STUDY ON THE P AND S WAVE VELOCITY STRUCTURE UNDER TANGSHAN REGION WITH DIGITAL EARTHQUAKE RECORD

The original P and S wave pattern collected directly from digital earthquake record is compared with theoretical seismograms got from Haskell matrix transmitting.Through adjusting model,the body wave velocity structure model for the medium beneath the stations is obtained.THe S wave velocity structure in this method,which is different from the results that transformed from other wave,is obtained from simulating the original S wave like we did with the P wave.Meanwhile we also got the P and S velocity medium model.It will provide more information when we discuss the characters of the medium. The digital earthquake records were collected from Tangshan telemetry network.We selected five short-period earthquake records of Douhe and Luanxian station,which the range of epicentral distance is 30°~60°,focal depth range is 100~600km.The results show that the layered structure of longitudinal wave coincides well with the transversal wave.It proves the results are reliable.The thickness of earth crust under both the two stations is 38km.The difference appears in the low velocity layer under 13km deep,where the low velocity layer of Luanxian is thicker than Douhe.The high and low velocity layer under the two stations appears alternately,and the difference from high to low in Douhe is more distinct than in Luanxian.The focus of Luanxian in Tangshan seismic area is shallower,but the main earthquake of Tangshan in 1976 took place in Doune area,so it is clear that the occurrence of earthquake is related to the amplitude between high and low velocity layer.From the view of high Poisson ratio,There are low velocity layers on the top of mantle in the region.The region is a swelling area of mantle and there may be some partial melt.