SKS波对地壳裂隙各向异性的响应--理论地震图研究

A study of synthetic seismograms for SKS-wave response to crustal fractured-induce anisotropy

穿透含裂隙、裂缝地壳8s视周期的SV波的理论地震图研究表明,当地壳平均裂隙密度高于0.01即横波各向异性高于1%时,非对称面内不同方位的SKS波均发生分裂;地震图中直接的记录显示是切向T分量上出现SKS波的振动,其振幅随地壳平均裂隙密度的增大而增强,甚至能与径向R分量上的振幅相当.局限于上地壳的强裂缝各向异性同样能引起SKS分裂.长周期SKS波分裂对地壳内裂隙、裂缝的分布缺乏分辨率.直立平行排列裂隙、裂缝使得SKS分裂T分量记录特征具有方位对称性,这来自于HTI介质中快、慢波偏振和到时差随方位变化的对称性;而倾斜裂隙、裂缝使得该方位对称性丧失.对实际观测SKS分裂的偏振解释需要考虑地壳裂隙各向异性,特别是断裂附近的强裂缝各向异性.

The study of synthetic seismograms of SV-waves that penetrate the crust containing fissures and cracks of 8 seconds apparent period shows that, SKS waves in different directions of asymmetrical plane will be split,when the average crack density of the crust is higher than 0.01, which means shear wave anisotropy higher than 1 %. It is directly showed in seismic records that vibration of SKS wave occurs in tangential component T, and its amplitude increases with average crustal fracture density, even comparable to the amplitude of the radial component R. The strong anisotropy limited to the upper crust can also cause SKS-wave splitting, and long period SKS- wave splitting does not have a good resolution for the distribution of fissures and cracks in the crust. Vertical parallel fissures and cracks make T component recording of SKS splitting have a characteristic of azimuth symmetry, which comes from the symmetry of fast and slow-wave polarization and arrival time difference in HTI medium varying with azimuth; while inclinedfissures and cracks make that azimuth symmetry lost. The actual interpretation of the observed SKS splitting polarization need to consider the crust fracture anisotropy, especially strong anisotropy nearby the faults.