| 1.IntroductionThe state of the interseismic strain accumulation in the causative fault zone is one of the important ways for the mid-long term earthquake prediction.The development of tectonic geodesy has provided important theoretical and technical means for carrying out this research.The eastern boundary of the Sichuan-Yunnan block and the southern section of the Longmenshan fault zone are important components of the middle section of the north-south seismic tectonic belt with most of the faults here behaving as sinistral strike-slip,thrust or thrust with strike slip.A lot of strong earthquakes have occurred in this area and the elapsed time of last strong earthquake is different for different fault sections which may indicate different stress and strain accumulation in different fault segments.Besides,there exists the first type of“gap”along the Anninghe fault zone and Zhaotong-Lianfeng fault zone from historical strong earthquake activity.Therefore,the study of the strain accumulation and locking status of these different faults segments is very important for mid-long term earthquake prediction of this area.Furthermore,it is only five years later after Wenchuan earthquake that Lushan earthquake occurred on the southern segment of Longmenshan fault zone,thus the study on the preseismic and postsemic deformation of Lushan earthquake is important to recognize the deformation characteristic of late interseismic period and the following seismic risk of the southern segment of Longmenshan fault zone.In this paper,based on the development of tectonic geodesy in decades,the interseismic deformation of different fault segments with different tectonic features in the east Sichuan-Yunnan area were studied.Firstly,the strike slip rate and locking depth of different segments of Anninghe-Zemuhe fault zone were inverted by Savage and Burford?1973?dislocation model by using GPS velocity during 1999-2007 and2009-2013 periods.Secondly,the Defnode dislocation model with GPS velocity as constraints was used to calculate the fault locking status of Anninghe-Zemuhe fault zone and Zhaotong-Lianfeng fault zone.Thirdly,the preseismic and postseismic deformation of Lushan earthquake was studied by GPS velocity and GPS timeseris.Lastly,the stress and strain states of the study area were analyzed by mathematical interpolation method?Kriging method and the Least squares collocation method?and Numerical manifold method.2.Main progress of this paper?1?The interseismic deformation of Anninghe-Zemuhe fault zoneAccording to previous studies and historical seismic rupture images,Anninghe-Zemuhe fault zone were divided into three segments which are Shimian-Mianning?Mianning-Xichang and south of Xichang segments.The possible affect of Daliangshan fault was also included in the process of dislocation model inversion.First of all,two kinds of GPS profiles are carried out which include and exclude the Daliangshan fault zone during 1999-2007 and 2009-2013.The results show that the strike slip rate and locking depth of Anninghe fault zone are affect by the motion of Daliangshan fault zone significantly.Thus,based on the GPS profile without Daliangshan fault zone,a reasonable strike slip rate and locking depth of Shimian-Mianning and Mianning-Xichang segments which are 7.3mm/a and 7.5mm/a of strike slip rate and 18.5km and 23.6km of locking depth during 2009-2013 were obtained.And the locking depth was consistent with the thickness of seismogenic layer.As there were not enough observations in 1999-2007,the result in this period is no meaning.The inversion slip rate of Zemuhe fault zone is about 5.7mm/a and4.7mm/a during 1999-2007 and 2009-2013 periods respectively,while the fault locking depth is 5km and 8km.The above result is consistent with the Seismic background of Zemuhe fault zone which may in early stage of the interseismic period.Based on the classical dislocation model,this paper deduces the dislocation model formula which takes into account the secondary fault.And the testing result showed that the inverted strike slip rate was approximate to the sum of the two fault slip rates and the locking depth would be enlarged when using classical model to invert two active faults.Therefore,the inversion depth of the above-mentioned inversion is beyond the brittle-ductile transition zone.Compared with the above two results,it can be seen that the motion and deformation of the parallel and sub-parallel fractures near the main strike slip faults have a significant effect on the slip rate and the locking depth of the main fault zone.Under the condition that the GPS station density is limited,artificially weakening the subordinate fault effect before inversing you can also get more reasonable results.Secondly,the inversion results of Defnode back-slip dislocation model show that the locking depth of Shimian-Mianning segment is about 20-25km and the locking coefficient is above 0.9,the deficit rate is about 7-9mm/a,then the far-field slip rate is7.8-10mm/a.The locking coefficient of Mianning-Xichang segment is about 0.6-0.8,and the deficient slip rate is about 5-7mm/a.The Zemuhe fault zone is not locked which is consistent with the results of the classical dislocation model.The inversion results also show that the Daliangshan fault zone has a significant effect on the inversion depth of the Anninghe fault.If the fault is not taken into account in inversion,the deformation caused by the Daliangshan fault zone will be accumulated to the Anninghe fault zone,then locking depth of Anninghe fault zone is deeper.However,the density of the GPS site cannot meet the accuracy and precision when considering all faults in one dislocation model,so one single fault model can get more reasonable results.?2?The