Abstract
In-situ stress measurements were made in a 450m deep well in tuff by the hydrofracturing technique. Six hydrofractures were created at depths between 263 and 436m. The results show that the maximum horizontal compressive stress, σHmax, increases with depth from 180 to 210 bars and that the minimum stress, σHmin, increases from 110 to 130 bars over the same depth range. The maximum and minimum stresses at a depth of 400m were determined to be 205 and 130 bars, respectively. A borehole televiewer was used to detect new fractures created by hydrofracturing. Two of six televiewer runs show clear new fractures and indicate the direction of maximum horizontal compressive stress to be N25°±10°E. This direction agrees well with those estimated from active faults, alignment of frank craters associated with Quaternary volcanoes, Quaternary dikes, and shallow earthquake mechanisms in the vicinity.
