DEPTH-DEPENDENT CRUSTAL ANISOTROPY AT MIDWESTERN HONSHU, JAPAN

Aligned cracks, possibly induced by tectonic stress, produce anisotrop y in the Earth's crust. Permeability and state of stress at several de pths in the crust can be estimated from the state of crack distributio n. We present clear evidence that crustal anisotropy changes with dept h at the Inagawa earthquake swarm area, mid-western Honshu, Japan. The observed fast shear-wave directions are parallel to the maximum horiz ontal compressional axes obtained from a hydro-fracturing test at a de pth of 0.80 km and focal mechanism solution at depths 4-8 km in the sa me area. These observations provide us the evidence that the crustal a nisotropy in this region is controlled by the tectonic stress. We esti mated the degree of anisotropy, that is, the ratio of the difference b etween fast and slow S-wave velocities, as a function of depth using t he layer-stripping method. The obtained value is 2% or less at most de pths but anisotropy of 11% is observed at depths 6-8 km. This high-ani sotropy layer has a local gradient in seismicity rates. We infer that vertical fluid flow from the high-anisotropy layer would be trapped in the low-anisotropy layer above, pore pressure would increase, and it could generate earthquakes. This correspondence suggests that the degr ee of anisotropy in the upper crust changes with depth and has some re lation to seismicity.