Pore-Pressure Rise, Frictional Strength, and Fault Slip: One-Dimensional Interaction Models

Abstract

SUMMARY Analytical solutions are derived for three modes of 1-D strike-slip frictional heating in a strip within a vertical fault zone that is embedded in an otherwise homogeneous medium. These include instantaneous and step-heating modes, as well as a preferred decay-heating mode in which the heating declines with time to a sudden end. Modelling results indicate that the evolution of temperature and pore-pressure rises varies with heating mode, but, at a time three times greater than the slip duration, the evolutionary paths for the different modes are practically indistinguishable. Generally the pressure front advances ahead of the temperature front by a distance of 1/D times the temperature-front distance, where D is the square root of the ratio of thermal to hydraulic diffusivity. A model based on the values of one plausible set of physical parameters suggests that temperature can increase by 100–320 K and pore pressure can rise by 5–16 MPa after 1 m slip under a mean resistive stress of 10 MPa. These estimates can be easily raised or lowered by 50 per cent, because some model-parameter values have uncertainties of one to two orders of magnitude. The estimated pore-pressure rise, being of the same order as the seismic stress drop, requires a large fault slip as a prerequisite. A lower pre-earthquake pore-pressure rise may bear a significant consequence on triggering earthquakes. Once a small aseismic slip begins, regardless of its initiating mechanism, the rise in pore pressure will weaken the frictional strength and lead to more slip. Pressure rise, fault weakening, and slip may reinforce one another through feedback processes that eventually escalate to earthquakes. Massive development of fractures during fault slip enlarges the pore volume; accordingly, the pore pressure is reduced and the frictional strength is raised. When the pore pressure drops below its pre-slip level, the slip stops abruptly. Thereafter, healing, sealing, compaction, and non-frictional heating re-initiate pore-pressure rise within the fault zone, setting the stage for another earthquake episode. Pre- and post-slip pore-pressure rises weaken the fault such that frictional heating is too low to yield an observable heat flow anomaly.