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

Friday, 19 December 2014
Silvia Mittempergher1, Steven A.F. Smith2, Francesca Remitti1, Alessandro Gualtieri1 and Giulio Di Toro3, (1)University of Modena and Reggio Emilia, Modena, Italy, (2)University of Otago, Dunedin, New Zealand, (3)University of Padua, Padua, Italy
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.