Crustal stress and strain measurements in a deep borehole are essential for studying plate subduction earthquakes. Especially in the case of the Nankai Trough earthquake, it is expected that crustal movements are detectable even on land, because the focal region caused by the subduction of the Philippine Sea plate is close to land. We have tried to search for a suitable field and a method for observations and investigations based on some hypotheses. Special emphasis is given to better understanding the hinge line as an adequate related field. The hinge line is located on the surface just ab...