Frictional Properties of Experimentally Sheared Gouges from the 2011 Mw 9.0 Tohoku-Oki Earthquake Fault Zone

Abstract:

Smectite-rich fault gouge recovered during IODP exp. 343 (J-FAST project) from the plate-boundary slip zone of the 2011 Mw 9.0 Tohoku-oki earthquake was deformed at slip velocities of 10 ^-5 - 3 m s ^-1 with theSlow to High Velocity Apparatus (SHIVA) at INGV, Rome. Experiments were performed “room-dry” (40-60% humidity, 8.5 -12.5 MPa normal stress) or “water-dampened” (0.5 ml distilled water, 3.5 MPa normal stress), with displacements up to 1m. Mineralogy and microstructures of pre and post-experiment material was investigated by quantitative X Ray Powder Diffraction (XRPD), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM).

The starting material is composed of smectite (beidellite, 55.8 wt.%), illite (17.2 wt.%), quartz (8.5 wt.%), plagioclase (7.4 wt.%), K-feldspar (7.1 wt.%) and kaolinite (5 wt.%). At all investigated slip velocities, water-dampened gouges have peak and steady state frictional strengths (0.04<μ<0.1) lower than room dry gouges and are velocity-neutral to velocity-weakening. Under room-dry conditions, the gouges are velocity-strengthening at intermediate velocities (0.001 – 0.1 m s ^-1 , 0.25<μ<0.35) and strongly velocity-weakening at slip velocities > 0.1 m s ^-1 (μ<0.1).

A detectable amount of amorphous material formed in room-dry experiments at low and high-slip velocities, likely by comminution and disordering of smectite.Room-dry gouges deformed at low slip velocities are foliated (P foliation) and cut by a series of shear bands lying either sub-parallel (Y) or at low angles to gouge layer boundaries (R). At high slip velocities, room-dry gouges contain a weak P foliation and a single, prominent Y shear. Deformed water-dampened gouges display homogeneous internal texture, lacking foliation or systematically organized fracture sets.

In room-dry gouges, velocity strengthening at intermediate slip velocities and a pronounced peak friction at high slip velocities, represent an energy barrier to seismic rupture propagation. The lack of such a barrier in water-dampened conditions is compatible with propagation of the Tohoku-Oki rupture in wet sediments to the trench and with large coseismic slip at shallow depths. The low friction measured in water-dampened gouges is consistent with the small thermal anomaly measured along the plate boundary fault.