Frictional heating and pore pressure rise due to a fault slip

Summary Accurate estimation of frictional stress is crucial to determining the bounds of pre‐ and post‐earthquake states of stress. A long‐term average value of 10 MPa, deduced from heat flow data in the San Andreas fault zone, is frequently cited, but it may or may not represent the resistive stress of an individual slip event. Based on 1‐D modelling, this paper deals with frictional heating, pore pressure rise, and migration of fluid, and speculates upon their consequences for triggering of earthquakes and emission of earthquake light. Analytic solutions are obtained for temperature and pore pressure rises due to frictional heating. Results are obtained for three heating models: I, instantaneous heating; IT, constant heating rate; and III, heating rate proportional to inverse square root of time. For the same total heat generation and physical parameters, solutions indicate that, for times less than twice the slip duration or distances within two thermal diffusion distances, temperature and pressure distributions are sensitive to heating models; at greater times and distances, they are insensitive to models. Though temperature rise due to 1‐m slip under 10MPa frictional stress may reach 200K or more on the slip surface, the rise is not measurable in practice at a distance beyond about 10 m from the slip surface. Estimates of pore pressure rise (0.2‐2MPa) depends crucially upon permeability and slip duration. Darcy flow may exceed 10 ‐7 m s ‐1 at one thermal diffusion distance; and immediately after the slip ends, reverse flow towards slip surface develops. Because the pore pressure front can advance beyond the temperature front by 100‐1000 times, measurement of pore pressure variation is an alternative to temperature measurement for estimating frictional stress. A rise of 500Pa is measurable at 330 m within 30 days for a fault slip with frictional heat production of 10MJm ‐2 . It is speculated that the propagating pressure front created by an initial slip, rather than the fluid flow itself, may weaken the frictional strength elsewhere and lead to additional minor slips and perhaps a larger earthquake. With large frictional stress and displacement, heating may also cause vapourization of water at shallow depth (<260m). However, the needed temperature rise for emission of earthquake light requires unusually large rates of frictional heating near the ground surface.