Study On Mechanisms And Characteristics Of Major Faults In The Eastern Boundary Of The Sichuan-Yunnan Block

The deformation of the Sichuan-Yunnan tectonic block which is located on the southeastborderland of the Tibetan plateau is very intense, where earthquakes occur frequently. Lateralextrusion of the Sichuan-Yunnan tectonic block has caused left-lateral shear deformation of theeastern margin. Seismologists using different methods obtained the important quantitative results.With the rapid development of geodetic survey technology, especially GPS, they not only providehigh precision data for the research on the deformation field of fault zone, but also constraint ofdynamic observation data to study the dynamic changes characteristics and the dynamicsmechanisms of fault zones. Therefore, to obtain the quantitative results of movementcharacteristics and strain accumulation characteristics of theXianshuihe-Anninghe-Zemuhe-Xiaojiang fault zone, it is necessary to combine the blockmovement model with the tectonic deformation quantitative analysis on the fault zone itself andnumerical simulation.There are two major analysis methods of the fault movement characteristics. One is thegeologic method and the other is the geodetic method. Many results about the fault zone havebeen achieved by these two methods. At the same time, the results different researchers obtainedare consistent in nature of deformation, but exhibit significant differences in quantitativecharacteristics. How should we explain these differences? Deformation of the fault zone is closelyrelated to the relative motion of the tectonic block, and there are differences in movementcharacteristics of different segments. Whether can we combine the deformation of the fault withthe tectonic block movement model to obtain more precise results? Deformation of the fault on thesurface has deep dynamic cause. Whether can we combine the finite element model withcontinuous deformation models (the least square collocation method) to obtain more rationaldynamic mechanism of fault deformation? In view of the questions above, the block movementmodel, the quantitative analysis method on the tectonic deformation of fault, the numericalsimulation method and continuous deformation model (least squares collocation) are appliedsynthetically in this work to the study of the fault movement characteristics of the easternboundary of the Sichuan-Yunnan tectonic block. (1) The research on movement characteristics of the major fault of the eastern boundary ofthe Sichuan-Yunnan tectonic block.This thesis introduces briefly the Xianshuihe-Anninghe-Zemuhe-Xiaojiang fault system,which serves as the major fault of the eastern boundary of the Sichuan-Yunnan tectonic block. Atthe same time, using GPS velocity field data from the period of1999-2007, the velocityresiduals of the Sichuan-Yunnan block, Bayanhar block and Southern China block are estimatedby the residuals and unbiasedness of the three models (the rigid rotation model (RRM), rotation inthe entire block with the homogenous strain model（REHSM）and rotation in the entire blocklinear strain model（RELSM）). Meanwhile, residual of the Sichuan-Yunnan block and refinedSichuan-Yunnan block are compared. This work also introduces the residual error analysis. Theresults show that the residuals of the RELSM, compared with RRM and REHSM models, are lessthan the other two models. It can be found that mean residual errors of RELSM is the minimumfrom the results of the mean residual errors calculation. The ratio of the GPS site which lies on therefined Sichuan-Yunnan tectonic block less than the sichuan-yunnan tectonic block are about58%.Compared with the residual of the Sichuan-Yunnan tectonic block, there is no obviousimprovement for the southern and northern Sichuan-Yunnan block. The main reason is thatRELSM involves the internal nonlinear changing process of the block. For REHSM, there isobvious improvement for the refined Sichuan-Yunnan tectonic block than the Sichuan-Yunnantectonic block. That is to say, it is more reasonable for the further division of the Sichuan-Yunnantectonic block by using REHSM to describe block movement characteristics and deformationcharacteristics. In conclusion, RELSM for description of the block is better than the other twomodels and whole movement of the block can be fitted better.Further, the fault is divided subtly into some segments based on the geologic method, and theXianshuihe-Anninghe-Zemuhe-Xiaojiang fault system is refined: Xianshuihe fault: northwesternsegment, the middle segment, the southeastern segment; Anninghe-Zemuhe fault is not refined.Xiaojiang fault: the northern segment, the middle segment, the southern segment. The left-sliprates of the fault zone are calculated using the three models, respectively. Slip rate of fault zonesobtained by the rigid rotation model: Xianshuihe fault: northwestern segment:11.5mm/a；themiddle segment:11.9mm/a; southeastern segment:12.1mm/a; Anninghe fault:9.9mm/a; Zemuhefault:8.9mm/a；Xiaojiang fault: the northern segment:10.4mm/a; the middle segment:11.4mm/a; the southern segment:11.3mm/a. The results show that slip rates of the Xianshuihe fault andXiaojiang fault obtained by the rigid rotation model seem to be the same as the geologic method，but the magnitude of the slip on the Anninghe and Zemuhe faults are larger than that by thegeologic method. This is due to the impact of the Greater Liangshan fault. It will be the same asgeology methods if the impact is removed. The results based on the linear strain model show thatthe slip rate of segments is significantly less than that obtained by the geologic method. This isbecause the results of the linear strain model involve the non-uniform change in the block. But itcan distinguish the differences between segments. The slip rate of the northeastern segment of theXianshuihe fault is greater than the middle and southeastern segment. This is because faultstructure of the northeastern segment is relatively simple, and that of middle and southernsegments is complicated. The slip rate of the middle and southeastern segment of Xiaojiang faultis greater than the southern segments, which is due to the absorption of the Qujiang-Shiping faultto the part of the block movement.