The sensitivity of smectite during brief and protracted heating intervals can provide crucial information about the temperature history of faults during seismogenic slip and creep. Pelagic-sourced smectite is the most abundant clay mineral that is incorporated into the slip zone that was drilled during the Japan Trench Fast Drilling Project (JFAST) Expedition 343 in the Japan Trench, located ∼820 m below seafloor. This study investigates the potential for abundant smectite to preserve a record of coseismic frictional heating associated with the Tohoku earthquake in May 2011 by laboratory examination of mineral hydration and dehydration in JFAST drill core samples during brief (maximum of 5 min) and protracted (5 h) heating sequences. Using a real-time heating stage that is connected to an X-ray diffractometer, we observe that (1) both brief and protracted heating causes reduction of water interlayers in smectite, (2) smectite recovers faster to the original hydration state after quick heating than long heating, and (3) nonrecoverable collapse of all smectite occurs at >200 °C for brief and protracted heating rates. Because hydrated, smectite clays are widely present in the fault rocks, we conclude that frictional heating within the slip zone of the Tohoku fault zone cannot have reached significantly elevated temperatures.

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