The 1999 Chichi earthquake (Mw = 7.6) was one of the most damaging earthquakes in the world over the past tens years. The earthquake was caused by thrust faulting that produced a visible surface rupture of 95 km in length along the front of the Western Foothills. This active fault separates Pliocene and Holocene deposits, and has produced easily identifiable and prominent geomorphic features in the past 0.5 - 0.7 Ma. Since the Chichi earthquake, several studies have attempted to characterize the geomorphic expression of the fault in Holocene terrace and fluvial deposits.
The Chichi earthquake was one of the largest earthquake events documented in central Taiwan during the past three centuries. The area contains many known active faults, such as the Changhua, Houli, and Chelungpu faults, which have displaced the late Pleistocene-Holocene deposits. Before the Chichi earthquake, the Chelungpu fault was not classified as an active fault in the Taiwan seismic catalogues, because there was no direct evidence of surface rupture. Hence, no detailed studies were completed on it. However, extensive paleoseismological research has been performed since the Chi-chi earthquake to better document the paleoseismic history. To delineate the timing and recurrence intervals of the Chelungpu fault, we excavated seven trenches across it, and these contained evidence of four surface-rupturing earthquake events during the past 2 ka, including the Chichi earthquake. We integrate data from a recent trench near Chushan with previous paleoseismologic results along the Chelungpu fault, and discuss their implications for the Late Holocene paleoseismicity history of the fault.
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 20-30o 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 24o 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 BP (E2), 710-950 cal yr BP (E3), 1380 - 1700 cal yr BP (E4), 1710 - 1930 cal yr BP (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 BP) 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.