We succeeded in an in situ measurement of the crustal stress by using the hydrofracturing method for the first time in Japan. It also was the first time in Japan that crustal stress measurements were made for the purpose of earthquake prediction studies.
Main parts of field experiments are to pump water into a selected interval of a measurement borehole, and to fracture the side wall of the hole by hydraulic pressure. A pair of “inflatable packers” is used to seal the interval. After the packers are recovered, the inclinations and azimuths of new fractures are observed with a “borehole televiewer”. The horizontal principal stresses and their directions are calculated from the records of pressure change and the fracture azimuth. Measurements were carried out in two boreholes 100m deep at Katsurajima and Miyajima in Okabe Town, Shizuoka Prefecture in Feb. -Mar., 1978. In the Katsurajima well, four measurements were made at depths between 57 and 100m and two sets of hydrofracturing data were obtained. In the Miyajima well three measurements were made at depths between 80 and 90m with satisfactory results.
The minimum and maximum horizontal compressive stresses at those experimental sites are calculated to be 32 to 53 bar and 58 to 79 bar, respectively. The stresses increase in proportion to the depth. All of the calculated horizontal stresses are greater than estimated vertical stresses due to overburden weight so that the least principal stress is vertical. The direction of the maximum compression was found to be N 40°E from televiewer observation of a new fracture at the 95m-depth. This direction is not in agreement with the regional strain field obtained from geodetic survey. On the other hand, earthquake mechanism studies suggest a complicated feature of the stress field in this area.
Further, relations between fracture toughness and depths of radial cracks on borehole walls are derived from the hydrofracturing data. The depths of pre-existing cracks on the borehole walls at the test sites are estimated to be several mm from the relation.
This experiment reveals that the hydrofracturing method is successfully applied to such an active and complicated orogenic zone as the Japanese islands. However, the investigation on the relation among the geodetic and seismic data and the in situ stress values is the subject for a future study.