Co-seismic displacements, folding and shortening structures along the Chelungpu surface rupture zone occurred during the 1999 Chi-Chi (Taiwan) earthquake

A nearly 100-km-long surface rupture zone, called Chelungpu surface rupture zone, occurred mostly along the pre-existing Chelungpu fault on the northw estern side of Taiwan, accompanying the 1999 Chi-Chi Ms 7.6 earthquake. The Chelungpu surface rupture zone can be divided into foul segments based on the characteristics of co-seismic displacements, geometry of the surface ru ptures and geological structures. These segments generally show a right-ste p en echelon form and strike NE-SW to N-S, and dip to the east with angles ranging from 50 to 85 degrees. The co-seismic flexural-slip folding structu res commonly occurred in or near the surface rupture zone from a few meters to a few hundreds of meters in width, which have an orientation in fold ax es parallel or oblique to the surface rupture zone. The displacements measu red in the southern three segments are approximately 1.0-3.0 m horizontally and 2.0-4.0 m vertically. The largest displacements were measured in the n orthern segment, 11.1 m horizontally and 7.5 In vertically, respectively. T he amount of co-seismic horizontal shortening caused by flexural-slip foldi ng and reverse faulting in the surface rupture zone is generally less than 3 m. It is evident that the co-seismic displacements of the surface: ruptur e zone are a quantitative surface indicator of the faulting process in the earthquake sourer Fault. The relations between the geometry and geomorpholo gy of the surface rupture zone, dips of the co-seismic faulting planes and the striations on the main fault planes generated during the co-seismic dis placement, show that the Chelungpu surface rupture zone is a reverse fault zone with a large left-lateral component. (C) 2001 Elsevier Science B.V. Al l rights reserved.