Characteristics of surface ruptures associated with the Chi-Chi earthquake of September 21, 1999

Abstract

The Chi-Chi earthquake, which reactivated the Chelungpu fault in central Taiwan, generated surface ruptures over 100 km long. Surface ruptures of the Chi-Chi earthquake can be divided into the north–south and the east–west sections. The north–south section extends for about 84 km, and the northern E–W section 16 km. The north–south section consists of a continuous main rupture and some branches. These are characterized by thrust motion with some strike–slip components. Topographic surveys indicate that the ground commonly uplifted as a single, continuous ramp-step surface, and deformation is mainly concentrated on the hanging wall. Fault slip data and GPS measurements show that the hanging wall along the northern segment of the Chelungpu fault moved northwest, but that along the southern segment of the fault, the hanging wall shifted to the southwest. Sandbox models suggest that the above dissimilarity in hanging-wall movement direction is mainly due to the presence of the Peikang Basement High. In the east–west section, the ruptures comprise a series of en echelon steep faults with right-step or left-step geometry, and trend ENE–WSW to E–W. Near the Ta'an River, the surface rupture orientation shifts from NE–SW to NW–SE and terminates at the Shuangtung fault. Here, the ruptures appear to be controlled by the 10–15-km-wide, NW-trending Sanyi-Puli shear zone. Clay modeling displays the development of E–W-trending ruptures when a N–S-trending fault meets a N30°W-trending weak zone.