strain accumulation characteristic of Zhaotong and Lianfeng fault zoneFirstly,the GPS velocity profiles show that the horizontal shortening rate of Zhaotong and Lianfeng fault zones is about 5-7mm/a and the shear rate is about1-2mm/a,which indicates that there are significant extrusion deformation accumulation in the Zhaotong and Lianfeng fault zones.Secondly,the calculated locking depth of the Zhaotong-Yiliang section is about 16km while it is about 10km in Huize-Zhaotong section based on Defnode dislocation model by using GPS velocity field in 1999-2007 and 2009-2013.Although Ludian 6.5 earthquake occurred at these fault zone,but the strain release is limited and the seismic hazard of the Zhaotong and Lianfeng fault zones did not alleviated.?3?The pre-seismic and post-seismic deformation of Lushan earthquake in the southern segment of Longmenshan fault zoneThe GPS velocity field from 2009 to 2011 before Lushan earthquake shows that the compressive strain accumulation rate is about 5.6×10-8/a which is much higher than that of 1.4×10-8/a before Wenchuan earthquake in the southern section of Longmenshan fault zone,and the GPS velocity also showed that the sinistral shear strain rate is about 3.6×10-8/a.The continuous GPS observation shows that the near-field compressive strain rate in the southern section of the Longmenshan fault zone is significantly lower than that in the west of the Maoxian-Wenchuan fault,and the extrusion deformation rate gradually decreased from west to east.The shear deformation mode is different as the west sides of Maowen fault behaves as dextral shear strain while the east area behaves as sinistral shear along strike N45?E.The above deformation characteristics indicate that the eastward movement of the Bayan Har block after Wenchuan earthquake provides a rapid loading to the southern section of the Longmenshan fault zone,and the extrusion deformation rate in the fault zone is relatively slow which may indicate relatively strong strain accumulation characteristics.The GPS velocity profile during 2013-2016 showed dextral shear strain along Maoxian-Wenchuan fault while sinistal shear strain along Guanxian-Anxian fault.The vertical component of GPS vectors showed stable eastward movement of Bayan Har block to Huanan block,and the east area to Maoxian-wenchuan fault showed continuous extrusion strain accumulation.Therefore,the southern segment of Longmenshan fault zone is still in strong extrusion strain accumulation.?4?Horizontal strain distribution in the studied areaFirstly,the Kriging method and the Least squares collocation method are used to obtain the distribution of the main strain rate in the study area from GPS velocity fields.The results show that the results of Kriging method are more likely affected by singular points and the continuity is not so good as the Least squares collocation method.The orientation of main compressive strain rate is about NWW-SEE in the east boundary of Sichuan-Yunnan block and the variation before and after Wenchuan earthquake is obvious.Secondly,the numerical manifold method?NMM?is used to simulate the deformation field in the eastern boundary of the Sichuan-Yunnan block.Besides the boundary constraints,the deep drag effect of Sichuan-Yuannan block in clockwise rotation is also considered.The simulated velocity field is highly consistent with the observed.And the simulated main strain rate showed the orientation of main compressive strain rate is about NW-SE which is consistent with the result of mathematic method above.The results of the maximum shear stress also show that the main fault zone in the eastern part of the Sichuan-Yunnan boundary is the gradient zone with the maximum shear stress,which reflects the strain accumulation characteristics.3.Major innovation?1?Based on the theory technical methods of tectonic geodesy,the interseismic crustal deformation of different active fault sections with different seismic background in the eastern boundary of the Sichuan-Yunnan block were studied.The results showed deeply understanding of the locking status of faults and strain strain accumulation in strike slip and dip-slip fault zones indicated by geodetic observations in the study area.Particularly in the new understanding of the variation of stress and strain state before and after the Wenchuan earthquake which indicated the tectonic loading effect of Wenchuan earthquake on the southern section of the Longmenshan fault zone and the deformation characteristics of the thrust type in the later stage of the interseismic period.?2?It is found that the movement and deformation of parallel and sub-parallel fault have a significant effect on inversion of slip rate and locking depth of the main strike slip fault.An modified dislocation model was deduced and the tests of theoretical and obsverd data suggested that the influence of the secondary fault activity can be weakened to a certain extent for the inversion of main strike slip faults by using the dislocation model.?3?Two kinds of strain calculation methods?Kriging method and Least squares collocation methods?were compared by calculating the strain rate distribution of the same area from the same GPS velocity fields.The Kriging method was found to be susceptible to outliers and nosises of observations,thus it is not suitable for the analysis of the overall trend of the strain rate in the larger region,and it is suitable for the recognition of the details of the strain rate field.?4?The numerical manifold method which can accurately describe the motion and discontinuous deformation of faults is used to quantitatively simulate the stress and strain state of the study area and new results are obtained. |
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