Nucleation and propagation of slow slip pulses on rate-strengthening faults

Elias R. Heimisson, Eric M. Dunham, & Martin Almquist

Published August 1, 2018, SCEC Contribution #8246, 2018 SCEC Annual Meeting Poster #187

Slow slip instabilities and transient aseismic slip have been observed in many settings, including the San Andreas system and subduction zones. Most theories for slow slip events postulate that instabilities initiate under rate-weakening friction, but are limited in rupture and slip speed by heterogeneities or additional stabilizing processes like dilatancy or a transition to rate-strengthening friction with increasing slip rate. However, slow slip occurs in regions where rate-strengthening friction is expected, based on experiments involving clay-rich minerals and temperature controls on the rate weakening-to-strengthening transition. Using linear stability analysis of perturbations about steady sliding with rate-and-state friction, we show that for two distinct destabilizing mechanisms, both involving coupling of slip and effective normal stress, perturbations can be unstable for sufficiently mild rate-strengthening friction. The destabilizing reductions in effective normal stress occur through poroelastic or elastic bimaterial effects. In contrast to the classic instability with rate-weakening friction, for which all wavelengths greater than a critical length are unstable, our instability occurs only for a range of wavelengths centered around a preferential wavelength of maximum growth rate. We derive expressions for the preferential wavelength, phase velocity, and growth rate that demonstrate that these instabilities are fundamentally different from earthquake-inducing instabilities. For example, the nucleation dimension is typically a hundred times larger than for earthquakes and instabilities develop over slip distances much larger than the state evolution distance. We augment our stability analysis with numerical simulations that couple quasi-static Biot poroelasticity and (nonlinear) rate-and-state friction. The simulations, initialized with small random perturbations about steady sliding, show emergence of a slip pulse that shares many characteristics of slow slip in nature, such as large slip patch dimension, rupture speeds much lower than elastic wave speeds, and low stress drop and total slip. The linear stability analysis provides insight into the parameter combinations controlling slip pulse length and propagation speed.

Key Words
slow slip, rate-and-state friction, poroelasticity

Citation
Heimisson, E. R., Dunham, E. M., & Almquist, M. (2018, 08). Nucleation and propagation of slow slip pulses on rate-strengthening faults. Poster Presentation at 2018 SCEC Annual Meeting.

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