Late Holocene Paleoseismicity of the Southern Part of the Chelungpu Fault in Central Taiwan: Evidence from the Chushan Excavation Site

Abstract

The geomorphic expression of the frontal Western Foothills in central Taiwan is usually defined by a late Holocene scarp that ranges from tens to hundreds of meters in height. This scarp is the product of displacement on a near-surface 20-30Њ east-dipping thrust fault, the Chelungpu fault, which ruptured during the Chichi earthquake. The large scarp height may correspond directly to the accumulation of successive surface ruptures. The Chushan site is located on the southern part of this fault where the Chichi earthquake produced a scarp with a 1.7-m vertical offset for the total vertical separation. Based on core-boring estimates, the vertical displacement on both sides is 7 m along a 24Њ east-dipping thrust fault. The results from our paleoseismic analysis indicate that five large paleoearthquake events have caused the large offsets during the past 2 ka. The radiocarbon age constraints of the paleoearthquakes suggest a clustering of 540-790 cal yr B.P. (E2), 710-950 cal yr B.P. (E3), 1380-1700 cal yr B.P. (E4), 1710-1930 cal yr B.P. (E5), and the 1999 Chichi earthquake. Events E3 and E4 have not been reported in previous studies and we did not observe event E1 (300-430 cal yr B.P.) at the site. Based on displacement and fault segmentation from the geologic features, we argue that the two new events may have occurred along the northern part of the Chelungpu fault. The vertical slip rate is estimated to be at least 3.9 ‫ע‬ 0.2 mm/yr over the past 2 ka, which is similar to the long-term estimation through a calculation of late Pleistocene-Holocene terrace elevations on the hanging wall.

Characteristics of Coseismic Thrust-Related Folding from Paleoseismic Investigation Responsible for the 1999 Chi-Chi Earthquake of Central Taiwan

“…Over the past ten years, we have excavated numerous sites and continuously cored borings on the Chelungpu fault, where the surface rupture shows surface deformation within the Holocene deposits; our results have provided quantitative data for late Holocene slip rates ( Fig. 1; Chen et al, 2001aChen et al, , b, 2004Chen et al, , 2007aOta et al, 2001Ota et al, , 2005Streig et al, 2007). Bedrock exposed along the Shihkang fault has not been characterized in previous paleoseismic studies.…”

Earthquake Probability Assessment for the Active Faults in Central Taiwan: A Case Study

“…For example, in the Chelungpu fault rupture cases, the north segment had not ruptured independently according to the paleo-earthquake results using trenching data (Chen et al 2007), so this article gave the north segment a lower weight than the south segment as well as the entire fault. In the TamaopuShuangtung fault rupture cases, because the length of the Tamaopu fault segment is too short to induce a large earthquake, this article gave 0.1 as the weight for the Tamaopu fault segment.…”

Time-predictable model application in probabilistic seismic hazard analysis of faults in Taiwan

“…supports the idea that the reoccurring ruptures on a fault can be described by a time-predictable model. The conclusion from paleoseismological investigations in Taiwan indicated that the behavior of rupture episodes at active faults adhere to a time-predictable model, based on a consistent slip rate level at which the earthquake occurs, such as the Chelungpu fault (Chen et al 2004(Chen et al , 2007 or the Longitudinal Valley fault (Yen et al 2008) etc. (as shown in Fig.…”

“…Seven trenches were excavated along this fault to investigate the history of large earthquake events. (Chen et al 2007; http:// cgsweb.moeacgs.gov.tw/). The average slip rate of the Chelungpu fault is 6.4 mm yr -1 .…”

“…It provided data that prove recurrence intervals to be consistent with an assumed probability function. Based on the fault displacement of the Chelungpu fault (reported by Chen et al 2000Chen et al , 2004Chen et al , 2007 and the Longitudinal Valley (reported by Yen et al 2008) fault in Taiwan, the magnitude of a paleoearthquake can be estimated using the relationship of average displacement to the moment magnitude by Wells and Coppersmith (1994). As shown in Fig.…”