Neotectonics in the southernmost part of Longitudinal Valley, eastern Taiwan and it's implications to the activity of Lichi Fault and Luyeh Fault

Longitudinal Valley Fault (LVF) in eastern Taiwan is known as the suture of Eurasian Plate and Philippine Sea Plate. It is the major seismic zone in Taiwan, and has a high convergent rate up to 30 mm/yr. In the southernmost part of the Longitudinal Valley, the LVF system branches to two distinct faults Lichi Fault and Luyeh Fault, they offset and uplifted the Holocene terraces and may fracture in the 1951 earthquake series. For lack of dating and firsthand evidence, there is just a rough definition of the activity of them, and the ambiguous of the fault trace is also controversial for a long time. The main purpose of this study is to solve the question about the fault activity, and further, we want to suggest the subsurface structural model of this area. According to the terrace dating, the uplift rates we estimated are 17~20 mm/yr and 3 mm/yr of the hanging wall of the Lichi Fault and Luyeh Fault respectively. In addition, we estimate the uplift rate of the Central Range Fault (< 4.1 mm/yr) that bounded the west side of Longitudinal Valley based on the assumption of the balance between river incision, aggradation, and tectonic uplift. Based on the field observation and the trend of GPS velocity profile, we assume Lichi Fault and Luyeh Fault are totally creeping that we can serve the short-term GPS velocity as the long-term crust motion rate. Then we simply reverse the vertical and horizontal rate of crust motion to the fault plane, and infer that the dip angle of the Lichi Fault is 53° in the upper part and decline to 30° below 7 km and the Luyeh Fault is 17° in the shallow depth of the valley. Concluding the result of this study, we confirm that Lichi fault and Luyeh fault are active through Holocene and may behave as creeping faults. And the fault geometry we inferred is close to that inferred from the aftershock distribution of 2006 Mw 6.1 Taitung earthquake.