利用数字地震记录研究唐山震区台下的P、S波速度结构

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

直接采集数字地震记录的原始 P波、S波记录波形 ,与利用 Haskell矩阵传递方法得到的理论综合地震图进行对比 ,通过不断调整介质模型 ,最后得到台站下方的体波速度结构模型 ,此方法中得到的 S波速度结构不同于通过其他波形转换而来的结果 ,而是同 P波一样从原始波形拟合而来 ,同时获得地震台下方 P、S速度介质模型 ,这为我们讨论地下介质的性质提供了更多的信息。数字地震波形资料取自于唐山市遥测地震台网陡河和滦县的短周期地震记录 ,共选用了震中距范围为 30～ 60°、震源深度为 1 0 0～ 60 0 km的 5个不同地点的深源远震记录资料。结果表明 ,台站下方的纵波和横波速度分层结构相当吻合 ,说明拟合结果相当可靠。两个台站下的地壳厚度均为 38km,速度结构在 1 3km的低速层处出现差异 ,滦县的低速层比陡河要更厚一些。两个台站的地壳速度结构都是高低相同 ,而且陡河的高低差异较滦县更为显著。唐山震区滦县部分的震源比陡河附近的浅 ,而 1 976年唐山 7.8级地震发生在陡河附近 ,可见地震的发生与壳内低速层的差异幅度有关。从泊松比高值来看 ,地幔顶部存在低速层 ,该区域是地幔上隆区 。

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.