Coda wave Q values of shallow media in Binchuan basin using dense array data

In order to obtain more accurate attenuation parameters of shallow medium and carry out more detailed ground motion simulation research in the field of engineering earthquakes, this paper uses the Sato single scattering model to study the coda attenuation characteristics of shallow medium in the Binchuan basin based on the dense array data and combined with natural seismic data. The results showed that with the increase of frequency, the coda Q value Q_c showed an increasing trend, which was in line with the frequency dependence of the Q value. The spatial distribution of Q_c values in this area has obvious lateral inhomogeneity. The stations located in the Binchuan basin in the central part of the study area have lower Q_c values, while the stations located in the hilly areas of the study area in the southwest and northeast directions have higher Q_c values, which is consistent with the velocity tomography results. The frequency dependence of the average Q_c in the study area is ${Q}_{{\rm{c}}} （ f ） ＝28.04{f}^{1.07}$. The Q₀ value obtained by the dense array data is lower than that obtained by the natural earthquake, which proves that the results obtained by the dense array data reflects the attenuation characteristics of the shallower medium. And a higher $\eta$ value means that the inhomogeneity of the shallow medium is higher than that of the deep medium, which is in line with the actual situation. The Q₀ value of the Binchuan basin is larger than the Q₀ value of the Songliang basin, the North China basin and the average sedimentary layer of the Chinese mainland, while the index $\eta$ is smaller than the $\eta$ value of the Songliao basin and the Chinese mainland, indicating that the information reflected by the dense array is between the near-surface and deep media. As the depth increases, the inhomogeneity of the medium gradually decreases, and the dependence of the Q value with frequency gradually weakens. Q₀ increases with the lapse time window of coda, and the index $\eta$ is the opposite. It is necessary to select a small lapse time window to ensure the accuracy of the results when the scale of study area is small and the magnitude of the selected natural earthquake events not large. In addition, local topographic and seismic-tectonic changes may lead to large differences in attenuation parameters and standard deviations. The useage of airgun source and near-field earthquake with small-scale dense seismic arrays can obtain more accurate attenuation parameters in shallow media, which provides a new idea for engineering application and shallow structure detection.