Ground Motion Characteristics Analysis Of Yangbi Earthquake Series In Yunnan Province In 2021

As is known to all,the damage of engineering structure is mainly caused by strong ground motion,and the significance of analyzing the characteristics of ground motion for earthquake resistance and disaster reduction is self-evident.Yunnan province is located in the southwest of our country,belongs to the earthquake high incidence area,moderate and strong earthquakes occur frequently.On 21 May,2021,an MS6.5 earthquake occurred in Yangbi,Yunnan Province,and the strong vibration station and intensity station recorded a lot of ground motion data of the earthquake,which can be used for us to study the ground motion characteristics of Yangbi.Based on the strong motion data,this paper first establishes the prediction equation of ground motion,then studies its directivity according to the change of the residual with the azimusion Angle,and finally simulates the main earthquake by the Two-stage Stochastic Empirical Green’s Function Method.Specific research contents are as follows:(1)Based on the strong motion records of Yangbi earthquake series,a ground motion prediction equation(GMPE)suitable for Yangbi area in Yunnan Province is established in this paper.And a reasonable and reliable method is used to process the obtained ground motion records,and the attenuation relationship between the horizontal peak acceleration(PGA),peak velocity(PGV)and the pseudo-acceleration response spectrum(PSA)with 5%damping ratio at different periodic points is fitted by random effects regression method.The attenuation relationship takes into account the effects of magnitude saturation,magnitude dependent geometric attenuation,linear site response,and inelastic attenuation.It is found that there is no obvious trend in the residual distribution map corresponding to the inter-event residual and the intra-event residual(path term and site term),and the mean value of the residual is near zero,which proves the robustness of GMPE.The results of comparison with the actual observed values of historical earthquakes in Yunnan province show that the predicted values are in good agreement with the observed values.The prediction results of this model are basically consistent with those of other domestic and foreign models(BSSA14,ASK14,ZB22 and W22),and the standard deviation is lower than those of these models,which proves that the model is reasonable and reliable.(2)In this paper,the rupture directivity characteristics for nine Yangbi earthquakes with abundant records in a good spatial station coverage were estimated based on the fitting method of rupture directivity effect.The results indicate that four earthquakes exhibited the rupture directivity effects.The four earthquakes are all characterized by the bilateral ruptures with different predominant rupture directions(southeast,northwest,and southwest),illustrating the rupture complexities of the earthquake sequence.Since the estimated rupture velocity(-2.2 km)of the mainshock is slow,its rupture directivity effect is weak and mainly affects the peak ground velocity.However,the rupture directivity effects for the other three earthquakes is very strong and the rupture velocities are faster than that of the mainshock.The dependency of the rupture directivity effects on the period were also observed in the four earthquakes.(3)The Two-stage Stochastic Empirical Green’s Function(EGF)Method is used to simulate major YangBi earthquakes.In the Yangbi earthquake sequence,nine small earthquakes with the same focal mechanism solution as the main motion were selected as the empirical Green’s function(EGF).The stress drop of main motion was calculated by elliptic model.The ratio of stress drop of main motion to small motion was set to 1 according to the self-similarity of the Yangbi earthquake and adjusted by the regression equation.According to The Two-stage Stochastic Empirical Green’s Function(EFG)Method,the yangbi main motion was simulated by using the stress drop ratio before and after adjustment,and the PGA and PSA residuals were calculated and analyzed under the logarithmic coordinate axis with the base of 10.At the same time,the results obtained by two-step random EGF method were compared with the actual observed values and the results obtained by the ground motion prediction equation.The results show that the results after adjusting the stress drop ratio are better,and the results of simulating the same station record with different EGF are quite different,and the effect of ground motion simulation is affected by the distance and the direction of rupture.Compared with GMPE,ground motion simulation has certain advantages in predicting ground motion.