新疆伽师地震区三维Q值结构

THREE-DIMENSIONAL Q STRUCTURE IN JIASHI EARTH-QUAKE REGION OF XINJIANG REGION

利用中国地震局地球物理勘探中心 1 998年在伽师地震区记录的天然地震资料 ,通过地震记录波形资料的衰减来反演伽师地震区地壳介质的三维 S波 Q值结构 ,并初步了解该地震区内地壳深部断层的大致形态 .首先 ,对精选的 45 0多次地震记录的 S波求出其振幅谱 ,称为观测振幅谱 ;然后 ,假设震源符合布伦圆盘位错模式 ,在对仪器和场地效应校正之后 ,用非线性阻尼最小二乘法对理论振幅谱和观测振幅谱进行拟合 ,从而得到观测吸收特征时间 ;最后 ,利用三维射线追踪方法 ,对理论吸收特征时间与观测吸收特征时间进行非线性阻尼最小二乘拟合 .在拟合过程中 ,速度模型保持不变 .速度模型是通过三维走时层析成像得到的 ,只修改 Q值模型 ,当拟合达到给定精度后 ,便得到最终的 Q值模型 .结果表明 ,在 1 0～ 1 8km的深度范围内 ,存在一个北东向的低 Q值条带 ;在 1 2～ 1 8km的深度范围内 ,存在一个北西向的低 Q值条带 .这与他人所做的北东向、北西向的两个低速条带较接近 .不同的是 ,低 Q值条带深度范围比低速带的更大一些 .

D S-wave Q structure in Jiashi earthquake region is inverted based on the attenuation of seismic waves recorded from earthquakes in this region in 1998 by the Research Center of Exploration Geophysics (RCEG), CSB, and a rough configuration of deep crustal faults in the earthquake region is presented. First, amplitude spectra of S-waves are extracted from 450 carefully-chosen earthquake records, called observed amplitude spectra. Then, after instru-mental and site effect correction, theoretical amplitude spectra are made to fit observed amplitude spectra with nonlinear damped least-squares method to get the observed travel time over Q, provided that earthquake sources conform to Brunes disk dislocation model. Finally, by 3-D ray tracing method, theoretical travel time over Q is made to fit observed travel time over Q with nonlinear damped least-squares method. In the course of fitting, the velocity model, which is obtained by 3-D travel time tomography, remains unchanged, while only Q model is modified. When fitting came to the given accuracy, the ultimate Q model is obtained. The result shows that an NE-trending low Q zone exists at the depths of 10~18 km, and an NW-trending low Q zone exists at the depths of 12~18 km. These roughly coincide with the NE-trending and the NW-trending low velocity zones revealed by other scientists. The difference is that the low Q zones have a wider range than the low velocity zones.