Study On Deep Crustal Structure Along Onshore-offshore Seismic Profile Across The Littoral Fault Belt In NE South China Sea

The northeastern South China Sea (SCS) is located in the jointing areas between the Eurasia and Philippine Sea plate. This area has been affected significantly by the Taiwan collision Zone since Miocene Epoch and therefore has intensive Neotectonic activities. The northern margin of the SCS is the key area to understand the forming and evolution history of the SCS due to its more detailed and comprehensive data.The onshore-offshore deep seismic experiment was carried out for the first time and filled the blankness of the seismic surveys in this transition zone between the South China mainland and northeastern SCS. The researches focus on a few of points as follow. The onshore-offshore deep seismic data (P-2001) were analyzed and processed; Three programs (EVT2SAC, RAW2UKOOA, SAC2SEGY) were written to convert data format; Different seismic phases were identified and their travel time arrivals were simulated to study the P-wave velocity crustal structure of this area; the location of the littoral fault belt was inferred. The results provided important information for the study on seismicities and evolution of the northeastern SCS.The significant innovation is that the onshore-offshore deep seismic experiment was carried out for the first time in northeastern SCS. We used 3-component Ocean Bottom Seismometers, portable land stations made in our country and marine air-gun seismic source. The seismic data obtained are in good quality and filled the blankness of the seismic surveys in this transition zone. All these provide a strong base for the study of the crustal structure.This area has the typical continental crust. The crust thickness is gradually decreasing southward along the onshore-offshore seismic line. Mainly the upper crust thins the crust thickness, while the lower crust changes little. The thickness ratio of upper crust (not including sediment layers) and whole crust decreases from 0.58~ 0.61 in the north to 0.46 ~ 0.48 in the south of Binhai (littoral) fault belt.Correspondingly the depth of Moho discontinuity from about 29.5 ~ 30.5 km to 25.0~28.0 km. The low-velocity layer (5.5-5.9 km-s"1) exists generally in the middle crust (10~18km) with about 2.5~4.0 km thick, which also thins seaward. No obvious high-velocity layer appears in the lower crust. The littoral fault belt in the crustal structure reveals as low velocity zone, which is located about 35km SE from Nanao station and corresponding with the gradient belt of gravity & magnetism anomalies. The depth of fault extends down close to Moho discontinuity. The littoral fault belt is a boundary between the normal continental crust of the South China and the attenuating continental crust of sea area. This littoral fault belt had important tectonic significance in the process of rifting and breakup. The tension fault belt made South China block detach.The studies on gravity, magnetism, seismicities, deep structure, fracture structure and focal mechanism show that fractures in trends NE, NEE and NW are main causative faults. The NEE tectonic is the main structure in the coastal region of the South China, but the NW fault system and the dense earthquakes zone can't be neglected. The low-velocity layer in depth from 10 to 18km is also one of the reasons that earthquakes happened frequently.