In-situ stress measurement in an earthquake focal area

Abstract

A 2-km-deep borehole was drilled into granitic rock where many shallow earthquakes, with focal depths from 2 to 15 km, have occurred. The drill site, Ashio, is 100 km north of Tokyo. Downhole testing and measurements were conducted five times: four times after each 500 m drilling and the fifth time after completing the 2000 m borehole. Measurements of in-situ stress orientation and magnitude were conducted by the hydraulic fracturing method, stress-induced well bore breakout analysis, and drilling-mud pressure induced hydraulic fracturing analysis. Breakouts and mud pressure induced hydraulic fractures were observed below 650 m and 1250 m, respectively. The circular well bore is maintained only in limited spots below 650 m because of breakouts indicating a large differential stress condition between the maximum and the minimum principal stresses. The differential stress is calculated at 90 ± 20 MPa at the depth of 2000 m based on the condition under which the breakout with some degree of width appears. It is interpreted that this large differential stress is representative of the regional crustal stress condition in the earthquake swarm area. Each spot of the circular well bore is always adjacent to a fracture zone. This suggests that the fracture zone has small differential stress. The stress values were measured where the well bore is circular by the hydraulic fracturing method. For example, the maximum and the minimum horizontal compressive stresses are about 35 MPa and about 25 MPa, respectively, at the depth of 1650 m; giving the differential stress of 10 MPa. The water pressure in pre-existing fractures was also measured, and found that they were nearly equal to the hydrostatic water pressure at the corresponding depths. The stress direction estimated from the azimuth of the breakouts and the hydraulic fracture is consistent with that estimated from the earthquake focal mechanisms. These results support the following conclusions. The differential stress is large in the earthquake swarm region. But, it is extremely small at narrow zones adjoining fracture zones. This is due to the relaxation of the differential stress at fractures of low frictional strength. However, water pressure is not so large that the low differential stress causes sliding of the fractures. Clay minerals seems to play an important role in decreasing of the frictional strength.