3-D Numerical Modeling Of Tectonic Stress Field In The Eastern Tibetan Plateau And The Seismic Hazard Assessment

The 3D stress state of the crust and its spatial-temporal variation are closely related to the occurrence of earthquakes.An in-depth understanding of the current stress state of the crust,on one hand,is important for studying the dynamic background and the gen-esis mechanism of earthquake;one the other hand,it also serves as important proof to discuss the related crust deformation.However,currents studies related to crust stress state are mainly limited to the shallow layer.Our understanding of the stress informa-tion?i.e.the stress value and orientation?in deep crust is still insufficient.In this regard,the 3D numerical modelling technique serves as a powerful tool to quantitively measure the dynamic and kinematic characteristics of the crust.This thesis is targeted to estab-lish the finite-element model including the complex faulting system for eastern Tibetan Plateau,and to retrieve the high-resolution 3D geomechanical model of this area by us-ing numerical modelling technique.This is achieved by an extensive collection of the current research data in this area from geophysical,geodetic and geological studies,and supplemented by focal mechanism data in key sections of the eastern Tibetan Plateau,to make sure the 3D numerical modelling results are consistent with observational data.The focuses of this thesis are that:?1?to study the consecutive stress distribution pattern at different depth level of the crust in eastern Tibetan Plateau and discuss the effect of the current stress state on seismic activity;?2?to get the absolute stress value and orientation in the crust and predict the stress value in areas where observational data are unavailable;?3?to reveal the uplift process and mechanism of the eastern Tibetan Plateau combined with results from rheology;?4?to discuss the role of the crustal flow model and the rigid block extrusion model and their contribution to the formation of the steep topography in the eastern Tibetan Plateau,thus to enhance the understanding of the continental dynam-ics of the eastern Tibetan Plateau.Meanwhile,the stress and slip rate of main fault zones will also be extracted to serve as the basis of analyzing the potential rupture segments and corresponding magnitude,to provide key reference information for quantitively assess the seismic hazard in the eastern Tibetan Plateau.The main achievements of this thesis are summarized as follows.?1?According to focal mechanism solution?FMS?result,the whole Longmen Shan fault zone and its surrounding area is in the thrust faulting stress field.The maximum stress axis??₁?is in near horizontal NW-SE direction while the minimum stress axis??₃?is near vertical.This reflects the continuous southeastern compression of the Bayanhar block.The FMS result indicates that the fault dip has an obvious change at around 10 km underground.This change is deduced to be caused by the shallow detachment layer in the crust,which promotes the upper crust shortening and the uplift of the Longmenshan fault zone.According to the fault geometry and the characteristics of the stress field and geomorphology,the Longmen Shan fault zone is divided into two segments with Beichuan city as the dividing point.The southwestern segment is in listric shape,with the dip angel steep at upper part and shallow at lower part.The northeastern segment is in transpressional fault geometry,with the dip angel shallow at upper part and steep at lower part.The Lixian fault is a tear fault,which adjusts the difference in southeastern thrusting slip rates of blocks with the Bayan Har block.The normal faulting earthquake within the Lixian area are due to the regional extension caused by the sinistral slip of the Lixian fault in NW-SE direction.Due to the continuous compression of the Bayan Har block,the stress accumulated in the southwestern segment of the Longmen Shan fault is relatively high.Thus,this area has high risk in occurring damagingearthquake.?2?According to modeled kinematics result,the eastern Tibetan Plateau moves in SE direction as a whole and relatively rotates in clockwise direction.The movement rate is high in its western part and decreases gradually towards its eastern part.The variation of movement rate in Sichuan-Yunnan block and the Bayan Har block is obvious.But in Sichuan basin and its north the Qaidam-Qilian block,the movement rate and direc-tion are relatively stable.The difference in movement rates of blocks is adjusted by the boundary faults of these blocks.According to the modeling results,the secular slip rate of the Luhuo-Daofu section in northwestern Xianshuihe fault is 12–13mm/a.In the mid-dle Kangding area,the slip rate is 10–11mm/a and in the southern Shimian area,the rate decreases to 7–8mm/a.The slip rate of the western part of the eastern segment of the East-Kunlun fault is 5-6mm/a and decreases to 3–4mm/a in its intersection area with the Longriba fault and further decreases to 2mm/a after passing through the Minjiang fault and the Huya fault in the east.The smallest slip rate of<1mm/a is in the eastern most edge of the East-Kunlun fault.Since the strike of the NE-SW faults is near perpendicular to the movement of the block in the study area,the corresponding slip rate of these faults is relatively low.For example,in the Longmen Shan fault zone,the slip rate is less than1 