中文摘要：

收集了甘肃岷县漳县Ms6.6地震序列固定台站和震后6个流动台站震相观测报告,提取了P波、S波走时信息,采用多台和达法计算该地震序列波速比,发现震后较强余震发生前波速比趋势性下降的特征.这对后续震情跟踪和趋势判定过程中强余震预测有一定的参考价值和指示意义.

英文摘要：

Collecting fixed stations and six ambulatory stations observational report of Minxian-- Zhangxian Ms6.6 earthquake sequence, extracting travel-time information of P-wave and S-wave, using the multi-station method to compute wave velocity ratio of this earthquake sequence. The results show that there is a decreasing trend of velocity ratio before strong after-shocks. It is of reference value and indicative significance on the short-imminent term prediction of strong after- shocks. Although the wave velocity ratio anomaly is still a controversial issue, domestic and foreign scholars have made some achievements in theory and practice. In 1928, the Japanese seismolo- gists Wadati Kiyoo first proposed the calculation method of wave velocity ratio： multi-station method. In 1962, the Soviet Union scholar Semionov published the first results of velocity abnor- mal in Garm region, found wave velocity anomaly and wave velocity ratio anomaly before earth- quake magnitude greater than 3.5. In the early 1970s, Japan, the United States and other coun- tries have also found a lot of wave velocity anomaly before earthquake. Especially American scholar Aggarwal found the change of wave velocity ratio anomaly before several small earthquake （the maximum magnitude is 3. 3} in earthquake group and proposed to explain the anomalous with DD mode. At the same time, domestic scholars got some useful results in this field. In 1970s, Feng et al. summed up in the evolution of the crust, physical properties of medium will produce a series of changes, such as the emergence of micro fracture, expansion, seismic waves through the earth＇s crust medium, and velocity will change accordingly, which is an important ba- sis for the study of the wave velocity ratio of the medium. Wang et al. （2011） made a further re-search of variation characteristics of wave velocity ratio before and after the Wenchuan earth- quake. Feng et al. （2009） studied the average wave velocity ratio in the northeastern margin of Qinghai--Tibetan Plateau, and f