| Longmenshan fault zone’s active faults are embodied in the Yingxiu-Beichuan faultwhich is central fault, the fracture zone of the fracture surface caused by the earthquake is aintraplate reverse fault type earthquake which has the most complicated structure and thelongest visibility displacement; Regional GPS displacement research shows that the crusthorizontal movements near the the seismic velocity anomalous regions in Wenchuanearthquake were mostly east-west movements, and that before the earthquake, theLongmenshan fault zone’s near field vertical deformation rate was characterized by low ratepressure, revealing that the fault zone were in a highly at the height of the "lock" state; Themeasured crustal stress data shows that the intensity and direction of the crustal stress changedbefore and after the Wenchuan earthquake, and the crustal stress of the fault zone’s hangingwall and footwall can have mutations and different release characteristics. Based on theunderstanding above, studying the distribution and mutation of the crustal stress of themeizoseismal area before and after the earthquake is of important academic value, at the sametime can provide reference on a macro level for engineering construction and planning in theearthquake zone.This article analyze from site investigation, regional geological prospecting, regionaltectonic environment data collecting and geological process mechanism, combined with theearthquake displacement change situation, the researched earthquake region’s GPSdisplacement characteristics and the characteristics of the measured geostress data anddistribution on the main fault zone in the Longmenshan region. The concrete method ofNumerical simulation is to simulate the fault process during the earthquake by adding somesoftening entity unit which has certain thickness,width and incidence in the intraplate faultingbased on the practical geological structure of the Longmenshan area;to establish the initial andboundary conditions of the experimental study on the basis of GPS displacement characteristicsof seismic region in Longmenshan area; Based on the measured data, to verify the rationality ofthe geological model with mathematics method, establish the tectonic pattern which conformsto the actual regional geological,and do an inversion research on the crustal stress fielddistribution characteristics and mutation regularity of the Longmenshan area before and afterthe earthquake. Then make a further study of the engineering effect of in-situ stress mutation,bring in a dynamic numerical simulation on the interface unit thrusting and strike-slip processwhen the the hanging wall and footwall of the fracture are in seismogenic process, study thein-situ stress mutation and release characteristics of crustal stress field and provide crustal stress materials for crustal stability assessment. The main research results are about the following sixaspects:(1) The Longmenshan study area velocity vector direction changed before and after theearthquake, the velocity vector of the whole study area before, on the whole, are north east tosoutheast; the velocity vector of the whole study area after the earthquake overall shift to thetwo plates of Yangtze and BaYanKaLa respectively move relative to one another to northwestand southeast. Compared the displacement vector of the study area before and after theearthquake,it appeared a right shift moving from south to north along the Longmenshan faultzone. The change of the maximum principal stress direction on the Longmenshan fault zonebefore and after the earthquake is piecewise, the angle of the maximum principal stressdirection in the southern section changed about15degrees after the earthquake,;in the middlesection,about50degrees and in the north section are about20degrees. The Longmenshan areais "Y" glyph structure pattern, affected by which the displacement vector direction will change.(2) According to the segmentation study of the main stress value of the Longmenshanfault zone, before and after the earthquake, the stress intensity of the south, middle and northsection has changed obviously. Before the earthquake the stress value of the south section ofLongmenshan fault zone is higher than the middle section,and that of the middle section isrelatively higher than the north section; After the earthquake, in the south and north section thistwo values are higher than the middle section, and the stress value of the south section ishigher than that of the north section.(3) Local shear stress concentration area appears before the earthquake, which is mostobvious in western area near Wenchuan, the shear strain increment of the study area isconcentrated in Sichuan-Yunnan block and the middle and southwest section of Longmenshanfault zone; After the earthquake the shear stress distribution is more homogeneous, its shearstrain is balanced, the shear strain increment of the study area extends to Sichuan-Yunnan block,Longmenshan fault zone, and most of the Yangtze plate area. With the release of seismicenergy, the earthquake stress difference in Longmenshan area is reduced relative to that beforethe earthquake, the principal stress value distribution of this area is evener, and tends to bebalanced, which is helpful to maintain regional stability. After the earthquake,the rigid Yangtzeplate which had been extruded all the way appears stress springback, locally characterized byincreased stress and stress concentration.(4) According to the change law of the stress value in the study area before and after theearthquake, we can see,before the earthquake, the stress value of the faults’ hanging wall andfootwall has continuous uniform change; And after the earthquake the hanging wall andfootwallof the study area appear discontinuous change along the central fault zone dividing line, alsoknown as stress jump, the stress mutation has something to do with the apparent fault hangingwall effect that the Wenchuan earthquake rupture fault appears. Comparing the stress statebefore and after the earthquake, we can see the maximum principal stress drop largely in thefootwall of the Longmenshan central fault zone, and in the hanging wall of the fault zone, themaximum main stress drop is small, local stress is even larger. Obviously, after the Wenchuanearthquake energy release is not complete, along the central fault hanging wall, a lot of largemagnitude happen, and the duration is long, showing that after the thrust type earthquakeenergy release slowly. Looked from the present stress level, the central fault footwall is under moderate stress level, the hanging wall of the fault zone is still in a high stress level.(5) According to the analysis of the influence that regional fault has on the in-situ stressarea around, we found that,before the earthquake, the stress value near the active fault increasesslightly as the distance from fault increases, the hanging wall and the footwall performs stably,indicating that the block of the study area keeps good integrity before the earthquake, in-situstress within a certain range has continuity; After the earthquake,the crustal stress value nearthe fault changes apparently, the stress value of the hanging wall decreases,but the stress valueof the region which is close to the fault zone in a certain range has increased, this is consistentwith the special high crustal stress proposed by predecessors.(6) Through the segmentation research on the hanging wall and footwall of Longmenshanfault zone found that each section stress is continuous and smooth before earthquake; After theearthquake,the stress value in the north section of the hanging wall of the Longmenshan faultzone increases slightly,the stress value of the middle section remains unchanged, and that inthe southern decreases,the stress value in most area of the footwall decreases; in the area whichis close to the fault zone, the stress value partially increases;in further area, the stress value isdeclining; the stress value increases slowly in the area which is more further,and is close to theaverage stress value in large area. It also indicates that the influence of faults have on crustalstress has a certain scope, outside the fault impact range, the stress level tends to be stable. |
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