Characteristics of Carbonaceous Materials in the Fault Zone of the Longmen Shan Fault Belt, China, by Raman Spectroscopy

Abstract:

On 12 May 2008, the Mw7.9 Wenchuan earthquake occurred in the Longmen Shan fault belt (China), and produced a 240km-long surface rupture along the Beichuan-Yingxiu fault. To retrieve faulting information of this destructive earthquake, the Wenchuan earthquake Fault Drilling Project (WFSD-1) was drilled to a depth of 1,200 m in 2009. The principal slip zone (PSZ) corresponding to the 2008 Wenchuan Earthquake was recognized at the depth of 589m and contained carbonaceous materials within fault gouge. Rock deformation experiments suggest that graphitization occurred within the PSZ by frictional heat of faulting.

Because graphite is an inerasable indicator of transient frictional heating, we aim at the investigation of the graphitization along successive carbonaceous-rich gouges enclosed PSZ collected from the WFSD-1 by using the Raman spectrometer. The Raman spectra of carbonaceous materials (RSCM) include the graphite band (1580cm^-1, G band) and the defect bands (1350cm^-1, D1 band; 1520cm^-1, D3 band). The R1 ratio of D1 over G is utilized to determine the degree of graphitization.

The results of RSCM analyses revealed that (1) black gouges was heated and pulverized; (2) one of the graphitized zones (low values of R1 ratio) was found at the depth of 589.22 m where was recognized as the PSZ related to the 2008 Wenchuan earthquake. Consequently, we surmise graphitization took place in the PSZ during the 2008 Wenchuan earthquake, and furthermore, the other graphitized zones could be signatures of transient frictional heating during ancient seismic events.