The aims of this research are to understand the deformation of the shallow structures (< 6 km depth), the crustal velocity variation of the western Philippine Sea Plate (PSP) and their relation to the arc-continent collision using Ocean Bottom Seismometers (OBS) and a multi-channel seismic (MCS) survey (together known as TAICRUST) offshore southeastern Taiwan. A seismic line, Line 23 which covers the Luzon Arc, the Huatung Basin, the Taitung Canyon and the western edge of the Gagua Ridge, is investigated. Prominent reflected and refracted arrivals form the sediment, the oceanic basement and the Moho can be seen in the OBS data. By applying the travel-time inversion of the stacked MCS data and the OBS data, the velocity-depth model is built sequentially from the shallow to the deep structures. Low RMS travel-time error and numerous travel-time picks demonstrate the accuracy and the high resolution of the model, respectively.<\p>

A deep basement beneath OBS stations 30 and 31, a narrow basement trough beneath the Taitung Canyon, and the long-wavelength bending of the oceanic crust near the western edge of the Gagua Ridge are found in the velocity model. Anomalously low velocity in the upper crust is also identified beneath the Taitung Canyon and near the Gagua Ridge. The former may result from the strike-slip fault while the latter may be generated from the uplift of the Gagua Ridge. According to the variation of the crustal thickness, the velocity model is divided into three portions with the distance larger than 74 km, between 23 and 74 km, and less than 23 km from its northwest end. The crustal thickness in the southeast portion (>74 km) is almost uniform at about 12 km. Similarly, the thickness of the upper crust in the central model (23-7 4 km) and the thickness of the lower crust in the northwest portion ( <23 km) remain uniform at about 4 km and 8 km, respectively. However, the lower crust in the central portion and the upper crust in the northwest portion gradually thicken northwestward. The maximum crustal thickness is about 24 km at the northwest end of the velocity model. The variations of the crustal thickness and the lateral velocity at a distance of 23 km from the northwest end of the model imply the eastern edge of the Luzon Arc. Furthermore, northwestward dipping of the Moho in the velocity model is consistent with other studies. The mechanism of crustal thickening in the western PSP is probably related to intra-plate deformation, thrust faulting and/or future subduction of the western PSP beneath the Luzon Arc.