Progress in the last 25 years and future perspectives in the low to high-velocity friction studies of faults; a time to shift from dry to wet experiments

High-velocity friction experiments in the last 25 years have contributed greatly to the understanding of the physical processes and mechanical properties of fault zones during seismic fault motion. But most experiments have been done at low normal stresses, under dry conditions, and at room temperature. A rationale for conducting experiments under controlled pore-pressure conditions is that specific fracture energy at a 100 MPa normal stress, extrapolated from the experimental results at low normal-stresses, is smaller than the fracture energy estimated for natural earthquakes by about one order of magnitude. Thus dry data are not consistent with earthquakes with respect to the specific fracture energy, and wet experiments may lead to a completely different framework of fault mechanics.

 We review nineteen existing friction apparatuses having high-velocity capabilities. Velocity regimes are conventionally classified into low velocity below 10^-7 m/s, intermediate velocity between 10^-7~10^-4 m/s, and high velocity above 10^-4 m/s. Then there are six high-velocity friction apparatuses, five intermediate to high-velocity apparatuses, and eight low to high-velocity apparatuses currently in use. Seven apparatuses can cover plate velocities on the order of 10^-9 m/s to high velocities to allow studies of earthquake nucleation to dynamic rupture processes. Six apparatuses are equipped with pressure vessels and experiments with pore pressures are becoming possible. Use of Ti-Al alloy with a thermal conductivity almost as low as that of rock will open a way to expand the high-normal stress capability dramatically.

 We also report a rotary-shear low to high-velocity friction apparatus at IGCEA, capable of producing plate to seismic velocities (44 mm/yr to 2.1 m/s), and velocity jumps by 10³ or 10⁶ by using five electromagnetic clutches without stopping the motor. A unique feature of the apparatus is a large specimen chamber where different specimen assemblies can be installed easily. We built two pressure vessels for pore pressures to 70 MPa; one at room temperature and the other at temperatures to 500ºC. We will report preliminary experiments under hydrothermal conditions in an attempt to determine frictional properties relevant to slow slip and high-frequency tremors in subduction zones.