平木鉱山, 宝殿採石場および土橋鉱山における地殻応力測定

Abstract

In order to investigate the stress state and the strain accumulation in the Earth's crust, andto presume the possibility of earthquake-occurrences, three dimensional absolute stress measure-ment and the stress gradient measurement were carried out at 3 sites in Northwest Kinki District.Respective results obtained by the two different measuring methods, hydrofracturing and stressrelief methods, do not always coincide three-dimensionally with one another due to the condi-tion of the measuring points, however, the azimuth of horizontal principal stress agree well withone another, and most likely reflect the tectonic stress of the region. Moreover, as for the resultsof the horizontal stress measurements by hydrofracturing method at respectiv depths, the scatter-ing is within 7 to 8 in the azimuthal angle. The regional differences observed are due not tothe accuracy of measurement, but probably to the local characteristics of the measuring sites.There is a considerable scattering in the stress values measured at the shallow part near theground surface. As seen in the results measured at Hoden, there appear low stress parts at in-tervals of approximately 20 m. This low values may be caused by regularly existing joints in thelarge rock mass and by low overburden pressure at shallower parts of the upper crust. The stressis constantly released at the large joints or small fracture zone through relative micro-displace-ment of each rock block. The stress depth gradient in this region is relatively lower than theWest Japan standard stress gradient. When the data at Hiraki measured in 1985 and 1989, especially their depth distribution, are compared, it is infererd that the principal stress azimuthand the stress gradient have been changing and increasing. This stress changes may be relatedto the seismic activities that continued since around 1986 near Kameoka in East 40 km awayfrom Hiraki.