Late Holocene paleoseismicity of the southern part of the Chelungpu fault in central Taiwan: Evidence from the Chushan excavation site

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 20-30 degrees 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 24 degrees 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. (U), 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 El (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 +/- 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.