SHEAR ZONE DEVELOPMENT AND FRICTIONAL INSTABILITY OF FAULT GOUGE

Abstract

Earthquakes are typical phenomena of frictional slip of geomaterials in nature. To evaluate
slip instability, shear development in a gouge layer or fault material has been investigated. However, the
quantitative relationship between slip instability and shear development has not been revealed because of
difficulty in quantitative observation of microstructures under high pressure. Hence, we aim to describe shear
development in a gouge layer energetically, and discuss the relation between shear development and slip
instability. To this end, we calculated shear angles by utilizing experimental data of gouge. As a result, this
study reveals that shear bands in a gouge layer develop at lower angles or almost parallel to rock-gouge
boundaries toward the occurrence of unstable slip, particularly under low confining pressure. Additionally,
variation in Riedel shear angles throughout gouge layers depends on confining pressures: Under low
confining pressures, heterogeneous localized shears trigger voluntary increase in strain. On the other hand,
under a high confining pressure, gouge layers deform homogeneously, and the whole of samples slips
dynamically. Clarification of shear development of geomaterials is useful for evaluating the occurrence of
frictional slip such as earthquakes and slope failures.