TEM microstructural analysis in a fault gouge sample of the Nojima Fault Zone, Japan

A fault gouge sample, recovered from 389 m depth of the Disaster Prevention Research Institute (DPRI) 500 m bore hole of the Nojima fault, which triggered the 1995 M7.3 Kobe earthquake, was examined with transmission electron microscopy (TEM). We observed different types of amorphous material containing tiny flakes of crystalline phases. TEM-EDX analysis of the matrix indicates a pure smectite composition suggesting that matrix and crystalline flakes have the same composition. The detection of tiny crystalline flakes in the amorphous material may confirm previous assumption from the alteration of pseudotachylyte into smectite in fault rocks.

► We characterize amorphous material in a fault gouge sample of the Nojima Fault. ► Microstructures were analyzed using TEM. ► We document the beginning of the alteration of pseudotachylyte into smectite.