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

The destructive 1999 Chi-Chi, Taiwan, earthquake (Mw 7.5) produced an approximately 100-km-long surface rupture, mostly along the previously recognized north-south-trending Chelungpu fault. Preseismic deformation in central Taiwan is realized from annually repeated Global Positioning System (GPS) data 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 the western boundary of the Central Range, is up to 25 mm/yr. The crustal deformation before the Chi-Chi earthquake was essentially a uniaxial compressional strain of 0.36 μstrain/yr in the direction of 114°. 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 hanging wall (eastern side) of the fault. There is a northward-increasing trend in the magnitude of the displacement vectors and a dramatic change in the direction of about 50° toward the east along the fault strike. In contrast, 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 horizontal offsets across the Chelungpu fault. Vertical offsets of 1.2-4.4 m with the eastern side up are also observed along the surface rupture. The uplift on the hanging wall decreases rapidly toward the east. It becomes subsidence 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 from about 10 km in the south to approximately 30 km in the north.