Fault Valving and Pore Pressure Evolution in Simulations of Earthquake Sequences and Aseismic Slip

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

Submitted August 2, 2019, SCEC Contribution #9328, 2019 SCEC Annual Meeting Poster #169

Earthquake sequence simulations are becoming widely used to explore faulting behavior, lithospheric stress profiles, and to interpret crustal deformation data. Many quantities predicted by these simulations--stress drop, slip per event, ambient stresses, and earthquake recurrence intervals--are influenced by the distribution of effective normal stress on the fault (total normal stress compressing the fault minus pore pressure). In most simulations, effective stress is specified a priori and held fixed for all time. Here we take a different approach by introducing a fault zone fluid transport model to our sequence simulations. Fluid ascends along a permeable fault zone surrounding a vertical strike-slip fault, with fluid transport governed by Darcy’s law. Permeability decreases due to healing and sealing processes in the interseismic period and increases due to cracking and dilatancy during fault slip. We also account for the decrease of permeability with increasing effective normal stress. For certain parameter choices, we observe fault valving behavior (Sibson, 1992) in which earthquake recurrence is influenced by cyclic build-up and release of fluid overpressure (pressures in excess of hydrostatic pressure). The system behavior is determined by dimensionless ratios of various time scales: the earthquake recurrence interval, the healing/sealing time scale, and diffusion times across the seismogenic zone. We have also identified a phenomenon of fluid-driven aseismic slip that occurs at the base of the seismogenic zone, in the form of an aseismic slip front that invades the previously locked seismogenic zone at a rate of order 100 m/yr. Ascending high-pressure fluids reduce fault strength, triggering aseismic slip, increasing permeability, and permitting further ascent of fluids. This fluid-driven aseismic slip might help explain plate unlocking behavior observed in subduction zones and possibly also slow slip. The modeling framework, once generalized to account for frictional heterogeneity, might also provide insight into swarm seismicity that has been linked to fluid migration as well as induced seismicity from fluid injection.

Key Words
earthquake sequences, fluids, pore pressure, permeability

Citation
Dunham, E. M., Zhu, W., Allison, K. L., & Yang, Y. (2019, 08). Fault Valving and Pore Pressure Evolution in Simulations of Earthquake Sequences and Aseismic Slip. Poster Presentation at 2019 SCEC Annual Meeting.


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