  In subduction zones, seismic slip at shallow crustal depths can lead to the
generation of tsunamis. Large slip displacements during tsunamogenic
earthquakes are attributed to the low coseismic shear strength of the
fluid-saturated and non-lithified clay-rich fault rocks. However, because of
experimental challenges in confining these materials, the physical processes
responsible of the coseismic reduction in fault shear strength are poorly
understood. Using a novel experimental setup, we measured pore fluid pressure
during simulated seismic slip in clay-rich materials sampled from the deep
oceanic drilling of the P\=apaku thrust (Hikurangi subduction zone, New
Zealand). Here we show that at seismic velocity, shear-induced dilatancy is
followed by pressurisation of fluids. The thermal and mechanical pressurisation
of fluids, enhanced by the low permeability of the fault, reduces the energy
required to propagate earthquake rupture. We suggest that fluid-saturated
clay-rich sediments, occurring at shallow depth in subduction zones, can
promote earthquake rupture propagation and slip because of their low
permeability and tendency to pressurise when sheared at seismic slip
velocities.
