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.

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On 6 February 2018, a M L 6.2 earthquake struck the east coast of Taiwan and caused more than 200 casualties in the Hualien area. The mainshock initiated at a shallow depth of 6.3 km and was accompanied by numbers of foreshocks and aftershocks. Coseismic ruptures widely occurred along the Milun Fault with various deformational structures and caused a variety of damage. Extensive field survey and drone-based images reveal the distribution of offsets and the characteristics of the surface ruptures. The major surface ruptures were mostly synthetic and antithetic Riedel shears interlinked by push-up moletracks or tensional fissures. The strike of the principal displacement zone evolved from northeast to northwest and defined a 7.5-km-long NS fault trace concave to the east. The vertical offset reached its maximum of 50 cm on the north segment and became insignificant toward south. Both on-fault single measurements and cumulative sinistral offsets estimated across the rupture zones showed the pattern of southward declining with the maximum offset of 77 cm in the north. The width of the damage zone increased from 1 -20 m to maximum 270 m in the south where the overstepping segments occurred. The development of step-overs is significant in the south, especially in the downtown area where slip is distributed across a transpressional area. Overall, the distinct features of the coseismic ruptures reflect the complex near-surface geology along the Milun Fault and offer new insights for future hazard assessments of the Hualien area.

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