Onshore/Offshore Wide-Angle Deep Seismic Profiling in Taiwan

Y.H Yeh,R.C Shih,F.T Wu,C.J Wu,H.Y Yen,C.S Liu,C.S Huang,C.C Liu,P.Z Chen,C.H Lin,B.S Huang

doi:10.3319/tao.1998.9.3.301(taicrust)

Abstract

Wide-angle reflection and refraction data are used to illustrate thecrustal structures in the central Taiwan area,across the Taiwan Orogenand in the westernmost part of the Ryukyu subduction system.The preliminarystructures were derived by modeling the data set collected usingonshore seismic recording instruments along the central cross-island highwayin 1995.Seismic signals were generated by the powerful airgun arraysof the R/V Ewing in the seas east of Taiwan.For the shallow structures,results from the forward modeling of the Pg phases show that strong lateralvariations of P-wave velocities in the upper crust were obtained.Velocitiesat the uppermost crust have a wide range from 3.0 km/sec to 5.2km/sec,and increase from 5.8 km/sec to 6.8 km/sec at the bottom of theupper crust.The thickness of the upper crust increases from 22 km beneaththe Western Central Range to 25 km beneath the Hsincheng Ridge,and it then dramatically decrease to only 10 km beneath the Hoping andNanao basins.For the deep crust structures,similar pattern of thicknessvariations,from 12 km to 20 km,was obtained.Velocities at the lower crustrange from 6.4-6.7 km/sec at its top to 7.2-7.3 km/sec at its bottom.Insummary,the crustal thickness beneath the island could be over 40 km,butonly about 25 km beneath the westernmost part of the Ryukyu forearc region.The thickest crust is not right beneath the highest mountains on theisland but with an offset of about 40 km toward the east.The result indicatesthat Taiwan has still not yet reached to its isostatic equilibrium yet.

Highlights

The island of Taiwan is located along a segment of the convergent boundary between the Eurasian Plate and the Philippine Sea Plate
For wide-angle seismic reflection data, since the offset between shot and receiver is too large and the phases are distorted, we cannot apply the conventional common-mid point (CMP) stacking method to form the image of a structure
Taiwan is located between the Eurasian Plate and the Philippine Sea Plate, where strong orogeny is well known; quality data were limited to details of the deep crust struc tures under the island

Summary

INTRODUCTION

Many scientists have been using seismic or gravity data to study the crustal structure beneath Taiwan They have established depth models of the continental. To examine the existence of the decollement as well as the shape of the Moho under Taiwan, an onshore/offshore wide-angle deep seismic profiling experiment was conducted at the same time. The results provide the first complete seismic images of the deep crustal structure be neath Taiwan, and lead to a better understanding of the Taiwan orogeny and subduction-colli sion system. The results of the wide-angle deep seismic profiles can provide us with better constraints for deeper structure study as well. Using the travel-time delays of the core phase, we will be able to interpret the regional tectonics of Taiwan These wide-angle seismic data will provide information on the lateral variation of the upper mantle beneath Taiwan. The more detailed results of the three profiles will be shown in other individual papers

FIELD WORK AND INSTRUMENTATIONS

DATA CONDITIONING

The Stacked Image

Result From the Ray-Tracing Forward Modeling

DISCUSSION AND CONCLUSIONS