3-DIMENSIONAL P-WAVE VELOCITY STRUCTURE UNDER THE BEIJING NETWORK AREA

An inversion study of 2447 teleseimic P-wave travel time residuals observed at Beijing network is given in this paper, using the spatial frequency domain expansion parameterization method proposed recently by the writers. It is proved that the new method can be effectively applied to real data. The results show that there are obvious lateral heterogeneities deep to 100 km beneath the area. The velocity anomaly images in the upper 50 km show certain correlation with the shallow geological features: high velocity zone beneath uplift areas and low velocity zone beneath depressional areas. Below the depth of 100 km, this correlation disappears gradually and the amplitudes of the anomalies decrease. Three prominent low velocity anomalies have been found within the studying area. Among them, the Tianjin-Tangshan anomaly has maximum amplitude. It extends about 100 km in horizontal direction and continues down to a depth of nearly 100 km. Below it, there is another low velocity anomaly. Almost all the strong earthquakes occurred within this area are situated in the transition zone between these high and low velocity anomalies.