| The12May2008Wenchuan Ms8.0earthquake, occurring on the Longmen Shanfault zone, has received much attention of international geosciences and motivatednumerous studies in China and elsewhere in the world. Some models have beenproposed to explain the mechanical genesis of this great shock, such as rigid blockextrusion, Lower crust channel flow and combination of multiple units. Meanwhile,many scholars have studied the tectonic stress field of the Longmen Shan fault zonethat would help address the genesis issue of the Wenchuan event. It is widely acceptedthat an EW-directed compression characterizes the fault zone, resulting in a complexstress environment. However, these existing research results are commonly based on asingle kind of stress data, or confined to some segments of the Longmen Shan faultzone and stress state at a certain depth. There is no comprehensive research on theheterogeneity of the tectonic stress field on Longmen Shan fault zone from south tonorth and from east to west and from shallow to deep. Besides, what is the variationrule of fault strength and stress size on the Longmen Shan fault zone? What is thecurrent seismic risk on different segments of this fault zone? And are there some newideas on the mechanism of the Wenchuan earthquake from the perspective of theregional tectonic stress field? It seems that previous work lacks intensive study onthese issues. Therefore, this thesis attempts to explore the relationship between theheterogeneity of the recent tectonic stress field on Longmen Shan fault zone and themechanism of the Wenchuan earthquake, which can help understand the tectonicdeformation and the dynamic process of the eastern margin of Qinghai-Tibet Plateau.In addition, this work also studies the fault strength of the Longmen Shan fault zonewhich has important reference significance and practical values to seismic hazard assessment in this region.This thesis is based on integrated use of various kinds of stress data, such asin-situ measurements, petroleum exploration stress data, coseismic displacement, faultslip and focal mechanism solutions. Firstly, it studies the variation characteristics oftectonic stress on the Longmen Shan fault zone and its adjacent areas. Then itanalyzes the strength and seismic hazard of the fault. Finally, this thesis discusses therelationship between the variation characteristics of tectonic stress on the LongmenShan fault zone and the mechanism of the Wenchuan earthquake. Last, and tries to putforward a tectonic model and the possible mechanic model for the Wenchuanearthquake. The contents of this thesis are summarized below.(1) Using focal mechanism solutions of moderate and large quakes in theeastern margin of the Tibetan Plateau, inversion is made to reveal the variationcharacteristics of tectonic stress on the Longmen Shan fault and its adjacent areas Theresults indicate that the maximum principal stress in the Longmen Shan area is nearlyin east-west direction and the minimum principal stress is almost in north-southdirection, implying a strike-slip stress state. In west, the maximum and the minimumprinciple stresses are north-east-east to south-west-west and north-north-west tosouth-south-east in the Bayan Har area, respectively. In east, the maximum principlestress is in north-west-west to south-east-east in South China, indicative of a stressregime of reverse slip. In north, the the maximum principal stress in Qaidam-Qinlingis in north-east-east to south-west-west as a compressional tectonic setting. And insouth, the maximum principal stress is in north-west to south-east as compressiondirection and the minimum principal stress direction is in north-east to south-west asextension, resulting in strike-slip in Sichuan-Yunnan. Thus, the direction of theregional tectonic stress field turns from NEE-SWW in the Bayan Har block to nearlyEW in the Longmen Shan block, and then to NWW-SEE in South China block,showing clockwise rotation from west to east in the spatial distribution. The directionof the maximum principal stress changes from NEE-SWW in Qaidam-Qinling blockinto nearly EW in Longmen Shan, and then into NW-SE in Sichuan-Yunnan, alsoshowing a clockwise rotation from north to south in the spatial distribution. Transition of stress direction among different blocks and about11meters crustal shortening and9-10meters mountain uplift on the Longmen Shan fault caused by Wenchuanearthquake indicate that the Bayan Har block is moving to southeast, pushing theLongmenshan block. Because of the strong stop of the Sichuan Basin, the motion ofthe Bayan Har block results in obvious crustal shortening and mountain uplift of theLongmenshan foreland thrust zone.