The Influence On The Regional Crustal Deformation And Stress Field Of The Wenchuan M_S8.0 And The Lushan M_S7.0 Earthquakes

1.OverviewThe long-term impact of coseismic and post-earthquake effects on the crustal deformation field,stress field,fault activity status,and seismic hazard in the surrounding area has attracted more and more attention from the international academic community and has become a hot topic for research in the past two decades.As the first high-angle reversed type 8-magnitude earthquake that occurred in the mainland since the instrument records,the occurrence of the 8.0-magnitude Wenchuan earthquake on May 12,2008 not only caused serious casualties and economic losses,but also had a continuous and significant impact on the regional tectonic deformation and fault stress accumulation.Five years after the Wenchuan earthquake,a7.0-magnitude Lushan earthquake occurred on April 20,2013 in the southern section of the Longmenshan fault zone,again causing people to discuss the future earthquake risk of the fault zone.Carrying out related coseismic and postseismic researches on both the Wenchuan earthquake and the Lushan earthquake not only favor to understand the rupture characteristics and interrelationships of the two earthquakes,but also has great significance on analyzing the change of regional crustal deformation and stress field and estimating the future seismic hazards of main active faults.Therefore,this paper takes the Wenchuan earthquake and Lushan earthquake as the research object,based on the classic dislocation theory and the viscoelastic constitutive relation,carries out the research on the changes of regional deformation field and stress field caused by coseismic dislocation and viscoelastic relaxation of the two earthquakes.Firstly,this paper constructs a 3D model of the seismogenic faults of the Wenchuan and Lushan earthquakes by means of comprehensive multidisciplinary data,together with the inversion using multiple coseismic observations,the coseismic slip distribution of the Wenchuan and Lushan earthquakes is obtained.Based on the constitutive relation of Burgers'viscoelastic body,with GPS time series as the constraint,with the distribution of coseismic slip in the Wenchuan earthquake as the initial condition,and with the help of finite element numerical simulation,the optimal short-term and long-term effective viscous coefficient of each block in both sides of the Longmenshan fault zone is determined.This paper calculated the regional deformation field changes caused by coseismic dislocations and viscoelastic relaxation after two earthquakes.From the perspective of the Coulomb stress evolution,the paper discussed the role of the Wenchuan earthquake and the Lushan earthquake on the subsequent aftershocks and the occurrence of a series of moderate and strong earthquakes in the surrounding areas,analyzed the Coulomb stress evolution of the major active faults in the surrounding area,and discussed the important fault section that is worth paying attention to in the future using the rate-state frictional constitutive law.2.Major achievements made by the paper?1?Seismogenic structure of the Wenchuan earthquake and the distribution of coseismic slip on faultsThe Wenchuan earthquake surface rupture,geophysical prospecting profile,and repositioned aftershock sequences were used as strong constraints for the deep and shallow structures.With reference to regional geological profiles,velocity tomography and magnetotelluric sounding results,a 3D model of the Wenchuan earthquake seismogenic fault was established through multi-profile interpretation,mainly including Beichuan-Yingxiu fault and the Guanxian-Jiangyou fault.Besides,an unknown anti-dipping fault has been identified instead of a simple extension of the Beichuan-Yingxiu fault,when the aftershocks zone traverse the Qingchuan fault.The inversion test concluded that the unknown fault may not have participated in the coseismic rupture process.Combined use of GPS,InSAR and strong motion data with relatively small postseismic effects as the constraints for inversion,a fault coseismic slip distribution model was obtained.The results showed that high-value areas with a maximum slip value of about 10 m were mainly concentrated in the vicinity of Hongkou-Xiaoyu-dong and Beichuan-Pingtong,and the maximum slip values around the north branch of the southwest section of Beichuan-Yingxiu rupture,Yuejiashan-Qingping,northeast of Nanba,and nearby Hanwang can reach about 4-6m.The fault movement pattern is dominated by thrusting motion in the Hanwang-Bailu section,and the thrust-domination gradually changes to the dextral strike-slip movement from the southwest to the northeast of the Beichuan-Yingxiu rupture.In addition,in the deep decollement,several high slip areas with a maximum value of about 4 m were identified.After analysis,they tend to be considered as coseismic rupture rather than mainly caused by afterslip.The inversion moment magnitude is M_W 7.9,and the simulation results are in good agreement with the observation data of the coseismic observations and the field surface rupture data.In addition,by conducting inversion