Stress Triggering Interaction Among The Large Earthquakes And Risk Of Future Seismicity In Yunnan

The process of seismogenesis is generally considered by the accumulation and release of stress on the seismogenic fault.Promoting or inhabiting the occurrence of subsequent earthquakes caused by the adjustment of the regional stress field during the coseismic,postseismic,and interseismic deformation is called stress triggering interaction.With accurate earthquake forecasting not yet achieved,Coulomb failure stress(CFS)becomes a crucial tool to investigate the triggering interaction between earthquakes and to assess the risk of future earthquakes.Subsequently the probability of seismicity calculated from coulomb stress change and background seismicity makes the risk assessment of earthquakes quantitative.The Yunnan region,located in a special location with strong plate movement and geological tectonic activity,is one of the most frequently seismically active regions in China.The large active fault zone areas in Yunnan such as the Red River fault zone,the Xiaojiang fault zone and the Lijiang-Xiaojinhe fault zone have occurred several M>6 earthquakes in history.The Ms6.4 earthquake that occurred on May 21,2021 in Yangbi,Dali Bai Autonomous Prefecture,Yunnan Province,is the largest earthquake in the region since 1976.Whereas the Weixi-Qiaohou-Weishan fault zone as the nearest fault to the location of the earthquake did not rupture significantly during this earthquake.Therefore,it is significant to investigate the stress triggering effects of historical strong earthquakes and active fault zones on Ms 6.4 Yangbi earthquake and that between strong earthquakes.And the research of stress accumulations on major active fault zones after the Yangbi earthquake are also important for the prediction and prevention of future earthquake hazards in Yunnan.This paper analyzes the influences of source fault model parameters,receiving fault geometry parameters,apparent friction coefficient,lower crustal and upper mantle media viscosity and calculation depth on the result of Coulomb stress change.The tests show that different parameters have different degrees of influence on the spatial distribution and value of stress change.It is necessary to determine the parameters of source fault model and receive fault accurately.The viscosity of lower crust and upper mantle media need to be selected with roughly appropriate values according to the research data.And the apparent friction coefficient can be adopted with empirical values.Based on the Coulomb failure stress theory,this study investigates the coseismic,postseismic,and interseismic stress triggering effects of 25 M>6 earthquakes distributed near the Red River fault zone,the Wuliangshan fault zone,the ChuxiongJianshui fault zone,the Xiaojiang fault zone,the Lancangjiang fault zone,the Nantinghe fault zone,and the Lijiang-Xiaojinhe fault zone since 1833 on the 2021 Yangbi earthquake.The results show that the postseismic and interseismic Coulomb stress accumulation reached 0.159 bar and 0.328 bar respectively.While the coseismic static stress unloading of more than 0.5 bar caused by the 1925 M7 earthquake on the Yangbi earthquake makes the complete stress triggering interaction not obvious.The stress triggering relationships among 25 strong earthquakes since 1833 under coseismic and postseismic effects are also researched in this study.The results show that the M7 earthquake of 1887,the M7 earthquake of 1925,the M6.25 earthquake of 1948,the M6.25 earthquake of 1951,the M6.2 earthquake of 1962,the M6.5 earthquake of 1966,the M6.5 earthquake of 2000,and the M6.3 earthquake of 2009 are subjected to 0.1-3.479 bars stress increase caused by all previous strong earthquakes,which suggests historical earthquakes have significant contributions to the occurrence of these events.In this paper,the coseismic stress change generated by the Yangbi earthquake is taken into consideration to calculate the total stress changes accumulation on 7 fault zones caused by 26 strong earthquakes and major fault zones under coseismic,postseismic and interseismic effects.The stress accumulation is high in the northern,north-central and southern segments of the Red River fault zone,the west-central and east-end locations of the Chuxiong-Jianshui fault zone,the northern and southern segments of the Xiaojiang fault zone.And the whole the Wuliangshan fault zone,Lancangjiang fault zone,Nantinghe fault zone and Lijiang-Xiaojinhe fault zone have high level of stress loading,which indicates a high probability of future earthquakes.The probabilities of occurrence of M>6 earthquakes between 2022 and 2038 on 7 fault zones are calculated with different friction state parameters and stress perturbation durations based on the seismic catalogs and stress perturbations.The results show that the northern segment of the Red River fault zone and some segments of the Lancangjiang fault zone have higher than 80%or even 90%of the maximum probabilities of earthquake occurrence which means high risk of future seismic activity.The risk of future earthquakes on medium probability level fault zones such as the Chuxiong-Jianshui fault zone and the Lijiang-Xiaojinhe fault zone should not be ignored.