Abstract. High-velocity friction experiments have beenperformed On a pair of hollow-cylindrical spe6imens of gabbro initially at room temperature, at slip rates fr'om 7.5 mm/s to 1.8 m/s, with total circumferential displacements of 125 to 174 m, and at normal stresses to 5 MPa, using a rotary-shear high-speed friction testing machine. Steady-state thetion increases slightly with increasing slip rate at slip rates to about ,.)00 mm/s (velocity strengthening) and it decreases markedly with increasing slip rate af higher velocities (velocity weakening). Steady-state friction in the velocity weakening regime is lower for the non-melting ease than the frictional melting ease, due perhaps to severe thermal fracturing. A very large peak friction is aIways recognized upon the initiation of visible frictional melting, presumably owing to the Welding of fault surfaces upon the solidification of melt paiehes. Frictional properties thus change dramatically with inerefising displacement at high velocities, and such a non-linear effect must be incorporated into the analysis of ear'quake initiation processes.

Large coseismic slip was thought to be unlikely to occur on the shallow portions of plate-boundary thrusts, but the 11 March 2011 Tohoku-Oki earthquake [moment magnitude (Mw) = 9.0] produced huge displacements of ~50 meters near the Japan Trench with a resultant devastating tsunami. To investigate the mechanisms of the very large fault movements, we conducted high-velocity (1.3 meters per second) friction experiments on samples retrieved from the plate-boundary thrust associated with the earthquake. The results show a small stress drop with very low peak and steady-state shear stress. The very low shear stress can be attributed to the abundance of weak clay (smectite) and thermal pressurization effects, which can facilitate fault slip. This behavior provides an explanation for the huge shallow slip that occurred during the earthquake.

Seismic faulting along subduction-type plate boundaries plays a fundamental role in tsunami genesis. During the Integrated Ocean Drilling Program (IODP) Nankai Trough Seismogenic Zone Experiment (NanTro SEIZE) Stage 1, the updip ends of plate boundary subduction faults were drilled and cored in the Nankai Trough (offshore Japan), where repeated large earthquakes and tsunamis have occurred, including the A.D. 1944 Tonankai (Mw = 8.1) earthquake. Samples were obtained from the frontal thrust, which connects the deep plate boundary to the seafl oor at the toe of the accretionary wedge, and from a megasplay fault that branches from the plate boundary décollement. The toe of the accretionary wedge has classically been considered aseismic, but vitrinite refl ectance geothermometry reveals that the two examined fault zones underwent localized temperatures of more than 380 °C. This suggests that frictional heating occurred along these two fault zones, and implies that coseismic slip must have propagated at least one time to the updip end of the megasplay fault and to the toe of the accretionary wedge.

[1] To determine the processes responsible for slipweakening in clayey gouge zones, rotary-shear experiments were conducted at seismic slip rates (equivalent to 0.9 and 1.3 m/s) at 0.6 MPa normal stress on a natural clayey gouge for saturated and non-saturated initial conditions. The mechanical behavior of the simulated faults shows a reproducible slip-weakening behavior, whatever initial moisture conditions. Examination of gouge obtained at the residual friction stage in saturated and non-saturated initial conditions allows the definition of two types of microstructures: a foliated type reflecting strain localization, and a non-foliated type composed of spherical aggregates. Friction experiments demonstrate that liquid-vapor transition of water within gouge due to frictional heating has a high capacity to explain the formation of spherical aggregates in the first meters of displacement. This result suggests that the occurrence of spherical aggregates in natural clayey fault gouges can constitute a new textural evidence for shallow depth pore water phase transition at seismic slip velocity and consequently for past seismic fault sliding.

[1] We conducted high-velocity friction experiments on clay-rich fault gouge taken from the megasplay fault zone in the Nankai subduction zone under dry and wet conditions. In the dry tests, dehydration of clay minerals occurred by frictional heating, and slip weakening is related to thermal pressurization associated with water vaporization, resulting in a random distribution of clay-clast aggregates in the gouge matrix. In the wet tests, slip weakening is caused by pore-fluid pressurization via shear-enhanced compaction and frictional heating, and there is a very weak dependence of the steady-state shear stress on the normal stress. The resulting microstructure reflects the grain size segregation in a granular-fluid shear flow at high shear rates. These results suggest that earthquake rupture propagates easily through clay-rich fault gouge by high-velocity weakening, potentially leaving the microstructures resulting from the frictional heating or the flow sorting at high slip rates.Citation: Ujiie, K., and A. Tsutsumi (2010), High-velocity frictional properties of clay-rich fault gouge in a megasplay fault zone, Nankai subduction zone, Geophys.