RESPONSE OF CLAY HYDRATION TO THE 2011 TOHOKU EARTHQUAKE IN THE SHALLOW PORTION OF THE JAPAN TRENCH

The sensitivity of smectite during brief and protracted heating intervals can give critical information about the temperature history of a fault during seismogenic slip and creep. Core samples from JFAST (Japan Trench Fast Drilling Project) Expedition 343 have been taken to characterize smectitic clay minerals in fault rocks of an active plate-boundary fault that produced a displacement of ~50 meters during the Tohoku earthquake of 2011. In order to analyze the swelling capacity of smectite during brief (2-5 min) and protracted (5 hours) temperature changes, we heated samples in intervals of 25° C from 25 to 225° C at different rates, using a temperature stage attached to an X-ray diffractometer.

Illite and smectite are the most abundant clay mineral types detected in the core. Clay size fractions 0.05 - 0.5 microns show pure smectite with a characteristic interlayer distance of 1.2 nm that increases to 1.7 nm after ethylene glycolization, indicating up to 3 water layers. 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 brief heating than protracted heating between 50-150° C, and (iii) non-recoverable collapse of all smectite occurs at temperatures > 200° C, regardless of the heating rate. Based on these results, we conclude that frictional heating cannot exceed a temperature of 225° C when smectitic clays are present in these fault rocks. With water available at depth and temperatures less than 200° C, swelling properties of smectite-rich fault rock are preserved. The occurrence here indicates that shear heating at the sampled site of the Tohoku earthquake was relatively low.