Inhomogeneous crustal structure beneath northern Miyagi prefecture, northeastern Japan, imaged by coda envelope inversion: Implication for fluid distribution

Three-dimensional (3-D) distribution of scattering coefficients in the crust beneath northern Miyagi prefecture of northeastern Japan is estimated using an envelope inversion method in order to investigate the short-wavelength inhomogeneous structure that cannot be imaged by conventional travel-time tomography. The inversion revealed two large scattering coefficient zones (LSZs) which are distributed beneath active volcanoes and around the mainshock fault of the 1962 M 6.5 Northern Miyagi earthquake. These LSZs merge in the lower crust, and the characteristic distribution correlates well with the previously reported seismic low-velocity zones, which are perhaps caused by fluids. We inferred that fluids are supplied from the lower crust to the upper crust along the LSZs, resulting in inhomogeneous structures such as localized distribution of fluid-filled cracks. It is expected that this distribution of fluids will cause strain and stress accumulation, affecting seismic activity in this region.