Geomorphic evidence for an active pop-up zone associated with the Chelungpu fault in central Taiwan

Abstract

The destructive Chi-chi earthquake ( M w=7.6, M L=7.3) occurred in central Taiwan on September 21, 1999. The surface ruptures extend more than 100 km, mainly along the Chelungpu fault. The northerly striking ruptures, however, branch and turn northeast from Shihgang to Neiwan. In this vicinity, a group of unpaired and polycyclic terraces along the Tachia River, known as Hsinshe River Terraces, is situated on the hanging wall of the Chelungpu fault. Many of the terraces were tilted, and the younger ones and the river beds “popped up” during the Chi-chi earthquake. The geomorphic evidence suggests that the terraces in the western group have been continually deformed by a pop-up structure with a hinge coincident with that of the 1999 pop-up deformation. It suggests that the pop-up structure of the Chi-chi earthquake has developed since the late Pleistocene and has migrated northward along with tectonic activity of the Chelungpu fault. However, the early deformation of this pop-up structure shows a kink-fold style. As this structure propagates northward, the Tachia River has been diverted around its plunging nose. Four cycles of tectonic uplift→channel migration→point bar formation have been recognized in the Hsinshe River Terraces and serve as a model for river response to growing tectonics. Furthermore, this study tentatively estimates the rate of 4.75±0.62 m/ka for tectonic uplift in the pop-up zone and 2.73±0.35 m/ka for the regional rate of uplift. Because the pop-up structure is a secondary structure associated with the main thrust, the Chelungpu fault, the rates estimated in this study give the lower bound of the vertical slip rate in this region.