Detection Of Buried Active Faults And Research Of Earthquake Risk In Daqing Area

Daqing is an important industrial city in Northeast China.It is a major city of Heilongjiang's Land and Sea Silk Road Economic Belt.With the high concentration of urban population and wealth in modern society,the seismic damage and hazard of direct earthquakes which are caused by active faults in urban areas are becoming more and more serious.Once a large earthquake occurs,it may cause extremely huge disaster consequences.Daqing is located at the intersection of Binzhou fault and Keshan-Da'an fault.A magnitude M5.1 earthquake occurred in Lindian County of Daqing,on July 25,2005,indicating that Daqing has the tectonic background of moderately strong earthquakes.In addition,Holocene faults have been found in the Yilan-Yitong graben,which breaks through the previous academia's understanding of seismic background and seismicity level in Northeast China.Therefore,it is necessary to detect and identify active faults in Northeast China,including Daqing,to accurately determine the seismic risk of Daqing and its adjacent areas,and to effectively reduce the loss of earthquake disasters that may be suffered.At present,the research results of geological structure in Daqing are confined to deep structures.As for the state of near-surface overlap of faults,which faults are active faults that may trigger moderate-strong earthquakes,and which areas will be seriously damaged after earthquakes,a series of these unsolved problems need to be further explored and studied.In view of these scientific problems,this dissertation takes the overlying faults in Daqing area as the research object.By means of collecting and interpreting petroleum geophysical data,shallow reflection seismic exploration and seismic risk assessment of target faults,a series of scientific problems,such as the distribution pattern of faults,the latest active age,the maximum magnitude and recurrence period of possible earthquakes in the future,have been identified.Through the research work of this paper,the following results have been achieved:?1?Thirty wells with large seismic profile have been interpreted and joined together.There are 20 wells in east-west direction and 10 wells in North-South direction.All of them are evenly distributed in the work area and cover the whole work area.?2?The distribution of inversion structural belts is controlled by half grabens and deep and large basement faults.Under the left-lateral compression-torsion stress field,NE-distributed and left-order anticlines are formed in advance,and then along the direction of half grabens and basement faults,long plains are formed.The NW trending graben is nearly parallel to the compressive stress,so there is no inversion.Under the action of left-lateral compressive torsional stress,T₀₂ dense zone rotates to NW direction and forms conjugate X-type faults?some new faults are NW direction?.According to the strength of the late inversion deformation,the inversion deformation can be divided into two ranges:the inversion deformation zone and the strong deformation zone.Similarly,according to the shallowest buried part of T₀₆ reflector,the strong inversion deformation zone is divided into two zones:the east one and the west one.The deformation intensity of the strong deformation zone in the west is stronger than that in the east.The inversion intensity of EW2-EW6 and EW10-EW11survey lines in the west is the highest,the fault activity is the strongest,and the possibility of detecting active fault is the greatest.?3?The Keshan-Da'an fault,determined by the shallow reflection seismic exploration section,is a thrust fault with strike northeast and incline to South east.We found out the detailed stratigraphic information and the distribution of the faults above the Nenjiang Formation using reflection seismic exploration.In the anticlinal structure overlying the main faults,we found many secondary faults,which did not put forward in the previous studies,and the activity characteristics of these secondary faults are also affected by the Keshan-Da'an fault.?4?According to the interpretation results of shallow reflection seismic profiles,combined with drilling and dating methods,it is concluded that the latest active age of fault in the target area is from late Early Pleistocene to early Middle Pleistocene; According to the detailed exploration data and comprehensive evaluation of various methods in this paper,the maximum potential earthquake of the target fault is M_S6.0.?5?Conservative calculation results show that the annual occurrence rate of MS6.0 is?_Ms6.0=0.0004,i.e.the recurrence interval is 2500 years.The methods and achievements of this paper can provide scientific basis for the urban development planning of Daqing City,the site selection of major engineering construction and the safe and efficient production of Daqing Oilfield.It can provide reference for other areas to carry out near-surface fault exploration,provide basic data for the follow-up study of shallow geological structure in this area,and fill in the blank of near-surface geophysical exploration structure research in this area.