Stress Evolution In The Southeast Margin Of Bayan Har Block And Simulation Of Spontaneous Rupture Of Ludian Earthquake

The Kunlun-Wenchuan earthquake sequence caused by Bayanhar block activities is the only group of strong earthquake sequence which magnitude over 7 in mainland China.This block is still the main region which strong earthquake ouccred in the next few years.The tectonic dynamic properties of the east edge of the Bayan har block from strike slip movement to thrust motion make the strong earthquake have correlation on two adjacent boundary.The study shows that the strong earthquake sequence in the eastern boundary of the block is the response to the strong earthquake activity in the north boundary,and the Xianshuihe fault zone on the southern boundary of the Bayan har block is also squeezed from the left to the East,However,the earthquakes in the southeast edge of the Bayan har block present two characteristics,one is that the strong earthquakes mostly occur in the east boundary fault zone of the Bayan har block,and the strong earthquake sequence in the east boundary is accelerated;and the two is that the Xianshuihe fault zone of the southern boundary has not occurred more than 7 earthquakes for more than 40 years after the 7.6 magnitude earthquake in the 1973.That shows a state of large earthquake missing in the earthquake sequence.Therefore,the seismic risk of the main fault zone in the southeast margin of the Bayan Har block deserves attention and research.The coseismic Coulomb failure stress plays an important role in explaining the distribution of aftershock sequences and analyzing the interaction relationship of strong earthquake sequences.For example,after the Wenchuan earthquake,Some researchers use the elastic dislocation model to calculate the variation of the Coulomb rupture stress of the Wenchuan earthquake,and estimate the future earthquake risk in the Longmen mountain area.Some researchers have calculated the Coulomb stress changes of the surrounding faults after the Wenchuan and Lushan earthquakes,as well as the viscoelastic relaxation stress field after these two earthquakes,their result shows the Wenchuan earthquake may effectively promote the occurrence of the Lushan earthquake.In the process of stress evolution,the tectonic stress loading between earthquakes is also very important.In the past,the theory of negative dislocation was used to calculate the stress accumulation in the past.Although the method is concise,it is assumed that the fault is completely locked and may cause certain error.In this paper,the regional tectonic activity of the block of Bayan har is considered as the boundary constraint of the three-dimensional finite element model.The viscoelastic material parameters of the medium are calculated by a comprehensive study of the inversion results,and the stress evolution of the research region before,between earthquakes and after the earthquake is simulated from the initial stress field,which aim to get more accurate and reliable result of stress field.Rate-state law is used to calculate the change of seismic rate caused by the change of Coulomb failure stress.By using Poisson probability process,the earthquake which magnitude over 6 occured probability of the research region and main fault in this area in the next 10 years after the Wenchuan earthquake and the Lushan earthquake is calculated.In addition to use the above quasi dynamic method to research the stress evolution and earthquake risk in the southeast edge of the Bayan har block.In this paper,the Ludian earthquake is taken as an example to study the spontaneous rupture dynamics process,which begins to slip from the nucleation area,the failure extend and stop to explore the main factors that affect the spontaneous rupture dislocation of the earthquakeResearch contents:(1)we have established a three-dimensional viscoelastic finite element model for the southeast margin of the Bayan Har block.(2)The stress evolution process is simulated on the main faults of southeast margin of Bayan har block since 1900,We also research the other strong earthquake whether trigger the Lushan earthquake or not,we compare the evoluted stress filed and seismic activity.(3)have constructed a viscoplastic model of Longmenshan faults.(3)We use rate-state law to research how the coulomb stress change influnent seismic rate after the Wenchuan earthquake and Lushan earthquake,we apply passion process to calculate the occurred probability of strong earthquake.(4)We use FAULT code which can depict friction law on falult to research the initial slip point and dislocation of Lushan earthquake,we discuss how the strength of fault after wenchuan earthquake affect the intial slip point of Lushan earthquake.(5)the spontaneous rupture model of Ludian earthquake is established by using the curved finite difference method,and the spontaneous rupture process of Ludian earthquake on the fracture of Bao Gu Xiaohe river is simulated,and the main factors that affect the spontaneous rupture process of the earthquake in Ludian are analyzed.Innovation points:(1)using the latest velocity and density inversion result to construct more realistic three dimension viscoelastic finite element model;(2)We get the background stress filed from intial stress filed after ten thousands years evolution to apply to simulate earthquake sequence;(3)The Coulomb failure stress which is applied in the rate-state law to get seismic rate is calculated by three-dimensional viscoelastic finite element model which fully consider the cosiesmic post seismic and interseismic stress change,At the same time,a more complete catalogue of earthquakes is adopted when considering the background seismic rate,which may make the estimation of future seismic hazard more accurate;(4).The dynamic simulation of the spontaneous rupture process of the Ludian earthquake was carried out,and the process of the inter earthquake stress accumulation was combined with the simulation of the coseismic earthquake rupture process.