Shear Wave Splitting : An Indication of change in crustal stress

我们在日本许多地震台站的微震波列中发现了剪切波分裂，即由定向微裂缝引起的有效各向异性的诊断。在近木地区，我们发现，在京都大学阿布山地震观测站的四个台站中，有三个台站的较快分裂横波的偏振方向在所有到达方位和所有入射角都小于临界角。这些偏振方向与由地震断层面机制获得的最大压力轴一致，也与代表解理或层理方向的地质构造总趋势的方向一致。这些结果表明，日本以下脆性上地壳存在裂隙各向异性，但我们不能区分这是反映了现今应力场诱导的裂隙分布，还是反映了地表地质的总体趋势。尽管较快和较慢的横波之间的延迟时间很难可靠地估计，但由于它们对内部界面的高度敏感，延迟时间可以解释为平行垂直裂缝分布的结果，裂缝密度约为0.04。四个台站剪切波偏振方向的一致性或不一致性说明了地表地形和近地表分层对剪切波偏振的影响。

We have identified shear-wave splitting, diagnostic of the effective anisotropy induced by aligned microcrackes, in the wavetrains of micro-earthquakes at many seismic stations in Japan.In the Kinki districts, we find that the directions of polarization of the faster split shear-waves are nearly parallel for all azimuths of arrivals, and for all angles of incidence less than the critical amgle at three of four stations of Abuyama Seismological Observatory, Kyoto University. These directions of polarization are consistent with the axis of maximum compression obtained from earthquake fault-plane mechanisms, and also agree with the directions of the general trends of geological structures which represent the orientations of the cleavage or lamination. These results suggest that crack-induced anisotropy is present in the brittle upper crust beneath Japan, as has been found elsewhere, but we could not distinguish whether this reflects the distributions of cracks induced by the present stress field, or results from the general trends of surface geology. Although the delay times between faster and slower shear waves are difficult to estimate reliably, because of their high sensitivity to internal interfaces, the delay times can be interpreted as the result of a distribution of parallel vertical cracks with a crack density of about 0.04. The consistency or lack of consistency of the directions of the shear-wave polarizations at four stations demonstrates the effects surface topography and near surface layring on the shear-wave polarizations.