Abstract
It is useful to express the state of crustal stress by the ratio (μm) of maximum shear stress to mean stress because the effects of depth on stress are compensated. Using the changing rate of μm calculated from repeated measurements of stress and present stress state in the upper crust, and assuming that shear fracture limit μc is from 0.6 to 0.7 by the Coulomb criterion and Byerlee's law, middle term (10-odd years to 1 year) prediction of earthquake might be possible. In the case of the 1995 Hyogo-ken Nanbu Earthquake, μm increased since around 1983 at both the Hiraki and Hoden stress measuring sites, and was estimated to have reached to 0.6 at the end of 1994. Measurements after the earthquake at both sites showed a decrease to the previous level of μm=0.2. Two months before the earthquake, crustal strain observation at the Rokko-Takao station indicated a decrease of the stress normal to the strike of the Rokko faults system, and at the same time the amount of seepage of under-groundwater increased in the Rokko-Takao observational tunnel. This also suggests an increase of hydraulic pore pressure in the fractured zone of the faults at depth.