comparisons based on different fault models published by previous researchers,it is believed that the fault model may control the primary characteristics of fault coseismic slip distribution,and may be the main reason that the largest coseismic slip amount range inferred by different scholars is from 7m to 14m.?2?Seismogenic structure of the Lushan earthquake and the distribution of coseismic slip on faultsBased on the simplified fault model,inversion was performed by supplementing more near-field coseismic observation data such as leveling and strong motion with GPS data,and compared with previous inversion results using GPS data alone.The results showed that the abundance of near-field observation data has a significant impact on the distribution characteristics of coseismic slip.Since the Lushan earthquake did not produce surface coseismic ruptures and the seismogenic structures are relatively complex,it is difficult to rely on surface geodetic data alone to infer the fault geometry parameters.Using the relocation results of the aftershock sequence with the the reference to the regional geology and geophysics information,a 3D fault model was obtained through multi-profile interpretation,which include the major fault F1,the secondary fault F2,the anti-dipping faults F3and F4,and the shallow decollement F5.Joint use of GPS,leveling and strong motion data to perform inversion tests based on the above fault model shows that the coseismic slip mainly occurs near the ramp of the main fault F1,and a certain amount of slip also occur on the anti-dipping faults F3 and F4.The coseismic fault movements are dominated by thrusting motions.F3 faults may also have a certain dextral strike-slip component,and secondary fault F2 may not participate in coseismic rupture.Although the shallow decollement F5 has a small amount of coseismic slip,its existence is very important for restricting the rupture below to continue to the ground surface,and adjusting and converting the thrust motion of the underlying structure.The moment magnitude obtained by the inversion is M_W6.5.The simulation results can well fit the surface coseismic observation data,and can explain to some extent the dynamics connection between the surface“sinistral”motion revealed by GPS and the pure reverse movement of deep faults.?3?Block viscoelastic structures on both sides of Longmenshan fault zoneTaking into account the significant viscoelastic differences between the Bayan Har block and the Sichuan Basin on both sides of the Longmenshan fault zone and the time-dependent characteristics exhibited by the viscous coefficients,a layered and horizontal block finite element model was established,based on the constitutive relations of the Burgers body.The Wenchuan earthquake coseismic slip distribution was introduced as the initial condition of the model by splitting nodes method,and the simulation results were fitted to the time series data of the six consecutive GPS stations in the Sichuan Basin.The short-term and long-term effective viscous coefficients of the middle-lower crust in the Sichuan Basin and the Bayan Har block are determined in turn.The results show that the short-term effective viscous coefficient of the middle-lower crust in the Sichuan Basin is 9.0×10¹⁸Pa.s,and the long-term effective viscous coefficient is 1.0×10²¹Pa.s;the short-term effective viscous coefficient of the middle-lower crust in the Bayan Har block is 5.0×10¹⁷Pa.s,and the long-term effective viscous coefficient is 9.0×10¹⁸Pa.s.?4?Impact of Wenchuan earthquake and Lushan earthquake on regional deformation fieldThe simulation results show that the coseismic horizontal displacement of the Wenchuan earthquake in the middle of the Longmenshan fault zone is mainly the opposite movement on both sides of the fault zone.The horizontal displacement on the two sides of the fault zone shows a clear turn of direction,showing obvious dextral strike-slip features in the north of the Longmenshan fault zone.The area with significant displacement is mainly in the vicinity of the rupture zone,and rapidly decays to the two sides off the rupture,and the attenuation on the Bayan Har block is relatively slow.Comparing the horizontal displacement field after 5 years and 10years after the earthquake,we can see that the horizontal displacement field caused by the viscoelastic relaxation effect after the earthquake is the further development of the coseismic displacement field,with a wider range of influence,and the morphological characteristics show the inheritance.The comparative analysis between considering and neglecting the lateral differences of the viscoelastic structures on both sides of the fault zone shows that the lateral differences of visco-elastic structures make the postseismic deformation field on both sides of the fault more asymmetric.The range of influence of the Lushan earthquake is basically limited to the southern section of the Longmenshan fault zone.The horizontal displacement field of the coseismic motion shows the opposite movement on both sides of the fault zone and the two-way escape along the strike near the fault zone.The horizontal displacement field after 5 years