Imaging of Arc-Arc Collision in the Ryukyu Forearc Region Offshore Hualien From TAICRUST OBS Line 16

A velocity model constructed using ocean bottom seismometer(OBS)data collected along TAICRUST Line 14 indicates that the Ryukyu Arcbasement(velocity＞～5 km/s)extends beneath the Nanao forearc basin to adepth of 18 to 20 km.Furthermore,our velocity model indicates that theRyukyu Arc basement extends west to at least 122°E,near the west end ofline 14,within about 50 km of the Coastal Range of eastern Taiwan.Thewestward extent of arc basement to 24°N,122°E eliminates the possibilityof a large northward transform offset of the trench as commonly shown.Beneath the Nanao Basin the depth to the base of this forearc crustal layeris consistent with the position of the thrust boundary between the subductingPhilippine Sea Plate(PSP)and overlying Ryukyu Arc as determined byearthquake studies.In the western portion of the line the probable base ofPSP crust appears to be at depths greater than 30 km.The velocity modelis based on data recorded by seven,4-component OBSs deployed at nominal25 km spacing.The seismic energy source for the OBS data was a 20airgun array operated from the R/V Maurice Ewing,which simultaneouslycollected deep-penetration multi-channel seismic reflection data.We constructedthe velocity model using forward and inverse travel time modelingof reflected and refracted arrivals recorded at offsets of 0 to 95 km.Thecoincident MCS data were used to develop the starting model and the finalmodel remains consistent with the sedimentary layering and basement structureevident in the MCS data set.