The Chi-Chi, Taiwan, earthquake of 21 September 1999 was a large thrust earthquake that caused disastrous damage. The surface fracture trace runs along the Chelungpu fault, which strikes N5°E along most of the faulting area but turns to the northeast while approaching its northern end. It finally presents a horsetaillike faulting system and bends to an east-northeast direction at the northern end. Both strong-motion and Global Positioning System (GPS) data recorded large displacements in the northern part. In this study, we use a multifault model to simulate the observed large strong-motion and GPS data. The results of the joint inversion reveal that in the southern part, slip occurred mainly in the shallow portion, reaching a maximum about 20 m on the ENE fault, where the rupture propagated to the deep part of the fault. There was obviously slip vector rotation from south to north, such that the slip on the branched fault plane was almost purely reverse faulting. The seismic moment, a total of 2.7 × 1020 N m, was released in less than 40 sec during the mainshock. Aftershocks occurred mainly in areas where little or no slip occurred. Slip vector rotation can be explained by the complexity of regional tectonic stress. Changes in tectonic stress at the northern end of the fault inhibited the northward propagation of the rupture and favored the rupturing of the ENE fault.

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