Fluid pressurisation and earthquake propagation in the Hikurangi subduction zone

TL;DR

This study investigates how fluid pressurisation in clay-rich fault rocks influences earthquake propagation in the Hikurangi subduction zone, revealing that shear-induced fluid pressurisation facilitates rupture propagation.

Contribution

It introduces a novel experimental setup to measure pore fluid pressure during simulated seismic slip in deep oceanic clay-rich samples, elucidating the role of fluid pressurisation in earthquake dynamics.

Findings

Shear-induced dilatancy is followed by fluid pressurisation at seismic velocities.

Fluid pressurisation reduces the energy needed for rupture propagation.

Low permeability of fault materials enhances fluid pressurisation effects.

Abstract

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.