岡山市万成における地殻応力の深度勾配の測定(続報)

Abstract

In November 1990, the stress distribution until as deep as 200 m was measured on Mannari granite block in Okayama City. The Borehole was extended until 300 m deep in 1991, and the stresses between 200m and 300m were further measured at 9 points by the hydrofracturing method. As a result, it was made known that the stress value is low and nearly lithostatic state until 170 m deep. At the depth around 175 m, discing phenomenon was observed. Then around 190 m deep, an old concreted fault came out and the stress value began to increase sharply in deeper part.The ratio (μm) of the shear stress value to the horizontal mean stress value takes from 0.3 to 0.4 in deeper than 180m. The horizontal shear strain changes in the vicinity of Okayama City obtained by geodetic surveys have been as small as 10×10-6(1×10-7/yr) in past 100 years, and seismic activity is extremely low as well. It is absolutely impossible to occur shear fracture if μm = 0.4 or smaller, or under such a stress state even though the hydrostatic pore pressure is taken into consideration.Okayama area is, though unexpectedly under high stress state, supposed to be a stable area with low shear stress and low stress changes.