Analysis of the tectonic history of Mizunami area, 1; Paleo-stress analysis of faults

Tagami, Masahiko; Tsuruta, Tadahiko  ; Matsuoka, Toshiyuki

Property and extent of a fault greatly influence the underground water-flow. It is important to understand the local tectonic history to predict prospective fault activity. This draft reports paleo-stress analysis of the Main Shaft Fault in Mizunami Underground Research Laboratory. Horizontal slicken-lines are widely observed on the fault gouge plane. The sense of movement was judged in every gouge sample. The multiple inverse method (Yamaji, 2000) is used to analyze paleo-stresses. The results show horizontal N-S trend maximum main stress and E-W trend minimum main stress. The Main Shaft Fault moved right-laterally when the basic dyke intrusion was occurred at 65 Ma. It was thought that NW directed Pacific plate subduction was parallel to the compressive paleo-stress at that time.