Frictional properties of Blueschist under in-situ conditions and implications for fault motion

Abstract:

The 2011 Tohoku-oki earthquake (Mw 9.0) nucleated at 24 km depth along the plate boundary. Before the earthquake, episodic tremor and slip occurred on a shallow portion (less than 20 km depth) in the Tohoku subduction zone (Kato et al., 2012; Ito et al., 2013, 2015). Although slow earthquakes often occur outside the conventional seismogenic zone in subduction settings (e.g., Schwartz and Rokosky, 2007), recent slow slip events within the seismogenic zone seem to precede some large subduction earthquakes. To understand such behaviour within the Tohoku subduction zone, it is important to determine the frictional properties of the corresponding fault rocks. With this aim, we conducted friction experiments on powdered samples of blueschist rocks, which are expected to be present at depth within the Tohoku subduction zone, employing (near) in-situ pressure and temperature conditions. Simulated gouges were sheared at temperatures of 22-400^oC, effective normal stresses of 25-200 MPa and pore fluid pressures of 25-200 MPa. We conducted velocity-stepping sequences (0.1 to 100 μm/s) to determine the rate and state friction parameter (a-b), focusing on the effect of effective normal stress on (a-b) to elucidate how this may affects the fault motion.

Our results show that at most conditions studied, (a-b) tends to decreases from positive to negative values with decreasing effective normal stress. However, neutral to negative (a-b) values were observed at a temperature of 200^oC over the entire effective normal stress range tested. Observed (a-b) values decrease with decreasing effective normal stress because of an increase in b with decreasing effective normal stress. Thus, the rate parameter (a-b) is significantly dependent on effective normal stress at all temperatures investigated, except 200^oC. Our experimental findings imply that low effective normal stresses, therefore high pore fluid pressures, promote negative values of (a-b) and hence the potential to nucleate a frictional instability, leading to both slow slip events and earthquakes.