Crushed and fried: ductile rupture at depth due to grain size reduction and shear heating

Since their discovery in 1928, deep earthquakes have been the subject of extensive research to unravel their nucleation and rupture mechanisms. Due to the elevated pressures and temperatures at depths below 50 km, brittle failure becomes less likely and ductile deformation is favored. To date, there is no consensus on the mechanisms resulting in deep earthquake generation. Three main mechanisms (dehydration embrittlement, transformational faulting and thermal runaway) have been proposed to cause deep earthquakes, but neither of them has been sufficiently quantified to yield a definite answer under which conditions they are active.

Here, we explore the feasibility of the thermal runaway hypothesis using 1D and 2D thermo-mechanical models. In particular, we investigate the impact of grain size reduction in conjunction with shear heating to see whether grain size reduction and shear heating are competitive mechanisms (which would prevent thermal runaway) or whether they are collaborative. Our results show that the combination of both mechanisms facilitates thermal runaway and significantly reduces the stress required for the occurrence of thermal runaway. We then investigate whether this combined failure mechanism may explain the seismicity observed in regions of detaching lithosphere, such as the Hindu Kush and the Vrancea earthquake nests.