Block Motion And Deformation Of Boundary Faults Of Active Blocks In Eastern Bayan Har Block And Its Adjacent Regions

The Ms8.0WenChuan earthquake happened on the middle segment of the North-South Seismicbelt, the eastern margin of the Tibetan Plateau, and the Longmenshan fault zone in the southern part ofthe eastern boundary of the Bayan Har block. Many studies in this research region on the mechanics ofWenchuan earthquake showed that, before the earthquake, the eastward of the Bayan Har blockrelative to the Sichuan basin was higher than the slip rate on the Longmenshan fault obviously, thismaybe mean that there was velocity attenuation between the middle-eastern of Bayan Har block andthe Sichuan basin. As part of the South China block, the Sichuan basin is more stable than themiddle-eastern of Bayan Har block, so the velocity attenuation mainly occurred in the northwest of theLongmenshan fault zone-the middle-eastern segment of the Bayan Har block. The GPS data and theseismicity in the research area show that the velocity between the Bayan Har block and the Sichuanbasin was not continuous, but had a block motion characteristic. Then, how many subblocks are therein the eastern segment of the Bayan Har block, how they deformed, and where the dynamic sourcescame from? What is about the boundary’s motion, deformation style, and seismic risk? Quantitativestudy on the tectonic deformation filed in the eastern segment of the Bayan Har block would provideevidence help to solve the above questions, and it can deepen the understanding of the continentaldeformation and the uplift of the Tibet Plateau, learn more about the mechanics of the Wenchuanearthquake, and provide a reference for the hypothesis about the deformation characteristics andseveral dynamic models of the Tibetan Plateau. At the same time, it would benefit the evaluation ofseismic risks in the research area.Based on field investigation, processing and analysis of GPS data, motion characteristic ofmajor active faults, seismicity, and the geophysical background, and combined with the F-test, thiswork divided the eastern Bayan Har block into several subblocks. Then this thesis placed emphasis onthe motion characteristics and deformation styles of the subblocks, the deformation of the activesubblock boundaries, and the relationship among the block motion, boundary deformation andseismicity. The major research results of this thesis are presented below.(1) Division, deformation and tectonic implication of active blocks in the eastern segment of theBayan Har block1）Based on the concept of active blocks, this work made an analysis the major active faults andsecondary faults, historical earthquakes，field geological investigations, and other data. Then theeastern segment of Bayan Har block and its adjacent regions are divided into four first-order blocks:Bayan Har block(I)，South China block(II)，Sichuan-Yunan block(III)，and West Qinling block(IV).According to GPS velocities, data of geophysical fields and geomorphological features, the Bayan Harblock is further divided into three subblocks: Aba (I1), Maerkang (I2), and Longmenshan (I3)，and theWest Qinling block into two subblocks: Minxian (IV1) and Lixian (IV2), and then the relative independence among those blocks are checked by the F-test.2）The northwest of the Bayan Har block can be divided into three subblocks: the Aba subblock,the Maerkang subblock, and the Longmenshan subblock. The westward motion of these subblocksdecreases from west to east. The eastward motion of the Bayan Har block is decomposed by theLongriba fault and Minjiang fault, and the decomposition of the Longriba fault is most obvious withright slip~4.8±1.6mm/a, and the decomposition of the Minjiang fault is relatively weak, only about1~2mm/a. These probably indicate that the eastward motion of the Bayan Har block is discontinuous.3）The modern translation and rotation rates of those blocks were calculated using the GPS Datafrom the Crustal Motion Observation Network of China between1999and2007. The results show thataccompanying with the eastward or southeastward motion of the subblocks, they rotate around theEuler pole. This work computed the principal strain rates of each subblock in the esatern Bayan Harblock by using the Delaunay triangular method and the least square collocation method. The resultsshow that the rigidity of the subblocks located in the northwest of the Longmenshan fault are muchsmaller than that of the Longmenshan fault, from east to west the rigidity of subblocks becomesincreasingly stronger.4）The interaction between the India plate and Eurasia plate is the long-term acting force for theeastward or southeastward motion of the subblocks located in the eastern segment of the Bayan Harblock and adjacent regions. Except for the long-term acting force, each subblock also be affected bythe subblocks surrounded it and which are responsible for the rotations of these subblocks proper.