Abstract: Starting with the method and theory of multistation velocity ratio calculation,we thoroughly analyzed the differences between velocity ratios calculated by different formulae.Through mathematic demonstration,it is thought that the basic theory of two systems of calculation methods is coherent,and the discrepancy of velocity ratios mainly results from the selection of independent variable.In view of the effects of different seismographs with different periods at different stations,t_s-p is selected as independent variable in this paper.Based on the data of P and S wave travel times of local earthquakes recorded at 52 digital and analog stations located in Shandong Province and northern Jiangsu from Jan 2002 to Jul 2005,seismic events recorded at more than 4 stations are collected before calculation.The results show that,the average velocity ratio of whole area is 1.716,which is close to and a little higher than the velocity ratios at other areas in North China.To ensure reflecting the local feature,the studied area is divided into units of 1°×2°,and some far stations relative to epicenters are eliminated.Comparing velocity ratios of different units in the studied area,it is found that the values at 34°～37°,115°～119°are obviously higher than that of other units,which are located nicely at the two sides of Tancheng-Lujiang Fault,and it is supposed to be closely related to underground media properties and deeper focal depth.The main results obtained are as follows:(1)The discrepancy in velocity ratios by two systems of calculation methods is related to the collection of independent variable.In fact,the theory is consistent.So in the future calculation of V_p/V_s,it is reasonable to choose more accurate variation as independent variable;(2)The velocity ratios in Shandong area are a little higher than that in other areas of North China,especially at the middle and southern segments of Tancheng-Lujiang Fault,which should be correlated to the thin deposit layer at surface,distribution of granite and high pressure metamorphic rocks in upper crust at the two sides of the fault,and the averaged crustal effects due to deep focal depths in this area;(3)The residual rms error is closely related with the number of stations.When data is plenteous,we should pick out those seismic events which can not be located accurately and choose those events recorded by much more stations in order to assure the higher precision and credibility of results.

Key words: velocity ratio, P wave travel time, S wave travel time, correlation coefficient, Shandong area

摘要： 文中从多台计算波速比方法的原理入手,对不同公式计算的波速比差异进行了深入分析,认为在原理一致的条件下,波速比差异主要与拟合过程中自变量的选取及2组数据的相关系数有关。利用2002年1月至2005年7月山东省境内及江苏省北部共52个数字及模拟台站的地方震P波和S波到时资料,选取有4个以上记录台站的地震。整个研究区域的平均波速比为1.716,反映了这个地区的波速比相对于华北其他地区偏高。将研究区域划分为1°×2°的单元,并剔除距离震中过远的台站,以保证区域特征能得到反映。通过对比不同单元的平均波速比发现,34°～37°,115°～119°的平均波速比明显高于其他单元,而它们正好分布在郯庐断裂的两侧,应该与该地区上地壳的岩石性质及震源深度分布范围较宽导致的平均效应有关。

关键词: 波速比, P波到时, S波到时, 相关系数, 山东