Structural fabrics of carbon grains in a natural fault gouge reactivated by the 2008 Wenchuan earthquake

Abstract

Natural fault gouges reactivated by the 2008 Wenchuan earthquake are typically rich in carbon in shallow parts of the seismogenic fault zone. Although experimental evidence indicates that amorphous carbon can be changed to graphite during seismic slips, this transformation has not yet been observed in nature. We conducted a nanoscale investigation of a carbon‐rich co‐seismic gouge from a surface rupture related to the Wenchuan earthquake using high‐resolution transmission electron microscopy. We found that all mineral grains were wrapped in amorphous carbonaceous materials with sinuate and straight graphene layer stacks. The sinuate layer was the transient material (~0.3456 nm) formed by amorphous carbon transforming to graphite; the graphene layer was graphite flakes (0.3354 nm). This means that graphitization occurred on the mineral grain surfaces (asperities) in the shallow slip zones during previous earthquake cycles, which could decrease the friction strength of the co‐seismic fault gouge and explain the dynamic weakness of the shallow parts of the Longmenshan seismogenic fault zone.