Intermediate-depth (50-300 km) earthquakes commonly occur along
convergent plate margins but their causes remain unclear. In the absence
of pore-fluid pressures that are sufficiently high to counter the
confining pressure in such settings, brittle failure is unlikely. In
such conditions, the rocks could fail by the mechanism of progressively
self-localizing thermal runaway(1), whereby ductile deformation in shear
zones leads to heating, thermal softening and weakening of rock(1-3).
Here we test this hypothesis by focusing on fault veins of glassy rock
(pseudotachylyte) formed by fast melting during a seismic event, as well
as associated ductile shear zones that occur in a Precambrian terrane in
Norway. Our field observations suggest that the pseudotachylytes as well
as shear zones have a single-event deformation history, and we also
document mineralogical evidence for interaction of the rocks with
external fluids. Using fully coupled thermal and viscoelastic models, we
demonstrate that the simultaneous occurrence of brittle and ductile
deformation patterns observed in the field can be explained by
self-localizing thermal runaway at differential stresses lower than
those required for brittle failure. Our results suggest that by
perturbing rock properties, weakening by hydration also plays a key role
in shear zone formation and seismic failure; however, thermal runaway
enables the rocks to fail in the absence of a free fluid phase.