CRUSTAL VELOCITY VARIATION OF THE WESTERN PHILIPPINE SEA PLATE FROM TAICRUST OBS MCS LINE-23/

The aims of this research are to understand the deformation of the sha llow structures (<6 km depth), the crustal velocity variation of the w estern Philippine Sea Plate (PSP) and their relation to the arc-contin ent collision using Ocean Bottom Seismometers (OBS) and a multi-channe l seismic (MCS) survey (together known as TAICRUST) offshore southeast ern Taiwan. A seismic line, Line 23 which covers the Luzon Arc, the Hu atung Basin, the Taitung Canyon and the western edge of the Gagua Ridg e, is investigated. Prominent reflected and refracted arrivals from th e sediment, the oceanic basement and the Moho can be seen in the OBS d ata. 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 numerou s travel-time picks demonstrate the accuracy and the high resolution o f the model, respectively.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 G agua Ridge are found in the velocity model. Anomalously low velocity i n the upper crust is also identified beneath the Taitung Canyon and ne ar the Gagua Ridge. The former may result from the strike-slip fault w hile the latter may be generated from the uplift of the Gagua Ridge. A ccording to the variation of the crustal thickness, the velocity model is divided into three portions with the distance larger than 74 km, b etween 23 and 74 km, and less than 23 km from its northwest end. The c rustal thickness in the southeast portion (>74 km) is almost uniform a t about 12 km. Similarly, the thickness of the upper crust in the cent ral model (23 similar to 74 km) and the thickness of the lower crust i n the northwest portion (<23 km) remain uniform at about 4 km and 8 km , respectively. However, the lower crust in the central portion and th e 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 l ateral velocity at a distance of 23 km from the northwest end of the m odel imply the eastern edge of the Luzon Arc. Furthermore, northwestwa rd dipping of the Moho in the velocity model is consistent with other studies. The mechanism of crustal thickening in the western PSP is pro bably related to intra-plate deformation, thrust faulting and/or futur e subduction of the western PSP beneath the Luzon Arc.