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The effects of frictional heating on the thermal, hydrologic, and mechanical response of a fault

University of British Columbia

The mechanical response of a fault zone during an earthquake may be controlled by the diffusion of excess heat and fluid pressures generated by frictional heating. In this study a numerical model is formulated which incorporates the effects of frictional heating on the thermal, hydrologic, and mechanical response of a small patch of the failure surface. This model is used to examine the parameters that control the fault response, and to determine their critical range of values where thermal pressurization is significant. The problem has two time scales; a characteristic slip duration and a characteristic time for thermal pressurization. The slip duration is set by the fault geometry. The characteristic time for thermal pressurization is set by the slip rate, friction coefficient, and the thermal and hydraulic characteristics of the medium. Results suggest that the fault width, and hydraulic characteristics of the fault zone and adjacent medium are the primary parameters controlling the mechanical response. For narrow zones with a low porous medium compressibility (<10⁻⁹ Pa⁻¹) and permeability (<10⁻¹⁸ m²), frictional heating can cause fluid pressures to approach lithostatic values and the temperature rise to stabilize at a maximum value dependent on the pore-dilatational and transport properties of the porous medium. Moderate slip events where shear strains exceed two cause substantial strain-weakening of the fault and, consequently, large stress drops, accelerations, and displacements. Both the dynamic stress drop and total displacement decrease for zones with larger compressibility, permeability or width. The diffusion of excess pore pressures during the latter stages of slip causes strain hardening and a decline in slip velocity. Whether the patch experiences substantial strain-weakening or acts as a barrier depends on the shear strain across the fault. Thus it is possible for the patch to act as a barrier for small earthquakes but not for large ones. If the compressibility or permeability exceed 10⁻⁸ Pa⁻¹ or 10⁻¹⁴ m², or the shear strain is less than one, then the effects of frictional heating may be negligible and the fault will exhibit no strain-weakening characteristics. Consequently, the patch acts as barrier that halts or resists further fault motion regardless of shear strain. Extrapolation of these results suggests that spatial variations in fault width and hydraulic characteristics will cause a heterogeneous stress drop and fault slip over the failure surface, explaining many of the features of active faulting (e.g., barriers, non uniform slip, rupture stoppage, random ground accelerations, strong motions, and frequency-magnitude relations).

Text

2010-08-06

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http://hdl.handle.net/2429/27139

Geological Sciences

University of British Columbia

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