Mechanochemical Effect on Maturation of Carbonaceous Material: Implications for Thermal Maturity as a Proxy for Temperature in Estimation of Coseismic Slip Parameters

• Published in: Geophysical research letters 2018-03, Vol.45 (5), p.2248-2256
• Main Authors: Kaneki, S., Ichiba, T., Hirono, T.
• Format: Article
• Language: English
• Subjects: Aliphatic compounds; Bonding strength; Chemical bonds; Chemical reactions; Comminution; Earthquake damage; Earthquakes; Fault lines; Friction; Geological faults; Heating; Infrared analysis; Lignite; Maturation; Maturity; Mechanochemistry; Molecular structure; Parameter estimation; Parameters; Rock; Rocks; Seismic activity; Seismic slip; Shear stress; Slip; Spectroscopic analysis; Temperature; Temperature effects
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1002/2017GL076791

Because shear stress in a fault is directly related to the frictional heat generated during slip, the thermal maturity of carbonaceous material has been used as a proxy for fault‐rock temperature. We used infrared and Raman spectroscopic analyses and friction and heating experiments to investigate enhancement of the maturation of lignite by shear damage (the mechanochemical effect) and explored the resultant implications for the use of the thermal maturity of lignite as a proxy for temperature. We showed that higher shear stress applied to lignite samples resulted in greater progression of the thermal destruction of aliphatic C─H chains and the survival of the stronger C─C bonds. Thus, we demonstrated that shear damage during earthquake slip causes mechanochemical enhancement of organochemical reactions related to the aromatization of lignite. Our results suggest that shear stress estimated from Raman spectroscopic analyses in previous studies might have been overestimated. Plain Language Summary Because the frictional heat recorded in faults is directly related to dynamic shear stress during earthquake slip, the thermal maturity of carbonaceous material in faulted rocks has been used as a proxy for temperature in fault‐rock studies. Friction‐induced comminution during fault slip distorts the molecular structure of carbonaceous matter and enhances the chemical reactions that lead to maturation; this is known as the mechanochemical effect. In this paper we present the first clear evidence of the important influence of the mechanochemical effect on the maturity of carbonaceous matter in faulted rocks. Our results will contribute to more accurate estimates of the parameters of earthquake fault slip and lead to a better understanding of the energetics of earthquakes. Key Points Organochemical anomalies in carbonaceous material may be a proxy for estimating fault slip parameters Shearing damage enhances maturation reactions of lignite at relatively low temperatures A temperature proxy of carbonaceous material should be used by taking mechanochemical effect into consideration