Seismic structure in the northeastern South China Sea: S-wave velocity and Vp/Vs ratios derived from three-component OBS data |
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| Authors: | Minghui Zhao Xuelin Qiu Shaohong Xia Huilong Xu Ping Wang Tan K Wang Chao-Shing Lee Kanyuan Xia |
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| Institution: | 1. Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China;2. Sanya Institute of Deep-Sea Science and Engineering, Chinese Academy of Sciences, Sanya 572000, China;3. Institute for Geophysics, University of Texas at Austin (R2200), 10100 Burnet Road, Austin, TX 78758-0000, USA;4. Exploration Department, China National Offshore Oil Corporation Ltd., Beijing 100010, China;5. Guangzhou Marine Geological Survey Bureau, Ministry of Land and Resources, Guangzhou 510075, China;6. University of Chinese Academy of Sciences, Beijing 100049, China;1. CAS Key Laboratory of Ocean and Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China;2. Department of Geophysics, Tohoku University, Sendai 980-8578, Japan;3. Institute of Deep-Sea Science and Engineering, Chinese Academy of Sciences, Sanya 572000, China;4. CNOOC Nanhai East Petroleum Bureau, Shenzhen, Guangdong 518054, China |
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| Abstract: | A nearly 500-km-long seismic profile with reflective and refractive wide-angle Ocean Bottom Seismometer (OBS) data and Multi-Channel Seismic (MCS) data was acquired across the northeastern continental margin of the South China Sea (SCS). The S-wave crustal structure and Vp/Vs ratios have been obtained based on a previously published P-wave model using the software RayInvr. Modeling of vertical- and horizontal-component OBS data yields information on the seismic crustal velocities, lithology, and geophysical properties along the OBS-2001 seismic profile. S-wave velocities in the model increase generally with depth but exhibit high spatial variability, particularly from the shelf to the upper slope of the northeastern SCS margin. Vp/Vs ratios also reveal significant lithological heterogeneity. Dongsha–Penghu Uplift (DPU) is a tectonic zone with a thicker crust than adjacent areas and a high magnetic anomaly. With a Vp/Vs of 1.74 and a P-wave velocity of 5.0–5.5 km/s, the DPU primarily consists of felsic volcanic rocks in the upper crust and is similar to the petrology of Zhejiang–Fujian volcanic provinces, which perhaps is associated with a Mesozoic volcanic arc. The ocean–continent transition (OCT) in the northeastern SCS is characterized by a thinning continental crust, volcanoes in the upper crust, and a high velocity layer (HVL) in the lower crust. The S-wave velocity and Vp/Vs ratio suggest that the HVL has a mafic composition that may originate from underplating of the igneous rocks beneath the passive rifted crust after the cessation of seafloor spreading. |
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