Correlation Between Medium Difference In Middle And Lower Crust And Strike-slip Strong Earthquakes Within Active Block

The Sichuan-Yunnan region has strong tectonic deformation and frequent strong earthquakes.Not only a series of strong earthquakes occurred in the boundary faults of active block,but also in the secondary faults within active block,such as the earthquake with a magnitude of 6.1 in the Miyaluo fault in 2008,the Ludian earthquake with a magnitude of 6.5 in the Baogunao-Xiaohe fault in 2014,and the Yangbi earthquake with a magnitude of 6.4 in the Weixi-Qiaohou fault in 2021.It has been shown that the seismic activities of the Miyaluo,Baogunao-Xiaohe and Weixi-Qiaohou faults are mainly of the strike-slip type and there are differences in the medium of the middle and lower crust on both sides of the faults.Therefore,the occurrence of strike-slip type earthquakes is not only closely related to the dynamical environment,but also associated with the difference of the middle and lower crustal media.On the basis of the above understanding,this paper aims to analyze the correlation between the medium difference between the two sides of the secondary faults within active block and the upper strike-slip earthquakes by using numerical simulations with the Miyaluo fault as an example.It can improve the understanding of the influence of the non-uniformity of the media within the active block on the seismogenic mechanism of the strike-slip earthquakes.The Miyaluo fault is located in the Songpan-Garzê block,striking NW and perpendicular to the Longmenshan fault zone.It is a transition fault that regulates the difference of SE crustal shortening in different sections of the Longmenshan fault zone.In order to study the correlation between the medium difference in the middle and lower crust on both sides of the Miyaluo fault and the upper strike-slip earthquakes,more reliable boundary constraints and sufficient conditions for testing the simulation are required.Accordingly,this paper firstly constructs a two-dimensional finite element contact model including the Longmenshan fault.The GPS results vertical to the compression rate of the Longmenshan fault are used as the initial constraint.The level uplift rate across the Longmenshan fault,the strong earthquakes period of the Longmenshan fault and the coseismic rupture slip are utilized as the test critria.We carry out several numerical simulation experiments to study the deformation pattern from the crust to the top of the upper mantle of the Songpan-Garze block and the sliding friction coefficient of the Longmenshan fault,so as to provide reference for the boundary loading and fault related parameters setting of the three-dimensional finite element contact model.Secondly,a three-dimensional finite element contact model containing the Longmenshan fault and Miyaluo fault is constructed.Under the same dynamic conditions,several numerical simulations are carried out to study the influence of medium non-uniformity on the crustal deformation by setting different magnitude differences of creep coefficients of the middle and lower crust on both sides of the Miyaluo fault.Finally,taking into account the differences in dynamical backgrounds on both sides of the Miyaluo fault,three numerical simulations are carried out by setting different creep coefficients of the middle and lower crust on both sides of the fault.The simulation results are compared with the sliding properties of the Miyaluo fault,the rupture characteristics of the Longmenshan fault and the regional geomorphology to study the medium distribution of the middle and lower crust on both sides of the Miyaluo fault and the correlation between the medium differences on both sides of the fault and its strike-slip type earthquakes.The main conclusions obtained from the above work are as follows:(1)The low-resistance low-velocity layer in the Songpan-Garze block can be used as a slip surface for material transport between the middle and lower crust and the upper mantle,which form a"decoupling" zone between the upper crust and the middle and lower crust.It can also promote the overall uplift of the Songpan-Garze block.However,the influence of the low-resistance low-velocity layer on the uplift is limited and not enough to form such a strong surface uplift.Considering the effect of low resistance and low velocity layer,the rate of surface uplift is most consistent with the long time level observation under the premise that the loading rate of the western boundary of the model increases linearly by 1.8 times from the surface to 100km depth.Under this dynamic condition,the middle and lower crust and upper mantle materials are strongly blocked by the Sichuan basin.The rapid rate reduction is mainly distributed within the Songpan-Garze block,while the rate reduction of the eastern edge of the Songpan-Garzê block and the Longmenshan fault zone is no longer significant.Thus,the center of surface uplift is located in the western Sichuan plateau rather than the Longmenshan fault zone.(2)When the sliding friction coefficient of the Longmenshan fault is 0.08,the coseismic vertical displacement at the fault outcrop,the thrust component of the fault coseismic rupture,and the recurrence period of the fault strong earthquakes are consistent with the existing research results.Therefore,the sliding friction coefficient of the Longmenshan fault may be around 0.08.(3)Under the same dynamic conditions,the results of 3D finite element simulation experiments show that the relatively low strength side of the medium in the middle and lower crust is more likely to absorb the horizontal compressional component during the interseismic period,resulting in a relatively strong vertical uplift.The greater difference in creep coefficients correlates with the greater the difference in the horizontal motion rate and vertical uplift rate of the upper crust.It is assumed that the creep coefficient of the middle and lower crustal media on the northeastern side of the Miyaluo fault is relatively large and the strength is relatively low.The horizontal motion rate of the northeastern side of the Miyaluo fault is smaller than that of the southwestern side during the interseismic period,which is consistent with the property of the left-slip of the Miyaluo fault.However,the coseismic displacement of the southeastern section of the Miyaluo fault in the parallel fault direction is larger on the northeastern side than on the southwestern side during the coseismic period of the Longmenshan fault,which is not conducive to the occurrence of left-slip earthquakes in the Miyaluo fault and is inconsistent with the occurrence of a series of left-slip earthquakes in the Miyaluo fault after the Wenchuan 8.0 magnitude earthquake.Therefore,the different dynamical backgrounds on both sides of the Miyaluo fault may be the underlying cause for the occurrence of the strike-slip earthquakes.(4)The compression component of GPS stations near the western boundary of the model perpendicular to the Longmenshan fault is interpolated and loaded to the western boundary as the boundary condition.Three numerical simulation experiments are carried out by setting the creep coefficient of medium in the middle and lower crust of the Miyaluo fault as high in the northeastern,the same on both sides,and high in the southwestern.The simulation results show that when the creep coefficient is relatively high on the northeastern side,the simulation results are in better agreement with the observation results.Therefore,the strength of the medium in the middle and lower crust on the northeastern side of the Miyaluo fault is lower.The relatively low-intensity middle and lower crustal media have an impact on the Miyaluo fault strike-slip earthquake breeding,the rupture characteristic of the Longmenshan fault and the regional geomorphological pattern.First,it elevates the left-slip rate of the Miyaluo fault during the interseismic period,causing the fault to accumulate more strain energy.The coseismic period of the Longmenshan fault caused another 0.3-0.6m left-slip displacement in the southeastern section of the Miyaluo fault,further triggering the left-slip type earthquakes in the Miyaluo fault.Second,it promotes the coseismic thrust in the Longmenshan fault on the southwestern side of the Miyaluo fault,but inhibits the coseismic thrust in the Longmenshan fault on the northeastern side of the Miyaluo fault.It affects the rupture characteristics of the Longmenshan fault.In addition,it reduces the difference in uplift rates between the two sides of the Miyaluo fault within the Songpan-Garzê block,but enhances the difference in uplift rates between the two sides of the Miyaluo fault at the eastern edge of the Songpan-Garze block,which affects the regional geomorphological pattern.