(2) Recent deformation of boundary faults in the subblocks in the eastern segment of Bayan Harblock1) Using the least residuals of GPS profiles across the boundary faults, this wirk obtained theoptimal locking depth of each boundary fault. The locking depth on the East Kunlun fault, theLongmenshan fault, the northwest part of the Xianshuihe fault, and Longriba fault are19.1±8.1km,13.6±3.9km,18.3±9.7km and15.8±5.0km, respectively, which are consistent with the lockingbackground on a large scale in the research area. The locking depth of the Aninghe fault is33.2±15.8km, deeper than the relocation results of small earthquakes along the fault zone, and it mayaffected by the Daliangshan fault zone and Mabian fault zone which are also included into the GPSprofile used to estimate the strike-slip on the Anninghe fault zone. According to the focal depths ofsmall earthquakes, the optimal locking depth of the Anninghe should be between the15~20km. Thelocking depth of the Zemuhe fault and Minjiang fault are8.3±3.6km and8.7±5.6km, respectively,relatively shallower than that of other faults. By and large, the optimal locking depths of the boundaryfault are mainly between8~20km, and the slip rates of these boundary faults are generally consistentwith previous studies, except the Aninghe fault which may be affected by the Daliangshan fault andMabian fault. 2) This work analyzed the activity of the block boundary faults on the eastern Bayan Har blockby using the Eular parameters, the GPS profiles, the arctangent fitting, and the baseline crossing thesefaults. The results show that the slip rates of the Ganzi-yushu fault, Xianshuihe-Xiaojiang fault, andDongkunlun fault are larger than that of the Longmenshan fault obviously, which is the blockboundary between the Bayan Har block and the South China block. And the whole eastern segment ofBayan Har block is in a compression environment. The GPS measurements after the Wenchuanearthquake (2009-2011) indicated that the kinetic property is the same as that before the Wenchuanearthquake. The northwest/northwest-west trending East Kunlun fault and Bailong jiang fault are ofleft-lateral strike-slip, and show compression characteristics. The normal components of the Minjiangfault, Huya fault, and the Mayanhe fault manifested that all these faults have extrusion characteristics,and a little slip rate. In a word, the results from the baseline across boundary faults show that themotion rates normal to the boundary fault are lager generally than the result from other methods. Andthis work found that there is at least one new GPS site in the baseline points at which the vertical rateis too large. It is difficult to judge the affection of the stability of the new GPS sites. It should be alsoconsidered that whether they were affected by the postseismic deformation of the2008Wenchuanearthquake.3）Comparing the results from the Euler vectors, GPS velocity profile, and the GPS baselineacross major faults, these work suggests that the motion characteristics of the boundary faults areconsistent with each other, but the vertical component of GPS baseline is larger than that divided fromthe Euler vector or GPS velocity profile. That is likely caused by the instability of the new GPScampaign site, for it is hard to make sure the affection of the stability of those new sites. Besides it isalso probably affected by the Wenchuan earthquake.（3） The relationship among Block motion，deformation of the boundary fault and the occurrenceof the Wenchuan earthquake1）In the eastern segment of the Bayan Har block, the rigidity of the Longmenshan subblock islower that of the other two subblocks, the Maerkang subblock and the Aba subblock, then the stressand energy accumulation in the Longmenshan subblock may be bigger than the other two subblocks,which is verified by the fact that large earthquakes happened in the Longmenshan subblock. With theeastward motion of the lower crustal flow, the upper crustal rocks may also move eastward, and thismay lead to the stress and energy accumulation mostly in the middle part of the Longmenshan fault.2）The Longmenshan fault is divided into three segments by the Maerkang subblock, theLongmenshan subblock and the Minxian subblock. The strain rate on the middle part of theLongmenshan fault is smaller than that on the southern part, and the deformation northwest of thefault shows that near the fault the strain rate becomes smaller, which may implied that it hasaccumulated high strain energy, and which is easy to generate surface’s ruptures. The change fromthrust motion to thrust with a right strike-slip component revealed by the surface ruptures and focal mechanisms of aftershocks of the Wenchuan earthquake maybe related to the change of the stressorientation on the middle part of the Longmenshan fault from southwest to northeast. All the lowervelocities appear between the Ma’erkang subblock, Longmenshan subblock, Minxian subblock andSouth China block, respectively, the lower strain rate and the highly locked character of theLongmenshan fault may have determined the lower slip rate of the Longmenshan fault.3） According to the division results of those subblocks, the motion and deformationcharacteristics, and the principal strain rate filed, this thesis suggests that the stress and energyaccumulation on the middle part of the Longmenshan fault zone is large than the south, and northsegments of the fault. In the strain rate filed, the direction of the compression strain rate is vertical tothe northeast trending Longmenshan fault, and this may might lead to the dominat thrust motion andlargest strain accumulation on this segment of the Longmenshan fault. And when it reached a criticalvalue, the great earthquake happened.