伽师强震群区上地壳三维速度层析成像

TOMOGRAPHIC DETERMINATION OF THE UPPER CRUSTAL STRUCTURE IN JIASHI STRONG EARTHQUAKE SWARM REGION

以伽师强震群为中心布设了一个 5 0km× 6 0km范围的三维人工地震透射临时台阵 ,接收来自不同方位的 8次爆破激发产生的地震波 ;利用所记录到的莫霍界面临界反射P波、S波走时 ,采用模型不分块反演技术 ,重建台阵下方上地壳三维P ,S波速度扰动图象 ,以及vP/vS 扰动分布图象 ;并结合伽师震群的地震活动分布 ,对该震群的成因进行了分析 .结果表明 ,研究区上地壳存在明显的不均匀性 .自 12km深度开始 ,在与震群震中相应的位置上 ,明显出现沿北北西向的高P波速度块体 ,在它的周围为相对低速分布 ,这种结构上的差异是伽师强震群发生的最直接原因 ;vP/vS 在相同的位置上为高值分布 ,则表明由于介质相对软弱可能造成震源体抗剪强度下降 .

A three-dimensional temporary seismic transmission array was arranged in a 50 km×60 km region around Jiashi strong earthquake swarm to receive seismic waves generated by 8 fires from different azimuths. With the inversion method without model blocks and using P and S reflections from Moho at critical distances, the 3-D images of P, S velocity perturbation and ratio v P/v S perturbation of the upper crust under the seismic array were reconstructed. Meanwhile, the seismicity of the Jiashi earthquake swarm was taken into consideration in the analysis of the seismogenesis. The results indicate that the upper crustal structure under the Jiashi strong earthquake swarm region is characterized by significant inhomogeneity both laterally and vertically. From 12 km depth, it is clear that there is an NNW-oriented high P-wave velocity anomalous body corresponding to the epicenter of the swarm with low-velocity anomaly around it, which is the direct cause of the strong earthquakes. High v P/v S is distributed in the same location, which may indicate the decline of shear strength of the source region owing to relative softness of the medium, this can be accounted as an explanation for the seismicity feature of the Jiashi strong earthquake swarm.