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Narrow, highly-comminuted shear localization features in faults, known as principal slip zones (PSZs), are commonly associated with large-offset seismogenic faults. In this study, laboratory friction experiments were performed using shale and slate gouges where deformation was encouraged to localize at the gouge–wall-rock boundary. The slate gouges develop a black, narrow PSZ composed of densely packed submicron particles that appear sintered while the spectator gouge remains largely undeformed. These PSZs form at subseismic slip velocities of ∼10 –5 m/s and with a calculated temperature rise of ∼3 °C. Instances of velocity-weakening friction, which is necessary for unstable fault slip, are only observed for slate samples with a PSZ; shale gouges, however, do not develop a PSZ and exhibit only velocity-strengthening frictional behavior. The development of a PSZ may therefore be a prerequisite for future earthquake slip to occur, rather than unequivocal evidence of past earthquake slip.

The very large slip on the shallow portion of the subduction interface during the 2011 Tohoku-oki earthquake (M w 9.0) caused a huge tsunami along the northeast coast of Honshu, Japan. In order to elucidate the mechanics of such tsunamigenic slip, the Integrated Ocean Drilling Program Expedition 343 (Japan Trench Fast Drilling Project, JFAST), was carried out one year after the earthquake and succeeded in recovering rocks constituting the active plate boundary fault. Our mineralogical analyses using X-ray diffraction reveal that the shallow portion of the fault zone that caused the earthquake is significantly enriched in smectite compared to the surrounding sediments, which may be intimately linked to the tsunamigenic shallow faulting. For comparison, we also analyzed mineralogical features of incoming sediments just prior to subduction, recovered on the outer rise of the Japan Trench (Site 436, Deep Sea Drilling Project Leg 56), and found a characteristic smectite-rich horizon in the uppermost ∼5 m of the pelagic clay layer. This horizon should be mechanically weak and will become the future plate boundary fault, as observed in the JFAST cores. The smectite-rich deposits are broadly distributed in the northwestern Pacific Ocean, and may therefore potentially enhance conditions for large shallow slip during earthquakes that occur over a broad area of the Japan Trench plate boundary, which would result in large tsunamis for this region.

A fundamental problem in fault mechanics is whether slip instability associated with earthquake nucleation depends on absolute fault strength. We present laboratory experimental evidence for a systematic relationship between frictional strength and friction rate dependence, one of the key parameters controlling stability, for a wide range of constituent minerals relevant to natural faults. All of the frictionally weak gouges (coefficient of sliding friction, μ < 0.5) are composed of phyllosilicate minerals and exhibit increased friction with slip velocity, known as velocity-strengthening behavior, which suppresses frictional instability. In contrast, fault gouges with higher frictional strength exhibit both velocity-weakening and velocity-strengthening frictional behavior. These materials are dominantly quartzofeldspathic in composition, but in some cases include certain phyllosilicate-rich gouges with high friction coefficients. We also find that frictional velocity dependence evolves systematically with shear strain, such that a critical shear strain is required to allow slip instability. As applied to tectonic faults, our results suggest that seismic behavior and the mode of fault slip may evolve predictably as a function of accumulated offset.