Preparations for the Coming Large Earthquakes at the Nankai Trough. Crustal Stress and Strain Measurements on Land for Studying the Nankai Trough Earthquake.

Abstract

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 above the boundary between creep and seismogenic-locked zones on the upper surface of the descending slab. If soft materials such as serpentine exist on the surface of the slab, deformation and flow here will become initiate an earthquake. The basic data of crustal stress and the rock physical properties were acquired by the scientific drilling of a borehole. This borehole was drilled to a depth of 550 m at Shingu city, Wakayama prefecture, on the Kii Peninsula. In-situ experiments such as crustal stress measurements by the hydraulic fracturing method were conducted in the well. Furthermore, we have developed an intelligent strain meter for measuring in-situ rock stress deeper than 500 m using the over-coring method. The final goal of this study is to monitor temporal changes of in-situ stress and strain, and to reveal their storage processes in relation to a great earthquake.