"Graphene oxide" lubricant in active fault discovered by Tohoku University: Possible link to slow earthquakes

A research group of Tohoku University and other institutions has discovered the lubricant substance "graphene oxide" in the Atotsugawa Fault System, which spans across Gifu and Toyama Prefectures. The friction coefficient of graphene oxide is an order of magnitude smaller than that of normal rock, making it highly slippery. The relative scarcity of earthquakes in the shallow sections of the active Atotsugawa Fault System may be because the fault is slipping slowly due to the action of graphene oxide. This mechanism might be related to "slow earthquakes," a phenomenon where energy is released incrementally along plate boundaries and other regions.

Fewer earthquakes occur in the sections of the Atotsugawa Fault System shallower than 7 to 8 km underground, and a phenomenon known as "fault creep" where the fault slips slowly has been reported. According to Professor Hiroyuki Nagahama, who specializes in Fault Mechanics at the Department of Earth Science of the Graduate School of Science at Tohoku University, it has conventionally been thought that graphite, which possesses a crystalline structure of stacked sheets of carbon atoms, and deep underground fluids act like lubricants, reducing fault friction and causing fault creep.

Second-year Doctoral Student Tomoya Shimada, who is studying Structural Geology in Nagahama's laboratory, focused on "graphene," a single-layer two-dimensional crystal of carbon atoms that can be extracted from graphite. He successfully discovered graphene oxide on the slip surfaces of the fault by utilizing Raman spectroscopy which analyzes material structures using scattered light from laser irradiation, X-ray photoelectron spectroscopy which analyzes elemental composition and chemical bonding states on material surfaces by measuring photoelectron energy, and a transmission electron microscope (TEM) which allows for the observation of substances at the atomic level.

The friction coefficient of graphene oxide is 0.01 or less, which is an order of magnitude smaller and slipperier compared to approximately 0.6 for normal rock and approximately 0.1 for graphite. Upon investigating the underground temperatures of the Atotsugawa Fault System, the researchers found that the region where graphene oxide can stably exist (temperatures of 200℃ or below) matched the region with fewer earthquakes. This suggests that the ultra-low friction of graphene oxide may effectively facilitate fault slippage, thereby influencing the low-seismicity zone.

Although the Atotsugawa Fault System where graphene oxide was found is an inland earthquake fault, carbon has also been reported to exist at plate boundaries, where it is thought to undergo graphitization as plates subduct. There is a possibility that graphene oxide is being generated there as well. By conducting detailed analyses of carbon in subduction zones moving forward, the relationship between graphene oxide and phenomena like slow earthquakes, where the fault plane slips slowly without causing perceptible tremors, may become clear.

This study was conducted in collaboration with Tohoku Gakuin University and was published in the scientific journal Nature Communications on May 12.