芦山M_S7.0强震前单台波速比变化特征研究

Temporal variation of v_P/v_S at single seismic station before the 2013Lushan M_S 7.0earthquake

在搜集和整理中国地震局四川数字地震台网2001年1月—2013年7月震相观测报告的基础上,采用单台多震和达测定法,重点分析研究了芦山地震前波速比时空变化特征.为保证计算结果的可靠性和稳定性,对所收集到的单台震相数据和计算结果的精度进行系统的筛选和限定,同时对出现异常的地震台,进行不同台站方位和距台站不同空间范围的波速比时空变化特征的分区细化对比研究.根据其存在的共性和差异,探讨预测孕震体空间尺度的可能性.结果发现,龙门山断裂带南段及附近地区的小金、蒙顶山、油罐顶、花马寺和五马坪等5个台站的波速比在芦山地震前均存在较明显的降低过程;小金、蒙顶山、油灌顶和花马寺等4个台站的波速比异常降低过程大约持续了3年半,震前除小金台未见明显回返过程外,其它3个台站均在震前1年左右出现回返特征;峨眉山台和五马坪台的波速比时间演化特征差异性较大,表现出一定的复杂性.从芦山地震前波速比时间变化特征的动态跟踪中还发现,强震前波速比异常区的范围具有向未来震中及其附近地区收缩的趋势.

Based on ＊he seismic phase data recorded hy Sichuan Digital Seismic Network from January of 2001 to July of 2013, this study analyzes the spatio temporal variation of wave velocity ratio Vp/Vs before the Lushan h4s7.0 earth- quake by using Wadati method of single station and multi-earthquake. To ensure the accuracy and stability of the calculated results, we carefully winnow the data. Moreover, we compare the wave velocity ratios calculated by using phase data recorded by the stations with different azimuths and different epicentral distances, so that we discuss the size of potential seismogenic body for future earthquakes. Our results show that, in the southern Longmenshan fault and its surrounding area, there was an obvious decrease in wave velocity ratio at five stations （XJI, MDS, YGD, HMS and WMP） before the Lushan Ms7. 0 earthquake. In particular, for the four stations （XJI, MDS, YGD and HMS）, there had been an obvious vp/Vs decrease for 3.5 years before the Lushan earth- quake~ except the station XJI, at other three stations appeared a recovery of vp/ vs ratio one year before the earthquake. The anomalous characteristics are largely different for the stations EMS and WMP, indicating some complexity. Tracing the temporal variations of ve/Vs, it is also found that the extent of the region with anomalous wave velocity ratio tends to concentrate at the future epi- center and its surrounding regions.