ANALYZING SEISMICALLY–INDUCED HYDROLOGIC CHANGES USING THE GEOGRAPHICAL INFORMATION SYSTEMS

A general Geographical Information Systems (GIS) visual model was developed to analyze the coseismic and postseismic hydrologic changes within the Choshuichi alluvial aquifers associated with the 1999 Chi-Chi earthquake (Mw=7.5), Taiwan. The main tectonic structure that ruptured in the Chi-Chi earthquake was the Chelungpu fault, located along the western portion of the Taiwan orogen, which is a result of the ongoing collision between the Eurasia plate and the Philippine sea plate. The maximum fault movement (~ 9.8 m) associated with the main Chi – Chi earthquake was among the largest displacements recorded in modern history. An earthquake of this magnitude caused great fluctuations in water table levels, stream discharge, and groundwater chemical compositions. Water table changes were mapped by importing a georegistered base map into ArcView as a shape file. Well locations in UTM coordinates and the corresponding water table changes were imported as a database file. Isotopic changes of groundwater were mapped by creating a point shape file, digitizing points, and importing an editable table directly in ArcView. The mapped patterns and magnitudes of water level changes and isotope shifts are distinctly different for the unconfined and confined aquifers in the proximal, middle, and distal alluvial fan, indicating that seismically-induced hydrologic changes were derived from different sources and mechanisms. Timely hydrologic data collected from the Chi-Chi earthquake integrated with the GIS mapping provides an effective means to analyze, store, and view a wide range of data, which proves very useful in visualizing seismically-induced hydrologic changes in aquifers.