In a number of subduction zones, earthquakes at 50–200 km depth define two dipping planes, separated by 20–40 km, that appear to merge downdip. Upper plane earthquakes are inferred to occur within the subducting oceanic crust, whereas lower plane earthquakes occur in the subducting oceanic mantle. Beneath northeast Japan, the results of a new finite-element heat-transfer model suggest that the lower seismic plane cuts across isotherms at a shallow angle. Lower plane earthquakes occur at ∼550–800 °C at 100 km depth and at ∼350–600 °C at 160 km depth. These conditions coincide with the dehydration reaction antigorite (serpentine) → forsterite + enstatite + H2O, which suggests that lower plane earthquakes may be triggered by dehydration embrittlement, which in turn suggests that the subducting oceanic mantle is partially hydrated. Serpentinization may occur in the trench–outer rise region, where faulting may promote infiltration of seawater several tens of kilometers into the oceanic lithosphere. If this hypothesis is correct, current subduction-zone H2O budgets may significantly underestimate the amount of bound H2O entering the “subduction factory.”

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