Geodetically observed surface displacements of the 1999 Chi-Chi earthquake near Southern Termination of the Chelungpu fault

Abstract

Using the Global Positioning System (GPS) we investigated coseismic deformation in southwestern Taiwan for the Rayli (7/7/1998, ML=6.2), Chi-Chi (9/21/1999, ML=7.3) and Chiayi (10/22/1999, ML=6.4) earthquakes based on data collected from 1996 to 2001. The GPS network includes 130 campaign-surveyed, 32 local tracking stations and 13 IGS (International GPS Service for Geodynamics) sites. The geodetic data were processed with GAMIT/GLOBK software. The results indicate that coseismic displacement vectors show opposite moving directions across the surface trace of the Chelungpu-Tachienshan-Chukou thrust fault. In general, the hanging-wall moves toward NW (or W, SW) as opposed to NE (or E, SE) for the footwall. The average amount of displacement is larger in the hanging-wall than in the footwall, with maximum displacement appearing at sties near the mainshock. An array of NW-SE trending left-lateral tear faults distributed in a zone of tens of kilometers wide are interpreted as transfer faults accommodating SW-directed, right-lateral thrusting in the vicinity of southern termination of the Chelungpu fault.