Nucleation of frictional instability caused by fluid pressurization in subducted blueschist

Pore pressure is an important factor in controlling the slip instability of faults and thus the generation of earthquakes. Particularly slow earthquakes are widespread in subduction zones and usually linked to the occurrence of high pore pressure. Yet the influence of fluid pressure and effective stress on the mechanics of earthquakes is poorly understood. Therefore, we performed shear experiments on blueschist fault rocks, which likely exist at depth in cold and old subduction zones, to investigate the influence of effective stress on frictional behavior. Our results show potentially unstable behavior at temperatures characterizing the seismogenic zone, as well as a transition from stable to unstable behavior with decreasing effective normal stress, which is mechanically equivalent to increasing fluid pressure. This transition is a prerequisite for generating slow earthquakes. Our results imply that high pore pressures are a key factor for nucleating slip leading to both megathrust and slow earthquakes.