基于地震背景噪声的四川威远地区浅层速度结构成像研究

Seismic ambient noise tomography for shallow velocity structures beneath Weiyuan,Sichuan

地震S波速度是防震减灾中场地分类、强地面震动模拟建模等的重要参数.地震背景噪声成像方法可以重建地下浅层三维结构以及探测浅层速度结构变化,为页岩气开采提供参考,有效规避地震灾害风险.本文收集了2015年11月至2016年2月间四川威远地区50个流动台站记录到的垂直分量连续波形记录,利用波形互相关提取2~6 s的基阶Rayleigh波经验格林函数,采用一维地壳结构模型拟合相速度频散曲线,获得了该区域近地表5 km以内的三维S波速度模型.结果表明,威远地区深度2 km以内的S波速度为2.0~2.7 km·s^-1;2.0~5.0 km深度的S波速度横向分布不均匀,西北地区大于2.7 km·s^-1,东南地区在2.3~2.8 km·s^-1之间,分别与背斜构造和较厚的盆地沉积层相对应.3个剖面图均表明S波速度随着深度的增加而逐渐增大,并且在台站S29附近的页岩气田钻井底部与2016年1月7日四川威远M L3.9地震震源位置较为接近,均处于松散的沉积盖层与较为坚硬的花岗岩基底的分界处,推测它们之间可能存在一定的联系.

Seismic shear wave velocity is an important parameter for site characterization and strong ground motion modeling.Reconstruction of the shallow 3D seismic structures based on Ambient Noise Tomography(ANT)provides key information to the variable shallow seismic structures,the exploitation of shale gas and earthquake hazard reduction and mitigation.In this study,we perform interstation cross-correlations of vertical-component continuous waveform recorded by 50 temporary stations deployed in Weiyuan,Sichuan Province from November,2015 to February,2016.We then measure phase-velocity dispersion curves of fundamental-mode Rayleigh waves in the period band of 2~6 s and invert 3D shallow seismic shear-velocity structures within 5 km.Our results show that shear velocities within 2 km are relatively lower at about 2.0~2.7 km·s^-1.Shear velocities at the depth of 2.0~5.0 km show some spatial pattern that they are faster at the northwestern of the study region(over 2.7 km·s^-1)and slower beneath the southeastern part(2.3~2.8 km·s^-1),which might be related to the Weiyuan anticline and the basin sediment respectively.The cross-sections show that shear velocities generally increase with depth.And the January 7,2016 M L3.9 Weiyuan earthquake occurred near the bottom of a drilling platform where there might be the boundary between the sedimentary basin and the granite basement,which indicates the possible connection between them.