A 3-DIMENSIONAL VP MODEL OF THE SOUTHEASTERN TAIWAN AREA AND ITS TECTONIC IMPLICATIONS

Using travel time data from local earthquakes and air-gun shots record ed by the Central Weather Bureau Seismographic Network, the transition from a typical subduction to a collision suture in the southeastern T aiwan area is imaged in terms of a three-dimensional Vp structure. The southern prolongation of the Longitudinal Valley Fault (PLVF), which is characterized by a sharp contrast in velocity on either side, is th e primary feature in the velocity structure. West of the PLVF, a high velocity volume exists from the surface to about 9-km in depth, which can be interpreted as being related to the Central Range. The Central Range structure seems to end near 22.2 degrees N beneath the Hengchun Peninsula. East of the PLVF, a major high velocity anomaly in the midd le- to lower-crust beneath the Southern Longitudinal Trough and Huatun g Ridge is observed. According to the velocity structure and the estim ated composition, the high velocity body could be the forearc oceanic crust, which might have been torn off and separated from the Philippin e Sea plate after the Luzon are was formed, and has been shortened dur ing the collision of the Eurasian and Philippine Sea plates. The other conspicuous feature of the Vp model is a clearly lateral velocity var iation across the Taitung Canyon from the surface to about 25-km in de pth, which might be associated with the segmentation of the Luzon are. Using the three-dimensional Vp model, earthquake events that occurred from 1990 to 1997 were relocated. Most of the relocated hypocenters i n the study area tend to lie on the locations where there is a greater gradient in the Vp model.