A velocity-depth model, developed using the travel-time inversion of data from Ocean Bottom Seismometers on TAICRUST OBS Line 16, in the western Ryukyu forearc region is described in this paper. Phase indentifications of refracted and reflected signals, initial model building from the shallow to the deep structures, and comparison with a synthetic seismogram have been used to reduce the non-uniqueness of the velocity model.
The velocity contours of the model indicate, from west to east, (1) a thick forearc sedimentary layer below the Hsincheng Ridge, (2) a wedgeshaped upper 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 the Hsincheng Ridge is found to be about 10 km in the model. Based on the similarity of P-wave velocities for the Luzon Arc, the Coastal Range and the Yaeyama accretionary wedge, the forearc sediment may result from the arc-continent collision and/or the bending of the western Ryukyu Arc. The upper and the lower boundaries of the upper crust (mainly the.Ryukyu Arc) 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 high velocity gradient with the west-dipping velocity contours beneath the Hoping Basin and a depression about 5 km deep below the western half of the Nanao Basin are thus found in the upper crust. We infer that the high velocity gradient and the depression in the upper crust may be generated by the collision of the northern Luzon Arc and the western Ryukyu Arc.
Both basement (forearc sediment and Ryukyu Arc basement) deepening and crustal thickening westward in this study are similar to those in velocity models from TAICRUST OBS Line14 and the onshore-offshore line across central Taiwan. The tectonic motions of the northern Luzon Arc, the western Ryukyu Arc and the subducting slab of the Philippine Sea Plate interpreted in this paper provide possible mechanisms for the induction of high seismicity in the western Ryukyu forearc region.