Images of Structural Variation at the Active Collision Boundary of Central Eastern Taiwan

The Longitudinal Valley (LV), located in eastern Taiwan, is considered to be the surface representation of the suture between the Philippine Sea plate (PSP) to the east and the Eurasian continental plate (EUP) to the west. High-resolution 3-D Vp and Vs models are determined in order to understand their relationship with seismicity and tectonic implication. High-quality P and S wave arrival times have been selected from data collected by the Taiwan Seismic Network (TSN) and the Portable Array for Numerical Data Acquisition II (PANDAII) in the Hualien area in central eastern Taiwan. A traveltime tomography package initially developed by Benz (1996) and later modified by Shen (1999) is used in the study. After the determination of reliable 3-D Vp and Vs models, all available earthquakes are relocated to delineate subsurface structures using a recently developed earthquake location program (Chen, 2005). The results of checkerboard resolution tests indicate both P- and S-wave velocity structures can be successfully recovered at least to a depth of 30km for the study area. Thus, significant structures beneath the LV and CR can be reliably recovered by a seismic tomography method under the given station-event configuration. Recovered velocity model along with the relocated earthquake hypocenters clearly depict the deformation structures associated with the active collision. A northwest dipping fault is outlined by the most concentrated seismicity and located at the boundary between the low and high velocity anomaly. A sudden increase in velocity to the east and a sub-vertical extension of the high velocity anomaly (> 6.8km/s) beneath the Longitudinal Valley and Coastal Range suggests that the major faults have juxtaposed different lithologies as a consequence of progressive plate collision. Comparisons among Vp, Vs and Vp/Vs models, relocated seismicity and recent GPS observations indicate the fault defined by planar seismicity may be the major boundary between EUP and PSP, where a significant amount of horizontal crust movement is consumed.