The Thermal Memory of Smectite and Implications for Seismogenic Heating during the 2011 Tohoku Earthquake in the Japan Trench

The sensitivity of smectite to temperature changes in shallow fault systems can provide information about the heating history of a fault during seismogenic slip or creep. Pelagic-sourced smectite is the most abundant clay mineral that is incorporated into the slip zone drilled during JFAST (Japan Trench Fast Drilling Project) Expedition 343 in the Japan Trench, located at ~820 mbsf. Chemical compositions analyzed by ICP-OES show a significant amount of Fe, and lesser Mg and K in these smectite minerals that are pelagic in origin. In order to investigate the significance of smectite during recently proposed high frictional heat associated with the Tohoku earthquake in March 2011, mineral transformations in JFAST drill core samples are examined during fast (5 minutes) and slow (5 hours) heating sequences. For both series, eight measurement-sets were analyzed, taken at 25° C pre-heating, increasing maximum temperatures in the range 50-220° C, and 25° C post-heating, using a real-time heating stage that is connected to an x-ray diffractometer. We observe that (i) both slow and fast heating causes reduction of water interlayers in smectite between 50-200°C, with a delay of water-release during quick heating at temperatures up to 150° C, (ii) smectite recovers more quickly to the original hydration state after fast heating than slow heating between 50-150°C, and (iii) non-recoverable collapse of all smectite occurs at ~200°C, for fast and slow heating, respectively. Based on these results, we conclude that distributed frictional heating of the Tohoku fault zone cannot have exceed a temperature of ~200° C, because smectitic clays are widely present in these fault rocks. Their occurrence indicates that shear heating at the sampled site of the Tohoku earthquake was relatively low, or restricted to an extremely narrow zone.