Michiyo Sawai

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.

Earthquake sequence simulations with measured properties for JFAST core samples

Since the 2011 Tohoku-Oki earthquake, multi-disciplinary observational studies have promoted our understanding of both the coseismic and long-term behaviour of the Japan Trench subduction zone. We also have suggestions for mechanical properties of the fault from the experimental side. In the present study, numerical models of earthquake sequences are presented, accounting for the experimental outcomes and being consistent with observations of both long-term and coseismic fault behaviour and thermal measurements. Among the constraints, a previous study of friction experiments for samples collected in the Japan Trench Fast Drilling Project (JFAST) showed complex rate dependences: a and a−b values change with the slip rate. In order to express such complexity, we generalize a rate- and state-dependent friction law to a quadratic form in terms of the logarithmic slip rate. The constraints from experiments reduced the degrees of freedom of the model significantly, and we managed to find a plausible model by changing only a few parameters. Although potential scale effects between lab experiments and natural faults are important problems, experimental data may be useful as a guide in exploring the huge model parameter space. This article is part of the themed issue ‘Faulting, friction and weakening: from slow to fast motion’.

Depth dependence of the frictional behavior of montmorillonite fault gouge: Implications for seismicity along a décollement zone

To understand the seismogenic potential of shallow plate-boundary thrust faults (decollements) in relatively warm subduction zones, water-saturated Na-montmorillonite gouges were sheared at a pore fluid pressure of 10 MPa, effective normal stresses (σneff) of 10–70 MPa, temperatures (T) of 25–150 °C, and axial displacement rates of 0.03–3 µm/s. The Na-montmorillonite gouges were frictionally very weak at all conditions tested (steady-state friction coefficient μss = 0.05–0.09). At T ≤60 °C, Na-montmorillonite showed a transition from velocity-strengthening to velocity-weakening behavior with increasing σneff, whereas at T ≥90 °C it was largely velocity-neutral or velocity-strengthening, irrespective of σneff. The rates of frictional healing (β) showed extremely low values (mostly <0.001) at all temperatures. Our results suggest that the existence of Na-montmorillonite in the decollement zone at Costa Rica and Nankai promotes aseismic slip, particularly at shallow depths, forming weakly coupled regions.