群発地震発生域直上における地殻応力測定

Abstract

Crustal stress measurements by the hydraulic fracturing method were carried out using a 1000m- and a 100m-deep boreholes drilled at Inagawa town, Hyogo prefecture, where earthquake swarm activities with very shallow hypocenters have intermittently occurred since July 1994. On borehole televiewer record, borehole breakouts were clearly observed at a number of depth intervals of the 1000m-deep borehole. The borehole breakout data as well as the hydraulic fracturing data was used to estimate orientation of the maximum horizontal compressive stress.Magnitudes of the horizontal stress down to 700m in depth are comparable to the standard stress gradients with depth in western Japan derived by TANAKA (1986). On the other hand, the maximum and minimum horizontal compressive stresses at about 950m in depth are about 70MPa and 40MPa, respectively, which are about twice as large as the standard stress gradients with depth in western Japan. The maximum horizontal compressive stress below 600m in depth is generally oriented E-W to NW-SE.