| The Southeast area of Gansu is in east margin of the Qinghai-Tibet block, itlocated in the intersection part of Tibetan Plateau, Loess Plateau and QinlingMountain. The structural relationship of the region is extremely complex. There are aseries of faults and folds with different strikes and distinct outline. The region belongsto the north and middle segment of the North-South Seismic Belt. Many mediumearthquakes occurred in history. The Southern Tianshui and Wudu M8.0greatearthquakes are the greatest event in history in this area. Their elapsed times are358years and133years. The surface deformation has been blurred except that landslidecan be seen because of later various human and natural factors. Meanwhile, it isimpossible for going on seismo-geological surveys now that the traffic is notconvenience because topography and geomorphology in the area is large fluctuations,and the tectonic is complex. Superimposed these factors make it is difficult to study indetail. So far, only a few journals contain some scattered research about the twoearthquakes, and the content is a few isolated words and phrases. It’s not satisfiedwith seismic resistance and risk assessment. We have chance to re-recognize the twoearthquakes supported by urban active fault surveying project in southern Gansu. Weget systemic research result based on various means. This provides a base for activefault investigation and earthquake zoning which are carried out, and recognizingoccurrence regularity of future strong earthquake.Seismology Bureau of Gansu Province layout seismological observationalnetwork that the about interval between two stations is70km.It have recorded massivemoderate and small earthquake. We think the two big earthquakes have relationshipwith them. Their formed by "long-term activity" in a large earthquake area possibly. Itdo not satisfy with meticulous study because location results is by seismologicalobservational network of Gansu province is only a preliminary results. So were-locate them using Hypo2000location programme. This provides guarantee forinverting refined characters of seismotectonic. We chooseed several strip-shapedzones according to clustered small earthquakes and then obtained fault plane parameters of the earthquakes such as strike, dip, location using data of denselypopulated small events Based on the simulated annealing and Gauss—Newtoniannonlinear inversion algorithms. We define the causative structure of the twoearthquake combined with the existing results of geology investigation, topographyand geomorphology character and the landslide distribution finally. On this condition,we discuss the causal mechanism based on considering relationship between deep andshallow structures. The main conclusions are as follows:1. We negate that NNE-trending Lixian-Luojiabo faults which is triggeringseismic fault of the1879Southern Tianshui M8.0earthquake by previous. We thinkthat NWW—trending Longxi-Yanguan is triggering seismic fault through acomprehensive analysis of topography and geomorphology character, landslidedistribution, fault plane solution, satellite image interpretation and so on. We find thatshallow tectonic is different from deep tectonic by contrasting them. This meansshallow slide from east to west under N70°E regional tectonic stress field.2. The seismic-source fault of the1879Southern Wudu M8.0Earthquake is a typeB. The geometric sizes of the focal fault is inferred to be a right-lateral slip faultunder NNW-SEE compressive stress, which is striking to NNE with high dip,30kmin length, and2-23km in depth. However, existing seismogeological observationsthree nearly parallel NNE faults compose surficial fracture. We think the earthquakeoccurred by shallow listric faults strike-slip or dip-slip under traction deep seatedhigh-angle focal fault.3.We realize clearly their structures and mechanisms by comprehensivemethods.But we still needs to study them,such as recurrence period, magnitude, focaldepth, coseismic dislocation and so on. |
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