Crustal Structure Beneath the Nanao Forearc Basin From TAICRUST MCS/OBS Line 14


A velocity model constructed using ocean bottom seismometer (OBS) data collected along TAICRUST Line 14 indicates that the Ryukyu Arc basement (velocity > ~5 km/s) extends beneath the Nanao forearc basin to a depth of 18 to 20 km. Furthermore, our velocity model indicates that the Ryukyu Arc basement extends west to at least 122E, near the west end of line 14, within about 50 km of the Coastal Range of eastern Taiwan. The westward extend of arc basement to 24N, 122E eliminates the possibility of a large northward transform offset of the trench as commonly shown. Beneath the Nanao Basin the depth to the base of this forearc crustal layer is consistent with the position of the thrust boundary between the subducting Philippine Sea Plate (PSP) and overlying Ryukyu Arc as determined by earthquakes studies. In the western portion of the line the probable base of PSP crust appears to be at depths greater than 30 km. The velocity model is based on data recorded by seven, 4-component OBSs deployed at nominal 25 km spacing. The seismic energy source for the OBS data was a 20 airgun array operated form the R/V Maurice Ewing, which simultaneously collected deep-penetration multi-channel seismic reflection data. We constructed the velocity model using forward and inverse travel time modeling of reflected and refracted arrivals recorded at offsets of 0 to 95 km. The coincident MCS data were used to develop the starting model and the final model remains consistent with the sedimentary layering and basement structure evident in the MCS data set.

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