A Seismic Refraction Profile Acress the Longitudinal Valley Near Hualien Taiwan

Abstract

The tectonic plate movement in eastern Taiwan is very active, especially along the Longitudinal Valley, which is believed to be a suture zone between two plates. Except for the high seismicity and rapid ground deformation, the geological literatures would normally suggest two high angle faults on two sides of the Valley, i.e., the Central Range fault on the west and the Coastal Range fault on the east. The structure of the Longitudinal Valley itself must represent a certain degree of complexity due to the tectonic compression. In January, 1994, we conducted a 7.5 km long refraction survey at 0.5 km intervals across the Valley with two 50 kg dynamite explosions at both ends. The purpose was to explore the detailed structure under the Longitudinal Valley and to detect two faults near the Hualien area. Very clear first arrivals were obtained, from which we are able to draw a concave shaped valley profile (2.0 km in axial depth and 5.0 km in width) of the valley basement. The eastern flank of the basement is steeper than the western flank with good indications of the Coastal Range fault located just below the axial place, but the Central Range fault itself is not so obvious. A transition zone, with Central Range materials under the west half of the Valley, is suggested. The axis of the Valley would be thus the suture line. This fault, along the axial suture line, should be named the 'Longitudinal Valley fault' instead of using an ambiguous name such as the Coastal Range fault. Except for the refraction measurement, we also used surface wave simulations to derive an S-wave velocity constraint for the Valley. It is found that the P-wave and S-wave velocities for most alluvia layers are 3.2 km and 0.90 km/sec, respectively. The basement under the valley has P and S velocities of 5.0 km/sec and 2.0 km/sec, respectively. A low velocity near-surface layer with Vp=2.1 km/sec, Vs=0.4 km/sec, and thickness=0.3 km is also found, which would adequately explain a large amplitude surface wave group with a 0.54 km/sec apparent velocity. The result of this study is consistent with the refraction survey carried out in 1973 by Tsai et al (1974). Although our velocity values are lower by 10%, the dimensions for the concave shaped basement of the Longitudinal Valley are in good agreement.