A Seismic Study of the Taiwan Orogen

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Abstract for proposal EAR0106786 (PH # 9x)Title: A Seismic Study of the Taiwan OrogenPI's: Wang-Ping Chen, Department of Geology and Mid-America Earthquake Center, University of Illinois, Urbana, IL 61801This is a seismic study of the Taiwan orogen, an important case of active collision zone that has been a focus of international research for the last 20 years. Since the devastating Chi-Chi earthquake sequence of 1999, seismicity increased remarkably in and around Taiwan. Hundreds of large- to moderate-sized earthquakes have been well recorded by arrays of state-of-the-art instruments, providing high-resolution data to investigate active arc-continent collision.We plan to study about 300 earthquakes, including precise determinations of epicenters, focal depths, fault plane solutions, and seismic moments, in order to address some key questions in active collision. The young age and fast convergence of the collision zone are particularly favorable for resolving: 1) if spatial variations reflect time-progression of collision; and 2) if reactivation of pre-existing, extensional structures is important in the early stages of collision. We will focus on these topics and cast our interpretation in the context of geological and geophysical information gathered from various sources.In addition, we will study diffuse deformation to the east of the suture zone, in the interior of the western Philippine Sea. Recent marine data showed a surprisingly old age (early Cretaceous) for the colliding edge of the Philippine Sea. This region, underlain by oceanic lithosphere, is often assumed to be mechanically strong, thus behaving as the driving edge of the collision. The proposed work will constrain the thickness of the strongest part of the colliding oceanic lithosphere, and potentially provide clues to the anomalously shallow depth of the old sea floor.