基于波场梯度法研究安宁河—则木河断裂带速度结构

Seismic velocity structure for the Anninghe-Zemuhe fault zone by wave gradiometry analysis

不同尺度的密集台阵为波场梯度法的发展提供了条件,与其他成像方法相比,波场梯度法在拓展应用上还相对较少.本文将波场梯度法应用于小区域的相对高频的地震波形获得高分辨率的速度结构.首先将台站数据网格化,利用网格点上波形数据得到相速度、方位角变化、几何扩散和辐射花样.研究区域为安宁河—则木河断裂带上的冕宁—西昌—越西区域,采用波场梯度法获得了该区域5~15 s Rayleigh面波的相速度,并反演出上地壳S波速度结构.结果显示沿断裂带浅层地壳的低速异常与张裂断陷沉积形成的昔格达地层特征相一致.安宁河断裂带附近有明显的高速异常,在不同深度高速异常都有着明显的变化,与研究区域花岗岩的分布相对应.不同深度高速体的分布可能与花岗岩体在上地壳的侵入有关.西昌地区上地壳的低速异常反映了较厚的沉积层.速度结构与实际地质构造基本吻合,表明利用波场梯度法可以有效地计算小区域高分辨率三维速度结构.本文将速度结构与不同深度的地质构造资料和震源分布相结合,为该研究区域上地壳的构造研究提供了可靠参考.

The dense seismic arrays in different scales are crucial for the Wave Gradiometry analysis(WGA).However,the application of the WGA has been limited compared to other imaging methods.In this paper,We apply the WGA to relatively high-frequency seismic waveforms in small-scale region to obtain high resolution velocity structure.Firstly,the waveforms from an irregular seismic network are gridded,and the waveforms on grid points are used to obtain phase velocity,azimuth variation,geometrical spreading and radiation pattern.The study region is located around the Mianning-Xichang-Yuexi area along the Anninghe-Zemuhe fault zone.The phase velocity of periods 5~15 s are obtained by using the WGA,and the S-wave velocity structure of the upper crust is inverted.The low-velocity zone of the shallow crust along the fault zone is consistent with the Xigeda stratum which is formed by the sedimentation along the tensional fault.There are high-velocity anomalies near the Anninghe fault zone and they correspond to the granite in the study area.The high velocity distribution at different depths may be related to the intrusion of granite in the upper crust.But the low-velocity anomaly of the upper crust near the Xichang area reflects a thicker sedimentary layer.The inversion results are basically consistent with the geological structure,and it suggests that the WGA can be used to calculate the high-resolution 3D velocity structure effectively in a small area.The velocity structure,combined with geological structure data and the seismicity distribution at different depths,provides crucial constraint on the tectonics of the upper crust of the study area.