Characteristics Of Active Structures In The Bohai Sea And Its Relationship With Seismic Activities

Earthquakes are closely related to active structures. More and more seismologists and geologists are attracted by submarine active structures because of large damages of earthquakes and tsunami caused by them. The Bohai Sea and its adjacent area is a strong earthquake region, where 6 earthquakes more than Ms6.0 have been recorded in recent 500 years. Additionally, this area is also the doorway of capital area and the oil and gas basement and many people live here. So it is necessary and significant to understand submarine active structures in the Bohai Sea.Based on new more than 3000-km-long single seismic data and combined with collected borehole data, earthquake catalogue and geological research results published, geometrics, kinematics and dynamics of the late Pleistocene-Holocene submarine active fault are analyzed in this thesis. Activities of these structures are documented and its relationship with earthquake in the Bohai Sea is discussed. The study shows that:(1)In the Bohai Sea, the late Pleistocene-Holocene active fault in shallow strata mainly developed in uplifts and nearby boundary faults of uplifts or depressions. There are two fault compact districts, one located in the northern part of Miaoxi depression and another in the east marginal area of Shaleitian uplift. The long axis of active folds is orientated N-S. Compared to Tertiary faults system, distribution and structural framework during the late Pleistocene-Holocene is similar to it, but the density of fault is much lager than of it. Most of these faults in shallow strata are normal faults with upstanding fault planes and tectonic movements of them mainly occurred first in the stage of 65-53 ka B.P., secondly in the stage of 20-10 ka B.P.. Vertical slip rates in these two stages were twice than in other stages.(2)Tan-Lu fault with NNE-trending manifests as several discontinuous faults in the plane view and it can be divided into two segments by the Bozhong-2 fault. The southern segment, of which the last movement occurred in Holocene, includes the eastern part and the western part. The eastern part is characterized with 4-8 km in width of fracture zone, strong deformation of sediments and negative flower structures along the fault. The western part is composed of four faults and their fracture zone ranges from 1.5 to 3 km. Some oblique ranged faults compose the north segment of Tan-Lu fault. The last movement of it occurred in the late stage of late Pleistocene. Sectional architecture and faults assembly in plane view indicate that Tan-Lu fault has a right-lateral strike slip and normal slip during late Pleistocene-Holocene. The average vertical slip rate is more than 0.06mm/a and reaches the max. 0.146mm/a. Since 20 ka B.P. the value is more than 0.08m/a.(3)Zhangjiakou-Penglai fault zone with NW trending is consisted of a group of NW trending faults in the Bohai Sea. Plane surfaces of these faults dip to S or N. Negative flower structure can be found along these faults. Last movements of these faults occurred in the late stage of late Pleistocene and Holocene. Since 128 ka B.P., they mainly exhibit normal slip with vertical rate ranging from 0.032mm/a to 0.124mm/a. The large vertical slip rate occurred first around 60 ka B.P. (the value is 0.11mm/a), secondly around 20 ka B.P. (the value is 0.124mm/a). Since 20 ka B.P. the value is more than 0.06m/a.(4)Many normal faults with NEE-EW trending are revealed by seismic profiles and most of them are characterized with upstanding plane surface and normal slip. The average vertical slip rate is more than 0.05mm/a during late Pleistocene-Holocene.(5)Buried depth of several principal discontinuities and thickness of sediments show that the depositional center is consistent with subsidence center and both of them are transferred from the west to the east and from the south to the north during late Pleistocene- Holocene.(6)The geodynamic mechanism of Cenozoic tectonic movements in the Bohai Sea is regional extension and superposed interaction of several blocks in north China. The regional tectonic stress field and reactivation of early faults are main controlling factors for late Quaternary active structural deformation.(7)In the Bohai Sea, most earthquakes often take place in the middle-upper crust(less than 20 km), and controlled by Tan-Lu fault zone and Zhangjiakou-Penglai fault zone. A newfound depression beneath the seabed at 3 m depth, with NE33°trending and 26 km in length and 3 km in width, gives the new evidence to demonstrate that Tan-Lu fault is the causative fault of the 1969 M7.4 earthquake.