| Summary: | 碩士 === 國立臺灣師範大學 === 地球科學系 === 103 === On 12 May 2008, the Mw7.9 Wenchuan earthquake occurred in the Longmen Shan fault belt (China), and produced 270 km-long and 80 km-long surface coseismic rupture along the Yingxiu-Beichuan fault and Anxian-Guanxian fault, respectively. To retrieve information of the earthquake physics, the Wenchuan earthquake Fault Scientific Drilling project (WFSD-1) was conducted to a depth of 1,200 m in 2009. In the WFSD-1, the fault gouge contains carbonaceous materials (CMs) within the active fault zone, and the principal slip zone (PSZ) corresponding to the 2008 Wenchuan earthquake was recognized at the depth of 590 m. Rock deformation experiments suggest that graphitization occurred and fault was weak during the 2008 Wenchuan earthquake.
The graphitization process was determined from natural and experimental observation, but the characteristics of CMs of the Longmen Shan fault remain unknown. Here we characterize CMs of fault zone materials from both WFSD-1 and outcrop with Raman analysis to unravel the associated mechanism of graphitization and fault mechanics.
The results of Raman spectra of carbonaceous materials (RSCM) of FZ590 are suggested to be resulted from frictional heating and shearing. Coincidentally, graphitization process seems to occur in the PSZ corresponding to the 2008 Wenchuan earthquake. We further infer other highly graphitizated zones could be a paleo-signature of ancient seismic events. The RSCM of FZ760 is similar to the one of FZ590, and it presumably suggests that the similar mechanisms were occurred. High degree of graphitization within FZ760 seems to be resulted from many coseismic events, and it suggests that FZ760 is mature fault zone. We surmise that during interseismic periods, the abundant CMs (high friction) in fault gouge of FZ590 could accumulate stress and result in large slips afterward, instead of occurring in FZ760 which contains abundant graphite (low friction) in fault gouge and can easily releases stress.
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