Seismic Activity Of The Yitong-shulan Fault Zone (changchun Segment)and Its Impact On The Development Of Changchun City

The Yitong-Shulan fault zone is a branch of the well-known Tanlu seismic zone crossing south ofChangchun city. The fault zone is mainly composed of two faults in the east and west, it islarge-scale, shows obvious geomorphological and geophysical characteristics, and strictlycontrols the Quaternary deposition. Since the last century, there happened three timesearthquakes between5-6level around the fault zone, which is considered to be a modern activefault.For the Yitong-Shulan fault zone (Changchun segment), what level the earthquake intensitywill reach, where the potential danger zone is exactly located, and how much impact onChangchun City are the issues that has always been the greatly concerned by the community.Meanwhile, along with the urbanization process in Changchun city, the population is quicklyincreasing, city boundary continues to expand, and fixed investments is continuing to grow, thusmakes seismic safety issues increasingly prominent. Therefore, it is significance to study seismicactivity of the Yitong-Shulan fault zone (Changchun segment).Now earthquake science is a worldwide problem, it is difficult to study and explore seismicactivity pattern. Earthquake usually occurs at the depth of a few kilometers to tens of kilometersbelow the surface, by the current limited capacity, human cannot intuitively understand thegeological structure and dynamic environment at this depth, therefore, geophysical methods isthe only way to solve.The earthquake is a response of the geological structure to the tectonic stress field, soexploring the laws of seismic activity should be based on the tectonic stress field and geologicalstructure. The study of tectonic stress field studies is a branch of Geomechanics, which is beyondthe current research scope, so this dissertation will analyze tectonic stress field of the Northeastbased on previous research results. The core content of this research will aim at the south ofChangchun City as the study area, process and interpret the geophysical data processing in thisarea, and study the morphology, distribution and activity characteristics of the Yitong-Shulanfault zone combined with the geological data. The paper also gives a preliminary judgment aboutthe upper limit of strong earthquake magnitude and seismogenic zone produced by theYitong-Shulan fault in the tectonic stress field. Finally I discuss the impact of the strongearthquake on Changchun City and propose some ideas and suggestions of disaster defense.The Yitong-Shulan fault zone with NE trend runs through the northern-margin fold belt ofthe North China plate in south of the study area and Songnen micro-plate in north of the studyarea. Two units located middle of the study area connect at Xar Moron River-Changchun-Yanjiline in the late Paleozoic-early Mesozoic. Therefore, the basement of southern Yitong-Shulanfault zone is related with deep Archean metamorphic crystalline basement of the North Chinaplate, while the northern basement belongs to early Paleozoic metamorphic crystalline basementof the Songnen microplate. Changchun section of the fault zone may be superimposed of these two kinds of basal parts, which has the characteristics of the composite substrate. After thereunification of the mainland, the study area has experienced two-stages evolution about theancient Pacific Ocean and the Pacific tectonic, the geological structure pattern has changedgreatly, it forms the basin-Ridge pattern with NE and NNE trend composed of the Songliao basin,Daheishan Horst, Yitong graben and Dan Hada terrane.The study of geological and geophysical data show that the basement of Daheishan Horstand Yitong Graben was part of the body Dan Hada terrane, the northwestern boundary of theDaheishan Horst extends to inside of the Songliao Basin and coincide with Siping-Changchunfault zone. The uplift of Daheishan Horst makes the eastern edge of the Mesozoic sedimentarystrata of the Songliao Basin tilted, while NW-trend faults in the region shows the activitiescharacteristics of dextral translation. The evidences show that in the late Cretaceous-earlyPaleogene, the Songliao Basin occurred strong crustal stretching, a large area of the eastern partof the basin uplifts, Daheishan Horst uplifts to be a ridge. At the same time, the NW-trend faultsrun right translation, which leads to the separation of Daheishan Horst from Dan Hada terraneand sinking of Yitong graben basement, it becomes the thick deposition, this process is similar tothe way of transform faults activities.Geological evidences show that the the separation of Daheishan Horst, the Yitong grabenbasement, and the Dan Hada graben is along the NW direction. Three parts did not occur relativedisplacement at the extension direction of the Yitong-Shulan fault zone (NE), it shows that theYitong-Shulan fault zone does not have the nature of the apparent slip. This conclusion limits thejudgment of the upper limit about earthquake magnitude of the Yitong-Shulan fault zone(Changchun segment).I studied1:200000gravity and magnetic data in the study area and processed the data byusing the upward continuation, magnetic to the pole, the derivative of vertical direction, and thederivative of horizontal direction. At the same time, the Euler deconvolution, structural-enhancedfiltering, and comparison and analysis of gravity and magnetic are used. Based on theseprocessing, the orientation, distribution, activity and other characteristics of the fault structure inthe study area are interpreted and analyzed. The results show that the study area exist a variety ofdifferent fault formed in different periods, the NE-and EW-trend faults has large scale, whichcontrol the distribution of intrusive rocks; the NS and NW-trend faults have secondary orassociated characteristics, which have small scale, geophysical characteristics are not