Thermal Reactions of Smectite Minerals at Earthquake Timescales

Abstract Frictional heating causes a transient temperature rise in fault zones during earthquakes that leads to metamorphic reactions within the fault rocks. The products of these reactions provide potential evidence of earthquakes in both the outcrop and drill core. Clay minerals such as smectites are commonly found in faults. When heated, they experience a sequence of reactions with increasing temperature: dehydration, dehydroxylation, and decomposition. Clay reactions have been suggested as rock record evidence of past earthquakes, but whether these reactions are fast enough to occur over earthquake timescales (seconds‐minutes) depends on unknown reaction kinetics. We conducted ramped and isothermal heating experiments up to 900°C on smectite clays using in situ X‐ray diffraction (XRD) to monitor reactions as they progressed. Dehydration was variable across experiments due to sensitivity to ambient humidity and therefore accurate kinetics could not be determined for this reaction. Dehydroxylation is not measurable using XRD. The extent of decomposition was systematic with heating duration and peak temperature, enabling the quantification of reaction kinetics. We couple our empirical decomposition kinetics to a model of fault heating to determine how much clay decomposition should occur in a single earthquake. We find that a large shallow earthquake with 70 m of slip at 810 m depth (similar to the 2011 Tohoku earthquake temperature measured through fault drilling) would cause 5%–10% decomposition. Since amorphous material is unlikely to persist and accumulate over earthquake timescales, a large temperature rise during earthquakes could occur without evidence of decomposed smectite in the rock record.