四川攀西地区介质衰减特征研究

利用四川区域地震台网的数字波形资料以及中国地震台网中心的有关震相数据，选取互相衔接的3段地震波的几何衰减模型，利用基于遗传算法的Atkinson和Mereu方法，反演得到四川攀枝花-西昌地区的介质品质因子Q(f)，并采用Brune的ω平方模型约束震源位移谱，使用Moya方法联合多台多地震数据求得该地区各地震台站的场地响应。结果显示，2008-05~2013-12期间攀西地区介质衰减模型为Q(f)=94.4f0.665 9，2014-01~2015-06期间该地区介质衰减模型为Q(f)=83.9f0.719 6，两个时间段的反演结果十分接近，且均呈现出低Q0和较高衰减系数γ的特点，即该地区地震波衰减明显偏高，同时Q对频率f的依赖较大；场地响应方面，岩石台基对地震波信号在不同频率点的放大作用不同，其中MGU、PGE和SMK地震台场地增益较大，YYU、YYC地震台场地放大倍数在1~2之间平稳变化，LGH地震台场地响应随频率增大而快速衰减。结合上述计算结果推测，攀西地区构造活跃、上地壳结构极不均匀、地震波能量衰减强烈是造成该区域Q0低异常的主要原因。

Study of Attenuation Characteristics and Site Response in the Panzhihua-Xichang Area

Using digital waveform data recorded by region seismic networks along with relevant seismic phase data collected by the China seismograph network, we calculate the attenuation characteristics and site response for the Panzhihua-Xichang area. The frequency dependent Q(f) is obtained by the iterative grid-search technique described by Atkinson and Mereu, based on a trilinear geometrical spreading model. The source spectra is determined by the model of Brune and the site responses of seismic stations derived by Moya’s method, which uses genetic algorithms. Our results demonstrate that the associated model for the regional quality factor for frequencies can be expressed as Q(f)=944f0.665 9 (from May, 2008 to December, 2013）and Q(f)=83.9f0.719 6 (from January, 2014 to June, 2015）for the Panzhihua-Xichang area. Site response results indicate that different stations show different amplifications. Site amplifications in the Panzhihua-Xichang area are between 1.13 and 6.93 times. For tectonically stable cratonic regions, the Q0 is usually high; in contrast, for tectonically active regions, such as the Panzhihua-Xichang area, the Q0 is generally low. Broken media caused by strong tectonic activity or upper-mantle heterogeneity and velocity structure may be the main reasons of low Q0 values in this area.