Thrust-related river terrace development in relation to the 1999 Chi-Chi earthquake rupture, Western Foothills, central Taiwan

Abstract

The 21 September 1999 earthquake (M W 7.6) produced a surface rupture that extended for 100-km in a general north-south direction in front of the Western Foothills, central Taiwan. The study area, located in a segment of the fold -thrust belt of central Taiwan, provides an important natural laboratory to study the relationship between active thrust deformation and fluvial terrace development. We recognize, from west to east, three subparallel thrusts, the Sanyi, Houli, and Shihkang faults in the northern part of the earthquake disaster area. The 1999 earthquake rupture occurred only along the Shihkang fault, whereas the other two faults were inactive. Along the Tachia River in the hanging wall of the Shihkang fault, eight well-developed terrace levels are observed, and can be subdivided into lateritic (Lt5-Lt1) and non-lateritic terraces (Ft3 -Ft1). Five levels of lateritic terrace (Lt5 -Lt1) are located in the hanging wall of the Houli fault, and a lateritic terrace (Lt5) is present in the hanging wall of the Sanyi fault. Stratigraphic and elevation correlations between these well-developed fluvial terraces in relationship to nearby geologic structures show that the Sanyi fault is the oldest structure. Lateritic terraces of at least 10,000 yr BP, for example, overlie the Sanyi fault, but correlative terraces associated with the Houli fault are offset. These types of terrace correlations allow us to define the progressive development of the thrust system within the Western Foothills. q

Subsurface Structure, Physical Properties, and Fault Zone Characteristics in the Scientific Drill Holes of Taiwan Chelungpu-Fault Drilling Project

“…2). Studies of terrace deformation around the Tachia River (Chen et al 2003) infer the existence of an in-active fault (called Houli fault) between the Sanyi and the rupture-A faults. Despite that the Chinshui Shale is probably buried in the core of the small anticline associated with the rupture-A fault ), the stratigraphic positions of both rupture-A and Houli faults are generally unknown since the bedrock was covered by terrace and alluvial deposits.…”

An Electric Resistivity Study of the Chelungpu Fault in the Taichung Area, Taiwan

“…These resistivities are consistent with the apparent lithology of gravel on the top and shale at the bottom on the walls of an evacuation about 30 m to the south of the profile. The basal layer is suggested to be the Kueichulin Formation based on the resistivity spectra of rocks, the rock formation sequence and comparisons with seismic sections conducted in the study area (Wang et al 2002;Chen et al 2003).The boundary between the Chinshui Shale and the Kueichulin Formation is suggested to be the Chelungpu fault. The geological section of the profile deduced from the resistivity structures is shown in Fig.…”

“…4c. This boundary is suggested to be the subsurface fault zone associated with the Chi-Chi earthquake and is called the ChiChi earthquake rupture; it can be correlated to the Shihkang fault (Chen et al 2003).…”

“…The very-low resistivity layer of the eastern part is suggested to be the Chinshui Shale for it has a very-low resistivity. The basal layer of the eastern part is suggested to be the Kueichulin Formation based on the rock formation sequence, resistivity spectra of rocks and a comparison between the resistivity profile and the seismic sections conducted in the study area (Wang et al 2002;Chen et al 2003). The geological section deduced from the resistivity structures is shown in Fig.…”

“…This earthquake was generated by the reactivation of the Chelungpu fault. This fault was suggested to be a thrust fault with easterly Neogene formations riding on westerly Quaternary formations along the base of the hills in west-central Taiwan and to be the southern extension of the Sanyi fault (Meng 1963;Chang 1971;Ho and Chen 2000;Chen et al 2001;Chen et al 2003). Before the Chi-Chi earthquake, only a few outcrops of the fault had been found and most of the fault line could not be traced due to weathering, collapse and coverage by debries.…”