The 1999 earthquake fault and its repeated occurrence at the Earthquake Museum of Taiwan, central part of the Chelungpu Fault

Abstract

The Chelungpu Fault, Taiwan, records the surface rupture by the 1999 Chichi earthquake and abundant information has been obtained on paleoseismology on this fault during the last 10 years. This study area, located on the central part of the Chelungpu Fault, is known as one of the type localities of the surface rupture, but no paleoseismological study was carried out. This paper reports the trench log across the Chelungpu Fault preserved at the Earthquake Museum of Taiwan and tries to reconstruct the faulting history at this site based on 10 bore hole data on the location of the museum. Trench log indicates the displacement of terrace deposits and underlying bedrock by the 1999 earthquake. Four subparallel faults are observed within 1.8 m width, corresponding to the location of the surface rupture. Estimated height difference of bedrock surface based on the bedrock height by bore hole data is more than 4.2 m, significantly larger than the offset by the 1999 earthquake (2-2.5 m). This fact implies that at least two earthquake events including the 921 earthquake occurred at this site. We also estimate the fault plane dips eastward at about 30 degree, based on the location and stratigraphy of one bore hole which penetrates to the terrace gravel on the footwall through the bedrock.