Abstract
In this study, we numerically simulated thermal pressurization of a fault and investigated temperature and pore pressure changes during seismic slip. Thermal pressurization of fault fluid during slip tends to reduce frictional resistance by increasing pore pressure along the fault. We used a coupled Thermo-Hydro-Mechanical finite element model to simulate thermal pressurization by implementing a simple constitutive law to couple pore pressure changes with temperature rise due to frictional heating. The effects of hydraulic diffusivity and slipping zone thickness on pore pressure and temperature changes were investigated. The parametric study results showed that the weakening rate increases by decreasing the slipping zone thickness. Lower hydraulic diffusivity was shown to induce larger reduction in the effective normal stress.
1. INTRODUCTION
Once fault slip is initiated, two mechanisms with opposite impacts compete to control slip and rate of slip (Segall & Bradley, 2012). Shear-induced dilatancy of the fault core tends to increase fault permeability and consequently, to decrease fluid pressure. In contrary, fault heating inclines to increase fluid pressure and to weaken the fault (Rice 2006, Segal and Bradly 2012). An earthquake occurs if the thermal weakening process during the fault’s early slip takes over the shear-induced dilatency due to the release of tectonic stress (Wibberley & Shimamoto, 2005). Two thermal mechanisms, referred to as flash heating and thermal pressurization, can weaken the fault and decrease frictional resistance along the fault (J. R. Rice, 2006).
Flash heating occurs in rapid slips and mostly depends on the slip rate (Rice 2006). This mechanism deals with highly stressed micro-scale contacts during slip and decreases the fault friction coefficient. The real contact area (i.e., slip surface) is the sum of contact areas of all the asperities which is a small fraction of the macroscopic contact area. Since the stress supported by the asperities is larger than the stress carried by the fault surface, sliding leads to a large heat production and weakening of the contact (Rice, 1999; Tullis & Goldsby, 2003; Prakash, 2004; Rice, 2006; Beeler et al., 2008).
