Coupled interactions of fluid-pressure and earthquake cycles: Numerical simulations of fault-valve behaviour

Weiqiang Zhu, Kali L. Allison, & Eric M. Dunham

Published August 14, 2018, SCEC Contribution #8549, 2018 SCEC Annual Meeting Poster #195

Fluids are well known to play an essential role in controlling effective normal stress and hence fault strength. While most models of earthquake behavior assume a fixed pore pressure distribution, geologists have documented cyclic changes in fluid pressure and migration of fluids along fault zones. In particular, the fault-valve model (e.g., Sibson, 1990) posits the development of fluid overpressure during the interseismic period, when fault zone permeability is reduced by solution transfer and related healing processes, which is then released immediately after earthquakes when permeability has been enhanced by shearing of the fault zone. Despite this evidence for fluid overpressure development and fault valving, quantitative modeling studies of fault-valve behaviour are still missing. In this study, we extend our 2D antiplane shear earthquake cycle code to simultaneously model fluid flow along faults and permeability evolution. The fault is governed by rate-and-state friction, and we assume Darcy flow along the fault zone with no fluid loss to the adjacent medium. We introduce a simple, linear permeability evolution model that captures two physical processes: a rapid permeability increase accompanying earthquake slip and a gradual permeability recovery with time. The evolution equation has a minimal number of parameters (e.g., minimum and maximum permeability and healing time scale), making it ideally suited for parameter-space exploring of fault valving. We use this fully coupled model to quantify conditions under which fluid overpressure can reach sufficiently high levels that the earthquake process is altered. We identify several relevant length and time scales, which are combined to form the dimensionless parameters that control system behavior. The simulation methodology could be extended to utilize more realistic permeability evolution laws and, as in other work by our group (Allison and Dunham, 2017), to account for viscous flow in the lower crust and mantle.

Key Words
fluid flow, permeability evolution, earthquake cycle simulation

Citation
Zhu, W., Allison, K. L., & Dunham, E. M. (2018, 08). Coupled interactions of fluid-pressure and earthquake cycles: Numerical simulations of fault-valve behaviour. Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)