Laboratory observations of slow slip events and related frictional behaviors

Abstract:

Long-term slow slip events (SSEs) on plate boundary faults, which last from months to years and typically have slip velocities of a few times plate tectonic velocities, are frequently observed downdip of the seismogenic zone. New observations locate SSEs to regions updip of the seismogenic zone or even within it. The entire brittle crust may therefore be prone to SSEs, therefore understanding the frictional mechanisms which cause them is highly important, especially in the context of how they relate to ordinary large-magnitude earthquakes. Recent experiments demonstrate that SSEs can be generated in the laboratory, when shearing velocities as low as the plate convergence rates are employed. This includes fault zone samples from the Tohoku region of the Japan Trench, and Costa Rica offshore Nicoya Peninsula. Samples from the megasplay fault and décollement zone of the Nankai Trough, however, did not generate SSEs in laboratory tests.

Velocity-stepping tests show that a possible cause for the generation of SSEs may be velocity-weakening friction that results in quasi-unstable slip. Samples of Carrara marble and Westerly granite sheared under the same conditions results in ordinary stick-slip behavior, which suggests that these samples are truly frictionally unstable and that in comparison the SSE-generating fault zone samples are slightly unstable or quasi-stable.

Previous work has suggested that slip-weakening friction could be a viable mechanism for generating slow fault slip (e.g. very low frequency earthquakes in the Nankai Trough) in velocity-strengthening material. Slip-weakening friction was also observed at low slip velocities in the SSE-generating fault samples and thus may have contributed to slip instability. However, in the Tohoku samples, the slip dependence of friction following a velocity step transitions from slip-weakening to slip-hardening within ~1x10^-7 and 3x10^-6 m/s. This may represent an arrest mechanism that limits the peak slip velocity keeping the event slow. The relative importance of velocity- and slip-weakening friction, and how they interact, requires further study.