近畿北部の地殻応力状態とその変化

Abstract

At Hiraki Mine in Hyogo prefecture, a three dimensional stress measurement by the hydrofracturing method was recently made in a 108 m deep tunnel.This was the 5th stress measurement at the same site since 1978.The results of this latest measurement in 1992 agree well with the results of measurement in 1978 (by stress relief method) and also in Apr. 1989 which was carried out by the three dimensional hydrofracturing method. The other two measurements in Feb. 1985 and 1989 were made by hydrofracturing method for measurements of stress-depth gradient.Those respective 5 data, except for one in April 1989, show that there is a gradual increase in the value of μm=(σh1-σh2)/(σhl+σh2) during the past 15 years.On the other hand, there is slight scattering by ±15°in the direction of the horizontal principal stresses. However, this also seems to indicate the periodic variation of about 8 years with double amplitude of approximately 25°.The peak and the bottom of this periodic variation also seem to correspond with the alternation of the seismic activities around NE-SW and NW-SE fault systems in the Northern Kinki district.These variations of the principal stress direction can be attributed to the changes of interaction between three plates, namely, the Pacific, the Philippine Sea and the Eurasian Plates.