Subduction zone earthquakes can propagate to the surface causing large seafloor displacements resulting in tsunamis. This requires the earthquake to rupture through clay-rich sediments of the accretionary wedge, which are largely aseismic. As found previously, the frictional properties of a range of wet clays at low slip velocity are velocity strengthening, thus inhibiting earthquake nucleation. However, at high slip velocity the same materials weaken almost immediately resulting in a negligible critical slip weakening distance and fracture energy. We interpret this behaviour as rapid thermal pressurization of the pore fluid within the clay gouge. The lack of fracture energy can explain how a large rupture, propagating from depth, might not be arrested by clay-rich, velocity-strengthening sediments, as is commonly seen. The results suggest that generally, earthquakes may be difficult to nucleate on mature faults dominated by clay, but the propagation of earthquakes through these zones is energetically very favourable.