Spatial distribution and characteristics of fracture zones near a long-lived active fault: A field-based study for understanding changes in underground environment caused by long-term fault activities

Abstract

Fracture zone development has a substantial impact on long-term changes in the underground environment because fracture zones are potential major groundwater flowpaths as well as a locus for mechanical disturbance by fracturing. To understand fracture zone development, we surveyed the spatial distribution and characteristics of fracture zones in areas around the Atotsugawa Fault, a long-lived active fault in central Japan. Within 500 m of the Atotsugawa Fault trace, the number of exposed fracture zones increases sharply and most fracture zones greater than 2 m width are concentrated. The width of the clay-rich fault gouge in the majority of fracture zones is less than 1 m. Fracture zones greater than 2 m width along the Atotsugawa Fault are composed mainly of a zone with numerous fractures and breccia, and are considered to act as a major conduit controlling regional groundwater flow. Based on rock features and deformation fabrics at meso- and microscopic scales, once fracture zones had formed, epigenetic deformation was concentrated in the older fracture zones. Fracture zones formed by fracturing along hydrothermal veins or fracturing and weathering that accompanied displacement along joints, schistosities or lithological boundaries are sparsely but widely distributed in the study area, though most of their width are less than 2 m. The existence of unfavorable hydraulic characteristics needs to be studied by engineered methods, for the purpose of site selection and design of underground facilities.