Consideration of the Activity of a Fault Based on Detailed Structural Analysis of a Fault Fracture Zone

Case Study of the Yamada Fault, Western Japan

Abstract

It is important to clarify the structure and constituent minerals of a fault zone in order to evaluate fault activity. Thus, we conducted a detailed structural analysis of a fault zone of the Yamada fault, observed in a granite outcrop, to understand the characteristics of active and inactive faults during the Quaternary.

The fault zone consisted of fault gouge zone, fault breccia, and cataclasite, and included a main fault plane and several minor fault planes. The main fault plane cut unconsolidated sediment layers and was associated with the fault gouge zone. The fault gouge zone exhibited a layered structure with 10 fault gouge layers characterized by composite planar fabrics indicating dextral slip and composed mainly of smectite. Thus, we infer that the gouge zone was formed after smectite crystallization by repetitive slip in the shallow part of the crust. The minor fault planes showed no clear relationship with the unconsolidated sediment layers in the outcrop; further, they were not associated with fault gouge, and were in direct contact with the cataclasite. Within the cataclasite, biotite that had been plastically deformed in the deep part of crust was recognized, and no structures indicating fault activity in the shallow part of crust were observed. Thus, we infer that the minor fault planes have been inactive during the Quaternary.