P-wave velocity structures of the crust across the Southern Taiwan Strait imaged by using air-gun data recorded from ocean-bottom seismometers

Abstract

Ocean-bottom seismometers (OBSs) were deployed at a shallow water depth (< 50 m) and recorded data from air-gun shootings with a large volume of about 8000 in(3) along HX07 (26 OBSs) in June 2015 and HX08 (12 OBSs) in June 2016 across Littoral Fault Zone and Taiwan Shoal in the southern Taiwan Strait. Travel times of the refracted and reflected waves from the hydrophone component of OBS data were picked and the travel-time errors (< 0.1 s) were reduced layer-by-layer to invert the P-wave velocity-interface models. Based on the OBS models along HX07 and HX08, the large lateral variations of P-wave velocity (4.8-5.8 km/s) in the upper crust, interpreted as normal faults, were imaged in Littoral Fault Zone and Dongshan Fault. These normal faults, extend to the middle crust, have caused two horst structures (Dongshan Platform, Taiwan Shoal) and a graben structure (Taiwan Strait Cenozoic Depression). In the southern Taiwan Strait, the shallowest depth of 4 km and 12.5 km respectively at the top and the bottom of the middle crust near Littoral Fault Zone became deep (8 km and 17 km) toward both sides below Taiwan Strait Cenozoic Depression and Dongshan Platform. The shallow middle crust near Littoral Fault Zone may have resulted from crustal bending. However, since the location of crustal bending is far from Taiwan orogeny, the crust might be bent by continental rifting. Based on the iso-depth maps consisting of 9 OBS models in the southern Taiwan Strait, the depth of the basement (1-2 km) and the bottom of the upper crust (about 5.5-6 km) became shallow below Taiwan Shoal. These structural highs have been affected by a loading of Taiwan orogen to conform the forebulge (the Taiwan Shoal) in a foreland basin system.