关键词: 波速比, 低速层, 强震区, 弱震区, 地质构造

Abstract: In this paper,based on available S wave and P wave velocity structures beneath 16 seismic stations in North China,wave velocity ratios for every velocity layer beneath these stations are calculated.According to their seismicity and geographic locations,these stations are divided into three types and discussed separately.In macroseismic area,the V_P/V_S ratio is systematically higher than that in microseismic area,and the V_P/V_S ratio of some low-velocity layers in mid-lower crust is greater than 1.8. In seismically quiet area or at interior of stable block,however,the V_P/V_S ratio is relatively stable for the whole crust and upper mantle,in many layers of which the ratio is less than the normal V_P/V_S ratio(1.732).In transitional area,such as the marginal area of basins or stable blocks,the number of layers with high V_P/V_S value becomes less,but the average V_P/V_S values beneath the stations are generally still greater than 1.732.Moreover,beneath the stations in microseismic area,a layer of more than10km thickness is recognized,which has never been found in macroseismic area and may reflect the old continental nucleus or crystalline basement.Stations in the same tectonic unit of transitional area have the same S wave velocity structure,which might be the result of regional tectonic activities. In addition,the contour maps of V_P/V_Sand S wave velocity at 10km,15km,20km and 25km depths were plotted and analyzed.The results showthat at 10km depth,taken the Taihang Mountains as a boundary,the V_P/V_Svalues in the western area are higher than those in the eastern area;but at greater depth,the V_P/V_Sis higher in the eastern area,and especially much higher in Xingtai earthquake area at 25km depth.It can be seen from S wave velocity images,that the velocity differences between the eastern and western areas separated by the Taihang Mountains are very significant all the time,and there is no evidence of mixing-up even at 25km depth.This may indicate that the Taihang Mountains is an evident material variation belt,which extends to a great depth. The apparent differences of V_P/V_Sin macroseismic and microseismic areas are of great significance to prediction of potential risk area of strong earthquake,while the differences in velocity structures and wave velocity ratio beneath various geological structures may provide some leads for the study of the history of local tectonic activities.

Key words: wave velocity ratio, lowvelocity layer, macroseimic area, microseismic area, geological structure