(2) In terms of various kinds of stress data and inversion calculation, this workobtains the fault strength and characteristics of tectonic stress of the Longmen Shanfault zone.First, the fault strength on the Longmen Shan fault and in the Sichuan basin arecalculated from hydraulic fracturing data of17holes and stress measurement datafrom12oil fields. The results shows that the friction coefficients of the middlesegment of the Longmen Shan fault are0.13-0.47, meaning a weak fault strength.Because of most stress accumulation has been released by the Wenchuan earthquake,the earthquake risk of this section should be low. However, the coefficient at the siteof Yingxiu is0.397198, a relatively high value. It implies the fault strength is stillstrong with high seismic risk after the Wenchuan earthquake, and the occurrence ofLushan earthquake on20April2013may be the result of continual release ofaccumulated stress on the fault. In the north segment of Longmen Shan fault, thecoefficient is0.31-0.50and the fault strength and seismic hazard risk is a little higherthan that of the middle segment. In the south segment, the coefficient is0.33-0.68,representing the largest fault strength and the highest seismic risk compared to themiddle and the north segments on the Longmen Shan fault. In addition, the calculatedfault strength on the three transverse sections perpendicular to the Longmen Shanfault indicates the friction coefficient is lower on the fault or near fault but higher farfrom the fault. It reveals that tectonic stress is released because of fault activities,which causes the strength on the fault is lower than the stress level of tectonic stress inthe far field region.Second, based on the twice of investigations of active faults and measurementsof slickensides at17sites on the Longmen Shan fault, this work determines17 tectonic stress tensors by inversion. Combined with8tectonic stress tensors from fieldinvestigations just after the Wechuan earthquake, this work analyzes thecharacteristics on Longmen Shan fault. Among the8fault slip sites on the front-rangefault,3tectonic stress tensors are of strike-slip regime, other5are reverse-slip regime.The maximum principle stress of all the sites except number16are NWW to SEE ornearly in EW direction, suggesting thrust from west to east on the front-range fault.On the Wenchuan-Maoxian section of the back-range fault, the tectonic stress tensorsof two sites have good consistency, both having NW-SE The maximum principlestress as reverse faulting. It indicates that the Wenchuan-Maoxian section of the faultis in the NW-SE compression. On the Yingxiu-Beichuan segment of the centralLongmen Shan fault,11tectonic stress tensors all show the maximum principal stressin nearly EW direction, but three different kinds of stress regimes,7of which arereverse-slip,1is strike-slip and the other3are normal-slip. On theBeichuan-Qingchuan segment of the central of Longmen Shan fault, the invertedtectonic stress tensors show that the maximum principal stress is in EW direction andthe minimum principal stress is in NS direction, implying dominant strike-slip withsome thrust-slip component.Third, the inverted results from coseismic displacements show there are3different sub-segments with different characteristics on the Beichuan-Yingxiu surfacerupture. On the south sub-segment, from Yingxiu to Qingping, the maximumprincipal stress is in EW direction and the stress regime is reverse-slip. On the middlesub-segment, from Gaochuan to Chaping, appears the same characteristic of tectonicstress tensors as the south segment. And on the north sub-segment, from Beichuan toShuiguan, the maximum principal stress is mainly in EW compressional direction andin NS extensional direction, dominated by strike-slip faulting also with somereverse-slip component, except at Leigu and Pingtong sites where the maximumprincipal stress is in NNE-SSW direction. The complex fault strike may be the causefor the stress direction rotation. On the Hanwang-Bailu surface rupture, except themaximum principal stress on Shaba sub-segment in NNW-SSE, other4sub-segmentsall have the maximum principal stress in EW direction with the same reverse-slip regime. On the Xiaoyudong surface rupture, the maximum principal stress is