
recovery of water level changes in an
aquifer took minutes to months, depending
primarily on hydrogeologic conditions of the
confining layers. The sign of co-seismic
water level change at a well varied with its
distance from the fault. The strain field
inferred from the co-seismic water-level
changes due to the Chi-Chi earthquake
suggests that volumetric contraction predominated in most of the footwall area;
whereas volumetric expansion prevailed in a
belted area adjacent to the fault zone.
On 22 October, another earthquake of
ML6.4 occurred near Chi-yi city. It
induced the groundwater level changes at
some monitoring wells in the Choshui River
alluvial fan and Chinan plain, up to 0.4m.
The scattering of groundwater level changes,
revealed the complexity of geologic
structures of Chi-yi region.