Knowledge of tectonic stress magnitudes and orientation is essential for understanding the crustal dynamics. The stress at the depths in concern can be measured from the results of in-situ tests carried out at those depths in drilled boreholes. Especially for the measurement at depth more than 1 km, hydraulic fracturing has been used generally, since, compared with the other methods, its procedure and equipment of in-situ test are quite simple and appropriate for operating in such long and narrow space as boreholes. With this technique, an interval of a borehole which is free of natural fissures is sealed off with a straddle packer system and then pressurized by injection of fluid to generate a tensile circumferential stress around the borehole. When this tensile stress exceeds the strength of a rock and the concentration of tectonic stresses by the borehole, fracture initiation occurs on the borehole wall. Then, the so-called breakdown pressure
is observed as the borehole pressure at the fracture initiation. In addition to this pressure, several characteristic pressure related to the tectonic stresses are observed in the borehole pressure — time history during the test. Consequently, the tectonic stresses are computed from those observed pressures by using the formulae of theoretical relations between the tectonic stresses and the pressures which are derived theoretically in advance.