High-Velocity Frictional Properties of Westerly Granite and the Role of Thermal Cracking on Gouge Production

Abstract:

With the advent of high-velocity rotary shear apparatus, several experimental studies have been conducted in the last decades improving our understanding of fault friction at seismic slip rates (0.1 < V_s < 10 m/s). Here, we present the results of a series of tests conducted on Westerly granite with the Slow to HIgh Velocity Apparatus (SHIVA - INGV Rome), coupled with a high frequency acoustic monitoring (4 MHz sampling rate). Experiments were conducted under normal stress (σ_n) ranging from 5 to 20 MPa and at V_s between 3 mm/s and 3 m/s. Additional experiments were conducted in the presence of pore fluid at equivalent effective normal stress.

In dry conditions, two friction drops are observed: the first drop at V_s > 0.1 m/s is explained by flash heating mechanism while the second drop is due to the formation and growth of a continuous melt layer on the fault surface. In wet conditions, only the second drop of friction is observed. Average values of the fracture energy are independent of normal stress and sliding velocity. However, measurements of elastic wave velocities travelling through the fault strongly suggest that higher damage is induced for 0.1< Vs <0.3 m/s for equivalent final displacement. This observation is also supported by acoustic emission (AEs) recordings. Indeed, most the AEs are recorded after the initiation of the second friction drop, that is, once the fault surface temperature is high, suggesting they may be due to thermal cracking induced by heat diffusion. In addition, the presence of pore fluid pressure (water) delayed the appearance of AEs, supporting the link between AEs and the production and diffusion of heat.

Using the thermo-elastic crack model, we demonstrate that damage can indeed be induced by heat diffusion. Our theoretical results predict accurately the amount of sample wear, supporting the idea that gouge production and gouge comminution is in fact largely controlled by thermal cracking. Finally, we show that this new fracture energy term is non-negligible in the energy balance so that thermal cracking induced during seismic slip, in dry conditions, could play a key role both in the evolution of the physical properties of the slip zone and the high frequency radiation.