Preseismic deformation and coseismic displacements associated with the 1999 Chi-Chi, Taiwan, earthquake

The destructive 1999 Chi-Chi, Taiwan, earthquake (M-w 7.5) produced an appr oximately 100-km-long surface rupture, mostly along the previously recogniz ed north-south-trending Chelungpu fault. Preseismic deformation in central Taiwan is realized from annually repeated Global Positioning System (GPS) d ata acquired during the 1992-1999 period. The total WNW-ESE shortening rate in the vicinity of the epicentral region, that is from the west coast to t he western boundary of the Central Range, is up to 25 mm/yr. The crustal de formation before the Chi-Chi earthquake was essentially a uniaxial compress ional strain of 0.36 mu strain/yr in the direction of 114 degrees. The GPS measurements taken 0.2-2.7 yr before and within 3 months after the mainshock were utilized to estimate the coseismic displacements. Horizontal movements of 1.1-9.1 m in the NW-NNW directions are observed on the hangin g wall (eastern side) of the fault. There is a northward-increasing trend i n the magnitude of the displacement vectors and a dramatic change in the di rection of about 50 degrees toward the east along the fault strike. In cont rast, much smaller SE-SEE movements of 0.1-1.5 m are found on the footwall (western side) of the fault. The GPS data show 2.4-10.1 m of total horizont al offsets across the Chelungpu fault. Vertical offsets of 1.2-4.4 m with t he eastern side up are also observed along the surface rupture. The uplift on the hanging wall decreases rapidly toward the east. It becomes subsidenc e at Sun Moon Lake and in the Puli-Wushe area. The stations on the footwall show subsidence of 0.02-0.26 m. The width of the uplift zone increases fro m about 10 km in the south to approximately 30 km in the north.