Study On The Temporal-spatial Evolution And Mechanism Of The Slip Distribution Along The Northwestern Xianshuihe Fault Of The Seismic Period

The Xianshuihe fault is located in Sichuan province,which belongs to the boundary zone of the Bayankala block and Sichuan-Yunnan block in the Tibetan Plateau.The fault is activity fault and high seismic hazard,meanwhile the earthquake occurs frequently in the surrounding area.The northwestern segment of the Xianshuihe fault had experienced many M⁷ earthquakes,such as the Bamei earthquake in 1893,the Renda earthquake in 1923,the Daofu earthquake in 1981,and the Luhuo earthquake in 1973.The surface rupture traces of these four earthquakes are located along the northwestern Xianshuihe fault with a total length of about 200km and lots of overlapping area.Based on the data of coseimic geological survey and the fault-crossing short-baseline,we invert the coseismic slip distribution and the spatial-temporal evolution of slip distribution of the northwestern Xianshuihe fault using the elastic half-space model and the least-squares method.Then we estimate the friction parameters of the fault,and analyze the physical mechanism of seismogenic process as the rate-state law.Finally,we construct a summary model of the friction properties with the non-uniform fault along the northwestern Xianshuihe fault for seismic hazard assessment.We have mainly completed the following work:?1?We collect the geological surface rupture data of the Renda earthquake in1923 and the Luhuo earthquake in 1973,and the fault-crossing short-baseline data,then eliminate the influence of coseismic and seasonal deformation by analyzing the fault-crossing short-baseline time series.Additionally,the two kinds of data are transformed to the east-west and south-north displacement,which provides data support for further inversion of seismic slip distribution model and spatial-temporal evolution of fault slip distribution.?2?Based on the elastic dislocation model and the least square method,the kinematic method is used to invert the coseismic slip distribution of the Renda earthquake and the Luhuo earthquake,and the spatial-temporal evolution of fault slip distribution.The results of this study show that the moment magnitude of the Renda earthquake is Mw 7.0 and the maximum slip is about 3.3 m,which is near the Dazhai Village,Luhuo County.The moment magnitude of the Luhuo earthquake is Mw 7.3and the maximum slip is about 3.7 m,which is near the Dandu county.The maximum slip of the two earthquakes is located in shallow part of the crust⁵ km deep,and their focal mechanism of the two earthquakes is similar and mainly belongs to sinistral strike-slip.The maximum slip of the fault slip distribution is 25.6 mm near Xialatuo from 1976 to 1980.0,which have released the total moment M₀=1.61×10¹⁷N·m?Mw 5.47?.The accumulated slip distribution of 1976¹990,1976²000,and1976²010 increased with time accumulation,and the maximum accumulated slip is140 mm,locate near Xialatuo.?3?According to the spatial-temporal slip distribution of the faults,we estimate the average fault slip rate and the postseiesmic spatiotemporal evolution of the slip accumulation.The results show that the Xialatuo zone is mainly affected by the Luhuo earthquake in 1973,which lead to obvious creep.The average slip rate was about 7 mm/yr from 1976 to 1980 and gradually decreased to 3 mm/yr from 1990 to2000.It is speculated that the fault slip mainly belongs to the long-term sliding rate after 1990,because of the average sliding rate from 1990 to 2000 is similar from 2000to 2010.Meanwhile,the postseismic effect of the 1973 Luhuo earthquake may continue until 1990.From the existing research of the Xianshuihe fault,we find that the research result of the long-term sliding rate has slight different using different data and numerical simulation methods.?4?Based on the Coulomb rupture criterion,we calculate the coseismic and postseismic Coulomb stress changes of the Renda earthquake by using our coseismic rupture model.We analyze the stress triggering effect of the Renda earthquake on the Luhuo earthquake combining with the average recurrence interval of the earthquake.The results indicate that the Coulomb stress change increase about 29.79 kPa in the epicenter of the Luhuo earthquake,which is caused by the coseismic and postseismic effects of the Renda earthquake,and reach the earthquake trigger threshold of 10 kPa.Therefore,we speculate that the Renda earthquake may trigger an effect on the Luhuo earthquake combing with the analysis of recurrence period of the earthquake.?5?Based on the temporal-spatial evolution of slip distribution of the fault and the Luhuo earthquake slip distribution,we determine the friction parameters of the fault using the Scholz analysis method and the Perfettini analysis method which combine with coseismic stress.The result provides a reference for the dynamic method for studying the physical mechanism of seismogenic.Our results show that the creep is significant near Xialatuo?B zone?,and the maximum friction parameter is about 0.052.Secondly,the friction parameter is about 0.01 near Zhuwo?A zone?,and the near Goupu?C zone?is about 0.008.When two different calculation methods are used to study the changes of friction parameters,the friction parameters are the same in shallow 0⁵ km,there are obvious differences below 5 km deep in the crust,and the total differences are RMS?0.002,since the all observation data are from near-field data.?6?According to the existing research on the crustal stratification structure in the Western Sichuan Plateau,we stratify the northwestern Xianshuihe fault and determine the distribution characteristics of the friction parameters and creep properties of the fault.We construct a summary model of the friction properties of the non-uniform fault to analyze the interaction between the seismic rupture and creep behavior.According to the fault property,fault activity,distribution of microseismicity,average recurrence period of earthquake,and geological investigations,we comprehensive conclude that there is seismic risk of M⁷ earthquake hazard near Laoqianning.