利用主动震源检测汶川地震余震引起的浅层波速变化

为了监测汶川地震后断裂带附近的波速变化,2008年6月,我们利用快速响应探测系统,在龙门山断裂带北端进行了为期3周的连续观测实验.实验使用电动落锤作为主动震源,GPS连续同步授时的高精度数采作为接收系统.利用记录到的高时间精度观测数据,结合互相关时延检测技术,计算了P波和面波走时的相对变化,来表征地下波速的变化.我们观测到两次面波波速的异常变化,变化幅度最高达到2%,远远大于气压变化所能引起的波速变化.结合该时段内地震活动记录,我们认为面波的波速变化是由在附近发生的两次地震事件的同震效应引起的,该观测结果同理论计算结果相符.

Using Active Source to Monitor Velocity Variation in Shallow Sediment Caused by the Wenchuan Earthquake

Seismic velocity near the fault zone is sensitive to the stress state within that area. After the Wenchuan earthquake, in order to monitor the velocity variation near fault zone, we conducted a field experiment in the north edge of Longmenshan fault, which lasted three weeks since July 6. In the experiment, an electric hammer was used as a repeatable seismic source and the seismic signals were recorded by eight short period seismometers composed of Guralp 40T sensor and Reftek 130B digitizer. The digitizers were timing continuously with GPS to reduce timing error. We calculate the travel time variation by the combination of absolute time and cross correlation time-delay measurement. The fractal velocity variations were obtained from travel time measurement. Two prominent drops up to～2 % were found, which were much more than the possible variation caused by barometric pressure change. We notice two moderate aftershocks with magnitude 4.9 and 4.7 occurring within 10km from the experiment field, when the two surface velocity drops showed up. We speculate the surface velocity drops may be caused by the coseismic effects of these two earthquakes. The theoretical calculation of the stress drop agrees with our measurement.