of the earthquake shows more prominent asymmetry of magnitude on both sides of the fault zone,in which the central region of the fault spreads fan-shaped to the east and the magnitude is larger,and the west side of the fault shows a fan-shaped contraction to the fracture.The east side of the fault has no obvious displacement.The Wenchuan earthquake dominates the combined effects of the two earthquakes,and the main affected areas are the northeast margin of Tibetan plateau,besides of the Longmenshan fault zone and the eastern part of the Bayan Har block and the Sichuan Basin.The postseismic effect of the Lushan earthquake offset the partial impact of the Wenchuan earthquake on the southern segment of the Longmenshan fault zone,resulting in a decrease in the magnitude of the southward,south-southeast horizontal displacement caused by the Wenchuan earthquake in the area,and causing the displacement direction closer to the southward.From the result of Wenchuan coseismic dislocation and postseismic viscoelastic relaxation effect on the deformation field and stress field in the Lushan seismogenic area,the occurrence of the Wenchuan earthquake is conducive to the subsequent Lushan earthquake.?5?Impact of Wenchuan earthquake and Lushan earthquake on regional stress fieldThe corresponding relationship between Coulomb stress changes caused by coseismic dislocations in the Wenchuan and Lushan earthquakes and the subsequent distribution of aftershocks was analyzed.The effect of the two earthquakes on the series of typical moderate-strong earthquake events in the surrounding area has been discussed.It is considered that the impact of the Lushan earthquake on several earthquakes is negligible.The Coulomb stress changes caused by the coseismic dislocation and postseismic viscoelastic relaxation of Wenchuan earthquake are favorable to the occurrence of the Kangding M6.3 earthquake on November 22,2014,the Kangding M5.8 earthquake on November 25,2014,and the Jiuzhaigou M7.0earthquake on August 7,2017,while causes some inhibition of the Minxian-Zhangxian M6.6 earthquake on July 22,2013,and has no significant effect on the Ludian M6.5 earthquake on August 3,2014.Comparing the results of different viscoelastic medium models and different Wenchuan coseismic slip models,it is believed that the choice of these models has a great influence on the Coulomb stress results.The comprehensive analysis suggests that the Coulomb stress changes due to coseismic dislocations in the Wenchuan earthquake are conducive to the occurrence of the Lushan earthquake,and before the Lushan earthquake,the comprehensive impact of the coseismic dislocation and viscoelastic relaxation effects of Wenchuan earthquake is still manifested in favor of the occurrence of Lushan earthquake.Judging from the impact on surrounding faults,the effect of the Lushan earthquake is relatively limited.It only causes a certain increase in the Coulomb stress in the rupture gap between the Wenchuan earthquake and Lushan earthquake,the south section of the Xianshuihe fault zone and the middle section of the Longquanshan west marginal fault.The impact of Wenchuan earthquake has a wider spatial range and greater magnitude.The combined effects of the Wenchuan earthquake and the Lushan earthquake caused the Coulomb stress on the rupture gap between the two,the north section of the Pingwu-Qingchuan fault,the east section of the Wenxian fault,the north section of the Longyiba fault,and the north section of the Guanxian-Jiangyou fault increasing significantly,and exceeding the triggering threshold of 0.01 MPa.Meanwhile,it caused the Coulomb stress on the east section of the Lintan-Tanchang fault,the east section of the Diebu-Bailongjiang fault,the west section of the Wenxian fault,the Huya fault,the south section of the Minjiang fault,the south section of the Pingwu-Qingchuan fault,the Maerkang fault,the middle-north section of the Beichuan-Yingxiu fault,and the north section of the Longquanshan west marginal fault significantly decreasing.The results of the model with and without consideration of lateral variations of the viscoelastic media on both sides of the Longmenshan fault zone are compared and analyzed.It is considered that the overall Coulomb stress obtained by the two models are basically the same,but ignoring the lateral differences in the media will make the Coulomb stress change value overestimate.Taking the Coulomb stress change as the stress disturbance and taking into account the differences in regional background seismicity,the possible seismicity rate and Poisson probability of earthquakes on the major faults in the surrounding area of the Longmenshan fault zone are calculated based on the rate-state frictional constitutive law.The results show that by 2023,the faults with a relatively high seismicity rate and Poisson probability of M?6.0 earthquakes are mainly in the middle section of the East Kunlun fault zone and the middle section of the Xianshuihe fault zone.The Coulomb stress changes caused by Wenchuan earthquake and Lushan earthquake are also positive on the two fault sections,so the seismic risk of them in the future is worth further attention.