Shallow Crustal Stress around Shikoku and Kii Region, SW Japan, inferred from Hydraulic Fracturing Tests and Borehole Wall Observations

摘要

Since 2006 FY, Geological Survey of Japan, National Institute of Advanced Industrial Science andrnTechnology (GSJ/AIST) has been constructing an integrated groundwater observation network inrnShikoku, Kii and western Tokai region, southwestern Japan, for investigating crustal activityrnpreceding the coming Nankai, Tonankai and Tokai earthquakes. Fourteen stations have already beenrncompleted. Three observation wells with different depths, usually 30, 200 and 600 m, were drilled atrneach station. Various information about shallow crustal stress was derived by experiments using thesernboreholes and the drilled cores.rnHydraulic fracturing stress measurements were conducted in the 600 m boreholes of five stationsrnand the 200 m borehole of one station, where granitic rocks were dominant. Flow rate and pressure ofrnthe injected water were monitored at downhole by transducers installed just above the measurementrninterval to detect the re-opening pressure precisely. Reverse fault type stress fields, where thernminimum horizontal stress is larger than the overburden stress, were obtained at two stations in Aichrnprefecture.rnBorehole breakouts (BBs) and/or drilling induced tensile fractures (DIFs) were widely observed atrneleven stations on the borehole wall images produced using borehole televiewer/camera. Thernorientation of maximum horizontal stress derived from BBs and DIFs as well as the hydraulicrnfracturing stress measurements dominantly ranges between E-W and NE-SW. This stress orientationrnis perpendicular or largely oblique to the relative convergence direction of Philippine Sea plate tornAmour plate. On the other hand, our results are roughly consistent with the focal mechanisms ofrnshallow crustal earthquakes in this region.