利用拉曼光譜探討龍門山斷層帶的碳質物特徵

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

黃致柔(Jyh-Rou Huang)

指導教授 ： 宋聖榮 ； 葉恩肇

國立臺灣師範大學/理學院/地球科學系/碩士(2015年)

https://doi.org/10.6345/NTNU202205565

摘要

西元2008年Mw7.9的汶川地震，分別造成了270公里的映秀-北川同震地表破裂及80公里的安縣-灌縣地表破裂。為了瞭解此次地震的機制及相關資訊，除了地表的研究，也啟動了汶川地震科學鑽探計畫（WFSD）。目前，已知深鑽計畫一號孔的岩芯內含有豐富的碳質物，並在可能之活動斷層帶內發現碳質物有不同的石墨化作用。另外，岩石力學實驗的佐證得知，龍門山斷層帶的碳質物會因斷層作用產生石墨化作用，而滑動時的動摩擦係數也相當低，表示滑動時產生弱化行為。雖然瞭解龍門山斷層帶內碳質物之石墨化作用與斷層作用有關，但目前並未有任何研究針對此斷層帶內的碳質物特徵進行解析。本研究採集深鑽計畫一號孔的斷層帶樣本（2008年汶川地震之活動斷層帶與深部斷層帶）及九龍槽溝地表露頭，並利用拉曼光譜分析來瞭解碳質物的特徵及相對應之石墨化程度，進而提供龍門山斷層可能的斷層演化及構造活動史。
碳質物拉曼光譜分析結果指出，汶川地震活動斷層帶的黑色斷層泥之碳質物特徵與斷層作用極為相關，造成石墨化作用的相關機制包含剪應力與應變，以及斷層滑移時所產生的摩擦熱之溫度。此外，實驗發現2008年汶川地震的主要滑移帶有較佳的石墨化碳質物；暗示其他碳質物石墨化程度較高之處為規模較大的古地震事件發生之位置。汶川地震深部斷層帶之碳質物特徵與活動斷層帶相似，包含著斷層作用所產生的摩擦熱之溫度及剪應力與應變。
總體來看，相較於活動斷層帶，深部斷層帶之碳質物有較高的石墨化程度，顯示著在龍門山斷層帶的滑移歷史上，深部斷層帶遭受過較多次的斷層作用，可能為較成熟的斷層帶，又因為此處石墨化程度高，摩擦係數較低，使應力不易集中，易以潛移的方式釋放。相對的，活動斷層帶可能為較年輕的斷層帶，碳質物的豐度較高，應力相對較易集中，進而產生較大的滑移，並伴隨著大地震。這或許可為2008汶川地震是在較淺部的斷層帶發生而非在深部斷層帶提供一個解釋。

關鍵字

汶川地震科學鑽探計畫 ； 碳質物拉曼光譜 ； 石墨化作用 ； 主要滑移帶

並列摘要

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.

並列關鍵字

the Wenchuan earthquake Fault Drilling Project (WFSD) ； Raman spectrum of carbonaceous material (RSCM) ； graphitization ； principal slip zone

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