千葉県富津市における水圧破壊法による地殻応力測定

抄録

Using the hydrofracturing technique, in-situ stress measurements have been made in a 450-m-deep well drilled through silty mudstone (the Miocene). Stress has been measured at ten points between depths of 101 and 437m.
Data show a steady increase with depth for both maximum (σ_{H max}) and minimum (σ_{H min}) horizontal compressive stresses, from 33 to 136 bars, and from 20 to 85 bars, respectively.
A borehole televiewer and an impression packer are used to detect new fractures created by the hydrofracturing, which indicate the azimuth of σ_{H max}. The impression packer detects new fractures more clearly than the borehole televiewer. The direction of σ_{H max} is N45°±10°W. This direction does not agree with those estimated from joints which have been developed in the Neogene formations (the maximum compressive stress axis is N-S or vertical), and tectonic strain analyzed from the first order triangulation survey (the maximum compressive strain axis is N-S). But the hydrofracturing result agrees well with the nothwestward motion of the Philippin Sea plate relative to the Asian plate.