Shear-wave splitting above small earthquakes in the Kinki district of Japan

We have identified shear-wave splitting, diagnostic of the effective anisotropy induced by aligned microcracks, in the wavetrains of micro-earthquakes at four stations of the Abuyama network in the Kinki District of Japan. We find that the directions of polarization of the faster split shear-waves are nearly parallel for all azimuths of arrival, and for all angles of incidence less than the critical angle at three of the four stations. 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 the four stations demonstrates the effects of surface topography and near surface layering on the shear-wave polarizations.