Rayleigh-wave phase velocity distribution in Shanxi region

In this work,seismic datas are taken from seismograms of over 100 earthquakes which are recorded from Feburary in 2009 to November in 2011 at 31 stations in Shanxi Province and other 6 broadband stations in adjacent areas,including Hebei,Henan,Shanxi and Neimeng Province.We obtained 350 high-quality Rayleigh wave phase velocity dispersion curves of fundamental mode by removing the duplicate paths and low quality data.The periods of the dispersion curves range from 8 to 75 seconds.Based on the method of Ditmar Yanovskaya,we obtained phase velocity dispersion maps in 33 periods with resolution ranges from 40 to 50 km.We analyzed the phase velocity distribution maps at four representative periods and the phase velocity section maps along three profiles.The phase velocity maps reveal the lateral heterogeneity of the velocity structure and the phase speed variation with depth in the crust and the upper mantle of the Shanxi fault depression zone.The phase velocity map at 10 s clearly shows the spatial differences between the rift zone and the uplift zones in the two sides,presenting low phase velocity anomalies in the maximum depression regions near the centers of several basins inside the rift zone.Moderate to strong earthquakes(M≥6)in Shanxi are mostly concentrated in the transitional zones where dramatic phase velocity changes occur at 15 s.At 20~26 s period,significant phase velocity difference can be observed across the latitude of 38°N,where phase velocity is higher in the south side than that in the north,which is in consistent with the variation pattern of Moho depth in the depression zone.This kind of velocity pattern keeps constant with the increase of period,which is consistent with the feature that the blocks in the north Shanxi is relatively weaker than those in the south.Along 113°E,the phase velocities at periods of 25~75 s are higher in the north side of latitude 38°N than in the south.This is consistent with the result of the lithospheric section in the same region,and the undulations of Moho depth shown in the other two profiles are also agree with the results from active source studies.