Late Holocene paleoearthquake activity in the Ruyeh fault in the Longitudinal valley system, eastern Taiwan

To improve seismic hazard assessment on creeping fault, it is important whether the fault have an ability to produce a big earthquake or not. The Luyeh fault with high possibility of creeping is reported to have been shown four earthquake events by trench excavation survey. The island of Taiwan straddles the boundary between the converging Eurasian and Philippine Sea plates. The convergent rate is about 8-9 cm/yr. The 150 km long, NNE-SSW trending, Longitudinal Valley in eastern Taiwan is considered an active collision boundary, and separates two quite different geologic provinces: the Central Range which is composed of the pre-Tertiary metamorphic to the west and the Coastal Range which consists of Neogene andesitic volcanic units, turbidite sediments and mélange to the east. The Longitudinal Valley fault (LVF) runs along the eastern rim of the valley and an east-dipping reverse fault. Our target area is located in the southend of the Longitudinal Valley, the LVF is divided into two fault zone in this area; Luyeh fault is western part trace. A major discontinuity of about 30 mm/yr on the rate of crustal motion across the Longitudinal Valley is attributed to inter seismic slip of the LVF. The Chieshan segment (north of Luyeh fault) is creeping and show a horizontal shortening of 17-19 mm/yr by creepmeter (Lee et al, 2001). Also Luyeh fault show a crustal movement of 12 mm/yr by GPS date (Yu and Kuo,2001). We had a trench excavation survey across the Luyeh fault. In this site, the fault can be made the significance of range facing scarp on the alluvial fan. We were able to anticipate that fine grain sediments with charcoal accumulated in the hollow, which was formed along the footwall side of the fault by displacement. Our trench size is The result is that we can found the deformed terrace gravel, tilted silty and sandy layer, and two faults. We confirmed four earthquake events on the trench wall by angular unconformity.