in EWdirection on both the south and north segments, but they have different stress regimes:the former is strike-slip faulting with some of reverse component, and the latter isreverse-slip faulting. The special phenomenon is seen on the middle segment of theXiaoyudong rupture. Though the stress regime of it is also reverse-slip faulting, themaximum principal stress is in NNW-SSE direction, rotating about50°clockwise. Itis presumably attributed to the complex fault strike of the middle Xiaoyudong rupture.In addition, the inverted results from coseismic displacements well reveal that theWenchuan earthquake experienced twice different rupture processes. The first isdominated by reverse-slip, which spread from the source northward along the centralfault of the Longmen Shan fault zone, and was separated into two branches at thenorth of Xiaoyudong because of the block of the Xiaoyudong fault. The two branchesspread along Yingxiu-Beichuan and Hanwang-Bailu ruptures, respectively. In theprocess of northward transmission, the seismic wave energy was gradually decreasingwhich made the reverse-slip component in the north of Beichuan on the central faultreduce. The second rupture process is characterized by right-lateral strike-slip, quicklyspread and attenuated to Beichuan along the Yingxiu-Beichuan rupture, and finallystopped near Qingchuan.The forth, the characteristics of variations in coseismic tectonic stress on theLongmen Shan fault are obtained by inversion calculation of2020focal mechanismsolutions of the relocated Wenchuan earthquake sequence. The results show that themaximum principal stresses in south sub-segment of the Yingxiu-Beichuan fault areall in EW direction at three depth intervals, i.e.0~5km,5~15km, and>15km. In thenorth sub-segment, the maximum principal stresses is in NE-SW direction at thedifferent three layers. It is obvious that the maximum principal stress rotates about40°anticlockwise from north to south. So, the characteristics of different three lays matchwell except the Longmenshan town and Gaochuan-Chaping-Beichuan sub-segmentsat the depth of D≥15km. The maximum principal stress at the two sub-segments are inNNW-SSE compressional direction, different from the stress direction at depth intervals of0＜D≤5km and5＜D＜15km. The possible reason may be theXiaoyudong fault and Leigu sub-segment with complex fault strike are just at theLongmenshan town and Gaochuan-Chaping-Beichuan sub-segments, respectively.(3) From the above results, this work reveal non-uniform characteristics oftectonic stress on the Longmen Shan fault zone. In the middle segment, the maximumprincipal stress is in NWW-SEE direction to nearly EW compressional direction.However, this stress changes its orientation in the north segment from EW directioninto NEE-SWW compressional direction. It means that the tectonic stress directionshows35°-40°anticlockwise rotation. Combined the characteristics of tectonic stressof the Longmen Shan fault and the relationship between the tectonic stress action andthe distribution and movement of active faults, this thesis tries to put forward atectonic model of “antithetic S-type sinistral compression”. This model explains thatthe Xianshuihe fault and Zemuhe fault both constitute an antithetic S-type structure onthe left-handed strike-slip fault zone, forming a tectonic compressional setting wherea series of faults with compressional or shear features developed. And under theaction of compression tectonic stress, the Longmen Shan fault and Longriba faultwhich strike in NE direction are characterized by reverse-slip with some right-lateralstrike-slip component, and the Minjiang fault and Huya fault striking in NNWdirection are also dominated by reverse-slip but with some left-handed strike-slipcomponent. In addition, under the influence of fault structure and the obstruction ofthe Sichuan basin, the tectonic stress direction in the compressional zone changescompared to the adjacent regions. From the aspect of tectonic stress characteristicsand the coupling of earth’s crust, the “rigid block extrusion” model seems to be ableto explain the consistency of the tectonic stress directions at different depths of thecrust, cannot explain the changes of stress directions on the Leigu, Pingtong andShaba sub-segments. Therefore, a comprehensive dynamic model but not the singlemechanism is required to explain the all the geological and geophysical phenomena ofthe Wenchuan earthquake. |
PDF Full Text Request