Experimental Constraints on Temperature Rise of Shallow Active Faults: A Case Study from the Nojima Fault, Japan

Several thermo-chronological methods such as TL, OSL, and ESR have been used to study fault activation history. The methods rely largely on basic assumption that chronological signals are reset due to brittle frictional processes, likely to frictional heating. Frictional heating rate and slip duration are critical to discuss whether frictional heat production was sufficient to cause complete resetting of the signals. At high velocities, shear stress may change with displacement due to frictional heating. Hence the analyses of temperature evolution assuming fixed frictional properties may not be rigorous. One of the direct approaches would be the high-velocity frictional experiment using natural fault material. Here, we report our recent experimental results performed on active fault zone material. Recently, a trenching survey of the Nojima fault, along which fault displacement during the 1995 Kobe earthquake has been recorded, was carried out at Funaki on Awaji Island, southwest Japan. Experimental samples were collected from the 10 mm-thick gray gouge zone, which develops at the boundary between cretaceous granite and the Quaternary sediments. Friction experiments were performed at a normal stress of 2.0 MPa under both dry and water saturated conditions with a constant slip velocity of 1.3 m/s. In the dry case, shear stress is initially at 1.2 MPa followed by slip-weakening with about 4 m slip to reach an approximately steady state shear stress at 0.4 MPa. In the wet case, shear stress is initially at 0.6MPa, followed by slip-weakening with 1 m slip to reach a steady state at 0.24 MPa. Frictional heating rate (power density as defined in Di Toro et al., 2011) was estimated to be 1.1x10^6 and 5.6x10^5 [Wm^-2], for dry and wet experiments, respectively. Temperature rise of the gouge at a displacement of 2 m was calculated using the finite-element method assuming that all the frictional work has been converted into heat. The maximum temperatures of the gouge were estimated to be about 460 and 260°C, for dry and wet experiments, respectively. The present analysis does not take into account the mechanical energy dissipated in comminution process. Further studies are needed to add additional constraints on the temperature rise of the gouge.