Thermal and metamorphic environment of subduction zone episodic tremor and slip

<jats:p>Episodic tremor and slip (ETS) have been detected in the Cascadia and southwest Japan subduction zones, where the subducting crust is relatively warm because of the young incoming lithosphere (<20 Ma) and modest plate convergence rates (∼40–60 mm/a). In the southwest Japan subduction zone, low‐frequency earthquakes occur on the plate interface at depths of 30–35 km beneath Shikoku where finite element thermal models predict temperatures of ∼425°C in the subducting oceanic crust and at depths of 30–40 km beneath the Kii Peninsula where predicted temperatures are ∼325°C. Warmer temperatures of ∼575°C are predicted at ETS depths beneath southern Vancouver Island in the Cascadia subduction zone, but here tremor also occurs within the overlying fore‐arc crust where temperatures are lower. In the southwest Japan and Cascadia subduction zones, subducting oceanic crust passes through the blueschist, greenschist, and amphibolite metamorphic facies where mineral dehydration reactions are complex. The different temperatures predicted for the two subduction zones suggest that ETS does not coincide with a specific temperature or metamorphic reaction. Several lines of evidence indicate that a free H<jats:sub>2</jats:sub>O‐rich fluid is present, at least transiently, in subducting oceanic crust and fluids released by prograde metamorphic dehydration reactions may help trigger or enable ETS within the subducting plate. Less clear is the role H<jats:sub>2</jats:sub>O may play in tremor observed in the Cascadia fore‐arc crust where free H<jats:sub>2</jats:sub>O may exist locally in faults and fractures, but retrograde hydration reactions are expected to consume H<jats:sub>2</jats:sub>O.</jats:p>