obvious.Yitong graben is Cenozoic structure, which deposit thick Cenozoic clastic materialbelonging to Paleogene strata, it can reach the maximum thickness of up to6000m. YitongGraben is the joint product of NW-trend, NE-trend faults and the basement activities.Sedimentary strata completely record the history of geological movement since Cenozoic in thestudy area. Artificial seismic profile data of Yitong graben in the study area show that grabensediments mainly have NE-and EW-trend faults. Two different trend fault distribution hasobvious spatial opposite characteristics, that is, NE-trend faults locate near the southeasternmargin of the fractured inside the graben, and they are located in the two tectonic units ofMoliqing rift and Gudian slope, while EW-trend faults are located at both sides of LuXiangfaults and inside the north Chaluhe fault depression. This feature reflects the difference of basal, fault activities, and local features of the tectonic stress field, it piecewise limits tectonic activitiesof the Yitong-Shulan fault.The seismic interpreted sections show that the basement pattern, fault distribution,depositional style and faults activities of Yitong graben display segment features. Moliqing faultdepression and Bo-Tai depression of Chaluhe fault depression has initial stability and postactivities, it shows the thick deposition, soothing occurrence. The basement and cover ofLuXiang fault depression have suffered strongly fold reverse, No.2fault has vigorous activityand control vertical drop near2000m at both sides of the sedimentary strata. Thesecharacteristics limit tectonic activity of the Yitong-Shulan fault zone.Geological activities in Northeast of China is affected and controlled by the positivesubduction of the Pacific Plate from west direction since the Cenozoic. Worldwide pattern ofplate tectonics changes a lot in the late Cretaceous-early Paleogene, Songliao Basin appearsstrong tectonic stretching, Daheishan Horst uplifted, Yitong graben appeared its shape. Secondadjustment of worldwide pattern of plate tectonics happened in the end of the Paleogene-earlyNeogene, Yitong graben was located in squeeze the environment and suffered tectonic inversion,strong deposition of graben ended its history.Neotectonic movement in Northeast of China since Neogene is characterized by verticalmovement of the fault block, it shows the formation of Songliao watershed and changes in theSongliao Basin river flows. In addition, volcanic of Moliqing fault depression and Shulan grabenis well developed, thus indicates the particularity of these two units of tectonic activities.Modern tectonic stress field in Northeast of China is mainly affected by the horizontalsqueeze by the Pacific plate subduction, but the direction of principal compressive stress in theNortheast of China deviate the direction of subduction extrusion of Pacific plate about30degree,which shows that the region has additional near NS-trend compression. According to the newclassification scheme for global tectonic units in recent years, the extrusion pressure may comefrom extrusion of the Eurasian plate acting on the northern margin of the "Amur" micro-plate.Under the combined effects of two stress field, tectonic stress field in the Northeast regionexist the possibility of a dramatic turning point, and may lead to significant changes of theintensity and frequency of seismic activity. In recent years, Shulan and Tonghe area in northernof the Yitong-Yilang fault zone shows geological evidence of a strong earthquake more thanseven level since the Holocene. The evidence shows people need to re-recognize seismicactivities of Yitong-Shulan fault zone and the entire northeast region. Accordingly, I think thatthe Yitong-Shulan fault zone exist geological conditions, which can occur strong earthquakemore than seven levels, but the modern tectonic stress field does not suit for the breeding ofearthquakes more than seven levels. Accumulating enough energy to produce a strongearthquake may need a very long cycle. The direction of tectonic stress field is good forstrike-slip translational motion in Yitong-Shulan fault zone, but strike-slip translation property ofthe Yitong-Shulan fault zone is very limited. According to these two constraints, I suggest theresults that shear earthquake from the Yitong-Shulan fracture zone will not have too much energy,the magnitude of earthquake has a maximum of six levels. From the constraints provided bygeophysical and geological data, one can determine that the earthquake risk zone of the Yitong-Shulan fault is located at Leshan to extravagance Ridge line in the Xizhi fault depression.Since last century, global earthquake disaster shows a trend of increasing, the reason lies inthe population explosion and the extreme imbalance of the population density, densely populatedcities are facing increasingly serious earthquake safety issues. China is deeply influenced byearthquake disaster in the world, the earthquake death accounts to one third of that of the world.The reason is not that our country always appears strong earthquakes, but that we have badearthquake-resistant structure, the most fundamental reason is the weak awareness of disasterprevention. With the acceleration process of urbanization, the seismic safety issues ofChangchun city increasingly become prominent Preliminary assessment shows that Changchunwill suffer some casualties and economic losses if the Yitong-Shulan fault zone occur earthquakeabout6.0.Disaster prevention strategies depend on awareness and ways of thinking. Earthquakedisaster prevention in Changchun City need to combine with integrated factors includinggeological conditions, the level of seismic activity, and economic conditions etc. The city need touse reasonable social and natural resources, depend on science and technology, rely on the ruleof law, rely on the strength of the whole society, and scientifically provide disaster preventionmeasures.