采用云南数字地震台网记录的2011年1月1日至2018年7月31日ML≥2.0地震的震相到时数据,利用单台多震和达法计算了云南地区51个台站的波速比,分析了2018年通海5.0级地震前波速比时空演化特征。结果 表明:(1)通海地震前震中附近波速比以高值变化为主要特征,有4个台站在高值变化前经历了较长时间的下降—回升过程;(2)波速比变化出现的时间与震中距有一定关系,距离震中较近的TOH台波速比开始下降、回升和达到高值的时间都比稍远的JIS,YUJ和GEJ台要晚;(3)2015—2017年云南4次MS≥5.0地震震中均位于发震前一年波速比均值降低或正负变化交界地区,此次通海地震也遵从了上述规律,震中位于2018年1—7月波速比升高和降低区域的交汇地带。初步分析认为这次地震前波速比升高可能是地震孕育过程后期震中周围有流体渗入,流体的参与使得前期因应力增强造成的微裂隙达到了水饱和状态,从而导致波速比vP〖JB<1/〗vS增大。
Based on the phase arrival time data of the ML≥2.0 earthquakes recorded by Yunnan Digital Seismic Network from Jan.1,2011 to Jul.31,2018,we calculated the wave velocity ratios of 51 stations by using the Wadati method of single station and multi-earthquake,and analyzed the temporal and spatial evolution characteristics of wave velocity ratios before the Tonghai MS5.0 earthquake on Aug.13 and Aug.14 in 2018.The results show that:(1)Before the Tonghai earthquake,the mian characteristic of the wave velocity ratio near the epicenter shows high value changes.The wave velocity ratio of four stations experienced the process of decline-recovery in a long period before the high value.(2)There is a certain relationship between the occurrence time of wave velocity change and epicentral distance.The time of wave velocity ratio begins to decline,rise and reach high value of the Tonghai Station which is closer to the epicenter is later than that of Jianshui,Yuanjiang and Gejiu Stations which are farther from the epicenter.(3)The epicenters of the four MS≥5.0 earthquakes in Yunnan from 2015 to 2017 located at the area of decreasing or the positive and negative changes of wave velocity ratio in one year before the earthquake.And the location of epicenter of Tonghai earthquake follows the same law,which is the intersection zone of increasing and decreasing wave velocity ratio from Jan.to Jul.in 2018.The preliminary analysis suggests that the increasing of the wave velocity ratio before the earthquake may be due to the fluid infiltration around the epicenter during the late stage of the seismogenic process.The participation of fluid makes the micro-crack caused by stress enhancement in the early stage reaches the state of water saturation,resulting in the wave velocity ratio vP〖JB<1/〗vS increased.
