Depth‐dependent crustal anisotropy at Midwestern Honshu, Japan

Abstract

Aligned cracks, possibly induced by tectonic stress, produce anisotropy in the Earth's crust. Permeability and state of stress at several depths in the crust can be estimated from the state of crack distribution. We present clear evidence that crustal anisotropy changes with depth at the Inagawa earthquake swarm area, mid‐western Honshu, Japan. The observed fast shear‐wave directions are parallel to the maximum horizontal compressional axes obtained from a hydro‐fracturing test at a depth of 0.80 km and focal mechanism solution at depths 4–8 km in the same area. These observations provide us the evidence that the crustal anisotropy in this region is controlled by the tectonic stress. We estimated the degree of anisotropy, that is, the ratio of the difference between fast and slow S‐wave velocities, as a function of depth using the layer‐stripping method. The obtained value is 2% or less at most depths but anisotropy of 11% is observed at depths 6–8 km. This high‐anisotropy 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 degree of anisotropy in the upper crust changes with depth and has some relation to seismicity.