Deformation Mechanism of the southern ending of the Chi-Chi earthquake

The 1999 Chi-Chi earthquake, Mw 7.6, which resulted a nearly 100-km length surface rupture, was caused by the activation of the low angle thrust Chelungpu fault. Previous studies show that in the southern segment of the Chelungpu fault, the N-S trending surface rupture turns into SW trending and becomes dominantly right lateral strike-slip faulting along the pre-existing Tachienshan fault. However, according to the coseismic GPS slip vector data and field surveys, we found that the NW-trending Luliao fault was also activated during the Chi-Chi earthquake and the SW trending surface rupture (Tachienshan fault) could extend to the west and along the Meishan fault, which was active at 1906, M=7.1. Therefore, we separate the southern end of the Chi-Chi earthquake into three blocks bounded by three coseismic active faults: the N-S trending Chelungpu fault with thrusting dominantly, SE trending Luliao fault with left-lateral strike slip faulting dominantly, and SW to EW trending Tachienshan-Meishan fault system (T-M fault) with right-lateral strike slip faulting dominantly. The strain compatibility of these three faults could explain the southern ends of the Chi-Chi earthquake. Furthermore, the deformation process could be derived according the characteristics of slip vector of surface rupture and GPS data. At first, the Chi-Chi earthquake ruptured southwestward and activated the Chelungpu fault and Tachienshan fault. Then, the Chi-Chi earthquake rotated clockwise to the NW direction and slipped northward accompanying the NW trending Luliao fault­Ýs development as the lateral ramp of the Chi-Chi earthquake. Finally, one month later the Meishan fault was triggered at October 22th, 1999.