(2) The research on strain accumulation characteristics of the major fault of the easternboundary of the Sichuan-Yunnan tectonic block.Firstly, I use the arctangent function to fit the GPS velocity field parallel to the fault zone onthe basis of the results of segmentation above, and to identify the possible fault zone deformationwidth. It can be found from the results that the interseismic deformation characteristics of thenortheastern and middle segments of the Xianshuihe are significantly different from other faultzones. The possible reasons are as followed: Firstly, the fault system near the Xxianshuihe fault(northeastern side) leads to asymmetry deformation on both sides of the fault. Secondly, thedeformation of the fault may be is complicated, which is to be studied further. The deformationwidth of the Anninghe fault zone is the maximum, which means that its locking degree is strongand locking depth the deepest, so earthquake risk is relatively higher. In this work, the resultsobtained are consistent with the other methods with results obtained by identifying faultearthquake gap segments and risk. Although the Xiaojiang fault zone is an active fault whichseismologists pay much attention, the risk of the Xiaojiang fault zone is weaker than the Anninghefault zone.Next, the Xianshuihe-Anninghe-Zemuhe-Xiaojiang fault strain accumulation is analyzedsynthetically according to quantitative analysis of tectonic deformation of the fault zone itself. The calculation results show following features:①E arthquakes usually occur in the faultzone in which the activity of the fault is strong, and the local segment which is locked is in arelatively high state of strain accumulation. The MFwhich reflects the strength of the deformationcan be worked out according to the deformation parameter formula. The MFvalues of segmentsare relatively all large, reflecting the intense tectonic deformation characteristics of the fault zone.②The TArepresents the proportion which is the total deformation of the shear strain rate and normstrain in vertical direction of the fault relative to torsion rate of the fault zone. The value of theAnninghe fault is close to0.9according to the calculation result, which shows fault plane tends tobe locking, and deformation is caused mainly by strain.③The abnormal strength of theXianshuihe-Anninghe-Zemuhe-Xiaojiang fault system donminated by strike-slip is dependsmainly on the abnormal intensity ratio of the shear deformation rate SA. According to thecalculation results, the Anninghe fault zone is the high strain rate accumulation segment in whichthe fault activity is relatively locked. It is consistent with the above results obtained bydeformation width and other methods.④The high strain accumulation segments are all in theAnninhe fault zone according to SAand TAobtained, namely, the shear deformation of the faultzone plays a leading role in the whole tectonic deformation.(3) The research on the dynamic motion characteristics of each segment–taking theXiaojiang fault for an example.First of all, the fault movement characteristics in space and time are analyzed based on theblock movement and linear strain model and combined with the GPS profile analysis methodusing the1999-2001,2001-2004,2004-2007three short-period GPS velocity field data, and thestate of strain accumulation of the fault zone is also analyzed.①Temporal and spatialcharacteristics show that the left-lateral strike-slip trend of segments of the Xiaojiang fault doesnot change. There is a slight extrusion trend on some segments, but the magnitude is very small.Therefore, the general movement feature of the Xiaojiang fault is a left-lateral strike-slipmovement. Its slip rate decreases gradually from the northern segment to the southern segment;the tensile rate of the northern and southern segment is relatively larger according to thefault-norm rate, mid-northern and mid-southern segments are slightly of extrusion characteristics.Compared with the GPS profile analysis and inversion method, the result obtained is relativelysmall, and also smaller than geologic results. This is because the linear strain block model involves the effect of non-uniform change process within the block, that is to say, the fault withinthe block also absorbs part of whole block movement, which makes the calculated results getsmaller. But the spatial variation characteristic of the results is consistent with other methods.②The strain accumulation characteristics of segments of the Xiaojiang fault indicate that thestrain in the northern segment is higher than the mid-northern segment, and the mid-southernsegments are not conducive to accumulate strain or their levels of strain accumulation are low, andaccumulation of the abnormal area is of high speed. The northern segment of the Xiaojiang fault isan abnormal zone which is relatively locked with relatively high shear strain.(4) The research on the motion and the dynamics mechanism of deformation in the easternboundary of the Sichuan-Yunnan tectonic block.The three-dimensional finite element model is established based on the FEPG finite elementautomatic generating system. The study area is divided in lateral direction, including theSichuan-Yunnan tectonic block, BayanKala block, Southern China block and fault zone. Invertical direction: the crust is divided into the upper crust and lower crust according to the depthof major earthquakes in this region. The thickness of the upper crust is15km, and the thickness ofthe lower crust varies depending on the block, because this work is only concerned withcharacteristics of crustal deformation, so vertical depth extends only to the lower crust. Theparameters of each block are obtained mainly based on the regional geological and geophysicaldata; With the Maxwell viscoelastic constitutive relation, boundary conditions are applied usingthe continuous deformation model (the least square collocation method). In the process ofapplying boundary, three blocks are used respectively. The block boundary displacementconstraints are determined according to the least square collocation method. The loading time is1000years, and time step is10years. The movement characteristics and strain accumulation ofthe fault can be studied by the method of numerical simulation.①The slip rate of each segment:the Xianshuihe fault is8.5mm/a; the Anninghe fault is5.3mm/a; the Zemuhe fault is5.0mm/a; theXiaojiang fault is8.9mm/a; the value is consistent with value obtained by geology method, so themodel is relatively reasonable.②The Anninghe fault is the region in which strain is relativelyhigh according to the strain results. This is consistent with the result of the surface analysis, andalso the result obtained by the other geologic methods.③It can be found that the lithospheric flowintensity of the study region, the terrain features of the region itself, and the geometry characteristics of the fault zone itself led to the long-term deformation characteristics.