Microstructures in a Fault Gouge along an Active Subsidiary Fault

Case Study of the Subsidiary Fault Associated with the Kamishiro Fault, Central Japan

Abstract

There are cases that subsidiary faults are distributed around main faults. Many surveys and studies have been conducted on main and subsidiary faults in the past, but none have focused on microstructural analysis of active subsidiary faults that slipped together with main faults during the Quaternary and have a high possibility of slipping in the future.

The 2014 Northern Nagano Prefecture earthquake （Mw 6.2） ruptured the Kamishiro Fault （main fault） and three subsidiary faults. We conducted a paleoseismic trench survey and microstructural analysis in order to reveal activity of one （latest slip plane） of the subsidiary faults and microstructures in a fault gouge along the slip plane. The trench survey identified that the latest slip plane had slipped at least twice during the Holocene. The microstructural analysis showed that a layered structure and gouge fragments were found in the fault gouge by using a helical X-ray CT scanner, polarized light microscopy, X-ray diffractometer, and transmission electron microscopy.