Depth Limits of Slow Slip Events at the Japan Trench: Insights from Friction Experiments under In-Situ Conditions

Abstract:

Episodic tremor and slip occurred just before the 2011 Tohoku-oki earthquake on a shallow portion (less than 20 km depth) in the Tohoku subduction zone (Ito et al., 2013). The area where slow slip occurred overlapped with the seismogenic zone. To understand such diverse slip behaviour around the Japan Trench, not only the major earthquakes but also the slow slip events, it is essential to reveal the frictional properties of rocks distributed at the Tohoku subduction zone. We thus conducted friction experiments using a rotary shear apparatus on smectite-rich pelagic sediments retrieved from the plate-boundary thrust during IODP Expedition 343. The sediments were disaggregated and simulated gouges were then sheared at temperatures of 20-200^oC, an effective normal stress of 50 MPa and a pore fluid pressure of 50 MPa. We conducted velocity-stepping sequences (0.3 to 100 µm/s) to determine the rate and state parameter (a-b) and investigated the effects of temperature and slip rate on slip stability.
Our results show that at low temperatures of 20 and 50^oC, the simulated gouges exhibit negative values of (a-b) with a background friction coefficient of 0.38, except at the highest slip rate of 0.1 mm/s. However, the gouges show neutral to positive values of (a-b) at temperatures of >100^oC with the same background friction coefficient as at lower temperatures. In addition, the value of parameter (a-b) depends significantly on slip rates: at temperatures of 20 and 50^oC it increases from negative to neutral (or slightly positive at 20^oC) with increasing slip rates to 0.1 mm/s, whereas it tends to decrease with increasing slip rate at temperatures higher than 100^oC. The downdip temperature limit of the slow slip events at Japan Trench (Ito et al., 2013) seems to be in the range between 100 to 150^oC. The transition in (a-b) value from neutral to positive, particularly at lower slip rates, occurs at the same temperature range. Hence, this could correspond to the observed downdip limit of the slow slip events. Furthermore, at the lowest temperature of 20^oC this transition also occurs, but with increasing slip rate. This frictional property may thus inhibit further slip acceleration even if the slip commences, and could thus represent the upper limit of slow slip events at the shallow portion of the plate boundary.