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Preliminary Crustal Structures Across Central Taiwan From Modeling of the Onshore-Offshore Wide-Angle Seismic Data
利用寬角震測資料模擬橫跨臺灣中部地殼構造之初步結果
並列摘要
A velocity-depth model,developed using the travel-time inversion ofdata from Ocean Bottom Seismometers on TAICRUST OBS Line 16,in thewestern Ryukyu forearc region is described in this paper.Phase identificationsof refracted and reflected signals,initial model building from the shallowto the deep structures,and comparison with a synthetic seismogramhave been used to reduce the non-uniqueness of the velocity model.The velocity contours of the model indicate,from west to east,(1)athick forearc sedimentary layer below the Hsincheng Ridge,(2)a wedgeshapedupper crust with high velocity gradients beneath the Hoping Basin,and(3)a depression within the western Ryukyu Arc below the Nanao Basin.The thickness of the forearc sediment(3-4.5 km/sec)beneath theHsincheng Ridge is found to be about 10 km in the model.Based on thesimilarity of P-wave velocities for the Luzon Arc,the Coastal Range andthe Yaeyama accretionary wedge,the forearc sediment may result fromthe arc-continent collision and/or the bending of the western Ryukyu Arc.The upper and the lower boundaries of the upper crust(mainly the RyukyuArc)are better defined by two velocity contours of 4.5 km/sec and 6.25 km/sec according to the contrast of the velocity gradient in the model.The highvelocity gradient with the west-dipping velocity contours beneath the HopingBasin and a depression about 5 km deep below the western half of theNanao Basin are thus found in the upper crust.We infer that the high velocitygradient and the depression in the upper crust may be generated bythe collision of the northern Luzon Arc and the western Ryukyu Arc.Both basement(forearc sediment and Ryukyu Arc basement)deepeningand crustal thickening westward in this study are similar to those invelocity models from TAICRUST OBS Line 14 and the onshore-offshoreline across central Taiwan.The tectonic motions of the northern LuzonArc,the western Ryukyu Arc and the subducting slab of the Philippine SeaPlate interpreted in this paper provide possible mechanisms for the inductionof high seismicity in the western Ryukyu forearc region.