Shear zone development and frictional instability of fault gouge

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.