Coseismic graphitization


2017-09-04 16:18:41



據統計,地球的地殼平均三天就會發生一次規模五以上的地震,然而,時間將這些斷層作用之相關產物破壞殆盡,導致地表具可辨認的地震化石相當匱乏,也使得斷層(快速)滑移的地質證據充滿爭論。我們使用富含碳質物的斷層泥進行不同條件之岩石力學實驗,發現在快速滑移且相對缺水的條件下,碳質物會轉變成石墨,而此作用稱為石墨化作用。由於實驗產物與2008汶川地震之滑動帶特徵類似,也說明龍門山斷層有著快速滑移之行為特性,並且被記錄保存著。由於石墨化作用是一不可逆反應,且石墨不易風化(意味著不易因時間被破壞)。我們認為,藉由探索斷層帶內碳質物及相關石墨化作用,可以瞭解過去之斷層行為,並廣泛應用在其他未完整之地震斷層區域。

ABSTRACT
One moderate- to large-magnitude earthquake (M > 6) nucleates in Earth’s crust every three days on average, but the geological record of ancient fault slip at meters-per-second seismic velocities (as opposed to subseismic slow-slip creep) remains debated because of the lack of established fault-zone evidence of seismic slip. Here we show that the irreversible temperature-dependent transformation of carbonaceous material (CM, a constituent of many fault gouges) into graphite is a reliable tracer of seismic fault slip. We sheared CM-bearing fault rocks in the laboratory at just above subseismic and at seismic velocities under both water-rich and water-deficient conditions and modeled the temperature evolution with slip. By means of micro-Raman spectroscopy and focused-ion beam transmission electron microscopy, we detected graphite grains similar to those found in the principal slip zone of the A.D. 2008 Wenchuan (Mw 7.9) earthquake (southeast Tibet) only in experiments conducted at seismic velocities. The experimental evidence presented here suggests that high-temperature pulses associated with seismic slip induce graphitization of CM. Importantly, the occurrence of graphitized fault-zone CM may allow us to ascertain the seismogenic potential of faults in areas worldwide with incomplete historical earthquake catalogues.


附件1: Paper