mm/a.The slip rate of Longriba fault is around 2.0–2.2mm/a,which is relatively high and adjusting the internal deformation within the Bayan Har block.The sinistral sliprate of the Huya fault with nearly S-N striking is around 1.2–1.4mm/a,which is much higher than that of the Minjiang fault in its west.According to the surface vertical movement modeling result,the study area is in a state of uplift,with the maximum uplifting value of around 0.9mm/a.But there is obvious subsidence zone in the intersection area between Xianshuihe fault and Longmen Shan fault and the north of the East-Kunlun fault neighbor-ing the Ruoergai basin.The maximum subsiding amount is around 0.7mm/a,reflecting the regional extension environment generated by the high-speed slip fault in its corner.As the main-controlled-fault of the Mianshan uplift,Huya fault absorbed the major part of deformation from the East-Kunlun fault.With the continuous compression of the Bayan Har block,Huya fault may affect the geometry of the Longmen Shan fault and lead to the geometric distortion in the intersection area between Beichuan fault and Huya fault.This is reflected by the outcrop on the surface of the Leigu fault in NW-SE direction.The fault system in NW-NNW direction jointly formed by Huya fault and Lixian fault intersected at high dip angel with the Longmen Shan fault.This fault system may affect the geometry of the Longmen Shan fault strongly and lead to the high dislocation and high intensity of Longmen Shan fault in Hongkou and Beichuan during the 2008 Wenchuan Ms8.0 earthquake.Therefore,it is of both scientific and practical meaning to study this fault system in depth.?3?According to the modeling result of the stress field,the Sichuan-Yunnan block and the Bayanhar block are in strike-slip faulting stress in general.The major part of the Sichuan basin,the Bikou block and Qadam-Qilian block are in thrust faulting stress field.The north in Bamei-Kangding section of Xianshuihe fault and the north of East-kunlun fault neighboring Ruoergai basin are in normal faulting stress field.The maximumstress axis of the study area??₁?is near horizontal and in the direction of E-W or NW-SE,which is similar to the maximum horizontal stress S_H.The direction of S_Hchanges obvious near the detachment layer.Therefore,it is recommended that when extrapolating the shallow stress into deep layer in the eastern Tibetan plateau,the effect of the detachment layer on deep stress should be fully considered.The modeled absolute stress results show that divided by Beichuan,the normal stress in the northeastern segment of Beichuan fault is higher than its southwestern segment,but the shear stress of the former is lower than the latter.This fact is considered to be the mechanical cause of the unilateral rupture of the Wenchuan Ms8.0 earthquake:the high normal stress in the southwestern segment makes it difficult to slip and easier to accumu-late strain energy.With the continuous southeastern compression of the Bayan Har block,the stress on the fault plane is increasing and when it reaches the critical rock strength,the initial rupture will develop in the southwestern segment of the Beichuan fault,and the con-secutive stress perturbation makes the rupture develops along the northeastern direction with decreasing normal stress.While the rupture was stopped easily in the southwestern segment,due to its high normal stress and low shear stress.That is why the rupture of the Wenchuan earthquake is unilateral.The analysis suggests that the normal faulting stress field in the Bamei-Kangding section of the Xianshuihe fault is related to the extension effect in Kangding where the sinistral slip of the Xianshuihe fault rotated clockwise.It is deduced that this stress field provides a channel for the uplift of the lower crust?high temperature and high pressure material in plastic state?along the Xianshuihe fault,which is conducive to the upward heat transfer and promotes the partial melting of upper crust.The normal faulting stress further creates space for the magmatic emplacement,which finally forms the Zheduo granite.Be-sides,the upward moving of the lower crust flow triggered by the normal faulting stress may also promote the rapid uplift of the southwestern limb of the Danba anticline at around12Ma.Therefore,it is considered the uplift of lower crust around Bamei-Kangding,the emplacement of the Zheduo granite and the rapid uplift of the southwestern limb of the Danba anticline are all related to the extension effect caused here by the slip of the Xi-anshuihe fault.And these phenomena can be regarded as a collective response to the eastward movement of the eastern Tibet Plateau.?4?With the Xianshuihe fault as an example,the potential rupture segment and the corresponding maximum magnitude of is analyzed.The modeled result indicates that,the Kangding-Shimian segment of the Xianshuihe fault is in transpressional stress field and the normal stress in its fault plane is high,which makes it difficult to slip and easier to accumulate higher strain energy and leads to the occurrence of large earthquake.The long-period slip rate of this segment is around 9mm/a and the last large earthquake was occurred 235 years ago.The approximated magnitude of earthquake can occur in this area is around Ms7.3.Therefore,attention should be taken seriously in this area and more monitoring and study of the fault activity in this area is in urgent need.