Study On Q Value Of Shallow Media In Binchuan Basin,Yunnan Using Dense Array Data

It is one of the core issues of engineering seismology and structural seismicity to quantitatively predict the earthquake damage that the engineering site may suffer,and to provide reasonable ground motion input for the seismic design of building structures and major projects,especially in the near-field and near-fault areas.Affected by the station density and the spatial and temporal inhomogeneity of strong earthquake distribution,there are few actual seismic records that meet the application conditions.Therefore,it is necessary to obtain the ground motion input by means of artificial simulation,which is suitable for different structures and sites.In the current ground motion simulation research,scholars pay less attention to the parameter quality factor(Q value)in the propagation medium.The Q value calculated from far-field natural earthquakes cannot reflect the media properties of local or shallow sedimentary layers well.Therefore,how to obtain an accurate Q value of near-field shallow media is a hot issue in ground motion simulation.Artificial hypocenters greatly make up for the shortage of natural earthquakes in spatial and temporal distribution,and the development of dense seismic arrays further improves the ability of regional-scale structure detection.Therefore,it is a new attempt to combine artificial seismic sources with dense arrays to explore the Q value distribution of shallow media,so as to provide more accurate Q value parameters for ground motion simulation.Based on the dense array data,this paper attempts to calculate the Q-value distribution of shallow media in the Binchuan Basin by using artificial airgun sources and natural earthquake events with small epicentral distances,and verifies the feasibility of using airgun sources to obtain the Q-value of shallow media.The spatial distribution of the Qc value of the coda wave in the Binchuan Basin is obtained,and the influencing factors in the calculation process are discussed quantitatively.These studies will provide more accurate medium attenuation parameters for near-field ground motion simulation studies in engineering seismology.The results of this paper mainly include:1.Based on the data of 37 stations randomly selected in the dense array of airgun sources in Binchuan,Yunnan,the Q values calculated by the amplitude-distance attenuation method and the coda normalization method are in line with the actual situation,which verifies the feasibility and rationality of calculating the quality factor of shallow media by the dense array data of airgun source:(1)The coda normalization method uses the ratio of the S-wave spectrum to the coda-wave spectrum of the same records,the stability of the results is higher than that of the amplitude-distance attenuation method.(2)Both topographic and local site factors may lead to abnormal amplitudes of stations,affecting the accuracy of the results.(3)The Q value obtained by using the dense array data of airgun source is smaller than the result of natural earthquakes,and is between those of the hard soil and sedimentary rock and is similar to the Q value in the North China Basin,corresponding to the shallow velocity structure in this area,which further proves that the plausibility and accuracy of the estimated Q value.2.Based on the data of 375 stations,the Sato single scattering model was used to obtain the coda attenuation characteristics of the shallow media in the Binchuan area combined with the airgun source and natural seismic data:(1)With the increase of frequency,the Qc value showed an increasing trend as a whole,which was in line with the frequency dependence of Q,and has obvious lateral inhomogeneity spatially.(2)The spatial distribution of Qc value is highly correlated with the distribution of velocity and lithology.(3)The information reflected by the dense array is between the near-surface and deep media.(4)The attenuation parameters of each station are quite different,and the frequency dependence index η value generally fluctuates around 1.0～1.1.Local topographic and seismic geological structural changes may also cause dramatic changes in attenuation characteristics.3.Different factors have different influences on the calculation results.The airgun source,smaller epicenter distance and narrower lapse time window of the coda reflected shallower media information than the traditional natural source method.Fro applications such as near-field ground motion simulation,which requires the regional average Q values of the media,the attenuation parameters of shallow media can be obtained with a more sparse station distribution.A relatively stable coda Q value can be obtained when the time window length is 5 s and the starting point is 2 times the S-wave arrival.(4)Under near-field conditons,the strong ground motion characteristics simulated by the shallow media attenuation Q value calculated in this study are closer to the actual records,and the simulation results are more accurate,which further proves that the method and data used in this study can accurately obtain the inelasticity of shallow media and provide more reasonable attenuation parameters input for engineering seismology.