Abstract
We determined the stress history in the forearc region of the Nankai trough subduction zone by examinug boreholes at the Miyama and Ichiura observation wells, located on the northern and southern margins of the northern unit of the Kumano Acidic Rocks, respectively, eastern Kii Peninsula, SW Japan. To estimate paleostress, the multiple inverse method was applied to fault planes identified within the core samples; to estimate the present-day stress, the borehole breakout method was used. The results obtained using the multiple inverse method indicate that the two observation wells have similar stress histories, involving E-W compressive stress followed by NE-SW extensional stress and finally N-S compressive stress. The present-day maximum horizontal compressive stress (σhmax) at the Miyama observation well trends E-W, consistent with the present-day regional stress in SW Japan and different from the final stage of stress determined by the multiple inverse method. The present-day σhmax at the Ichiura observation well trends NNE-SSW, similar the final stage of stress determined by the multiple inverse method. These findings indicate that the boundary between the areas of E-W compressive stress (characteristic of SW Japan) and N-S compressive stress (influenced by subduction of the Philippine Sea Plate) has gradually migrated southward over time.
