Tempo-spatial Distribution Of Marine Seismicity And Its Tectonic Environment Analysis Of China Offshore Seas

The China offshore(or coastal) seas are located in the western margin of the Western Pacific Ocean and include from the north to south the Bohai Sea, Yellow Sea, East China Sea and South China Sea and off east Taiwan ocean. The abundant oil and gas resource is found to occur in these Seas. During recent years, with the enlargement of exploitation and investment on the marine resources in China offshore Seas, the economy and population in China offshore areas are unprecedented flourishing. However, The China offshore Seas and adjunct regions on situated on the joint areas of several plates, and suffer the interactions between the Eurasian, Pacific and Philippine Sea plates, resulting in intensive seismicity there. The earthquake and associated secondary geological hazards would bring heavy impact on the local economy and social development. Therefore, understanding of the tempo-spatial distribution of marine seismicity and their seismogenic environment in the China offshore Seas can not only provide scientific direction for earthquake hazard mitigation, but also is significance for local economy and social development.In this thesis, we collected all the historic earthquake information from literatures and national seismic networks, and analyzed geological data including active faults, tectonic stress field and other geophysical data available in the China offshore Seas, to investigate the shallow and deep structure of the seismogenic areas, and attempted to link all these geological and geophysical results with different aspects to seismogenic environment. The goal aims to find the tempo-spatial pattern of marine seismicity and their controlling factors, to provide some directions for earthquake activity assessment in the China offshore Seas.Our results show that, in terms of the intensity of earthquake, the Bohai Sea, Taiwan strait and the northern South China Sea are the areas with intensive earthquakes, followed by the Yellow Sea, and the East China Sea is the least intensive. In addition, the active timing of the earthquakes is also uneven in these Seas. Two peaks are found in the Bohai Sea and south Yellow Sea, one is from 1548 to 1597 and another is since 1888, with a recurrence period of 300～400 yrs. The timing of seismicity in the East China Sea is relatively even and is mainly since 1900, with the concentration of recent 50 yrs, and the South China Sea also shares the similar timing with the East China Sea. Accordingly, the seismicity of China Offshore Seas is of prominent tempo-spatial differentiation.The locations of earthquake are close related to the geologically active belts including the faults and subduction zones. The strong earthquakes in the Bohai Sea are mainly situated on the joint areas of the Tan-Lu Fault Zone and Yan-Bo Fault Zone, and most of the earthquakes in the Yellow Sea occur on the transition zone of the southern depression and Wunansha uplift. Although the seismicity in the East China Sea is weak, the Ryukyu Islands-Okinawa trough in the east is the strongest seismic belt; the present earthquakes are basically found in the south Taiwan Island and the Philippine. Both seismically active areas are the subduction zones. In short, the geologically active zones are the carriers of the (potential) earthquakes, and the identification of the large scale fault zones in the China Offshore Seas is vital for analysis of earthquake activity there.Earthquake is actually the rapid release of stress within the crust and the analysis of tectonic stress fields concerned would provide stress background for earthquake activity. The modern tectonic stress field in the China offshore Seas is dominated with (or nearly) horizontal compression, and the stress direction changes from NE striking to NEE,EW, SEE and SE gradually from the north to the south, showing eastward distribution with fan-shape. The intensity of the tectonic stress is also non-uniform and characterized by strong in the north and weak in the south. This pattern of tectonic stress in China offshore Seas is a result of combination of escaping southeastward of tectonic blocks associated with the Indo-Asian collision in the Tibet Plateau and the NWW subduction of the Philippine Sea plate in the east. The collision related compression tectonics result in the formation of all kinds of faults in the eastern offshore areas, then trigger the earthquake.In addition, comparing the earthquake distribution pattern of China offshore seas with those geophysical maps of crustal thickness, gravity and aero- magnetism and heat flow, we can observe that the transition or gradient zones of crustal thickness are the earthquake active areas, meanwhile these zones are also the areas where the faults develop, along with high heat flow and gravity anomalies as well. It is well known that the geophysical gradient zones are scenarios of the change of composition, structure and physical properties in deep crust, where the strain and stress concentrate easily, facilitating the earthquake occur under tectonic stress.In conclusion, our study presented the tempo-spatial distribution of marine seismicity in China offshore seas and their seismogenic environment in a whole scale. It can be summarized as the shallow fault zone that is the display of transition zones of physical properties in deep crust, facilities the stress and strain concentrate and (re)activates to form earthquake, under suitable tectonic stress field. Nevertheless, the seismogenic process is one complex dynamic system, affected by the coupling of shallow structure and deep tectonics, and need further study in future.