Probing mechanisms of unsteady shallow creep on major crustal faults

Junle Jiang, & Yuri Fialko

Submitted August 14, 2018, SCEC Contribution #8568, 2018 SCEC Annual Meeting Poster #188

A number of major crustal faults exhibit geodetically detectable shallow creep that involves both spontaneous and triggered slip events along tens of kilometers-long fault sections. Notable examples include the Ismetpasa segment of the North Anatolian Fault (NAF) in Turkey and the Superstition Hills (SHF) and the Southern San Andreas (SSAF) faults in Southern California. Various mechanisms such as conditionally stable friction and dilatant strengthening of fault zones have been proposed to explain the slow slip phenomena, but how aseismic slow slip relates to fault slip during earthquakes is unclear in these interpretations. Using observations of the co-, post-, and inter-seismic slip on NAF, SHF, and SSAF, we explore consistent mechanisms of the associated slow slip and the rheology and seismic potential of shallow crustal faults. We consider 2D models of faults governed by depth-dependent effective stresses and standard rate-and-state frictional properties, with a velocity-neutral layer sandwiched between the surface layer with velocity-strengthening frictional properties and a deeper velocity-weakening zone, and potentially subject to pore compaction and dilatancy. For the interseismic period, we find that models with different rheologies can reasonably well produce slow slip events with displacements on the order of millimeters and periods of months, similar to observations. However, during seismic ruptures, the behavior of the shallow fault parts in these models is significantly different. In purely frictional models, shallow fault areas cannot arrest earthquakes efficiently while simultaneously promoting large-scale creep events that can propagate to the surface in the interseismic period. In models with dilatancy, the extent of coseismic shallow rupture is reduced due to dynamic fault strengthening, leading to small coseismic slip and robust postseismic slip at the surface, in a better agreement with available observations. By combining model ingredients tailored for each seismic-cycle phase, we reproduce the overall behavior for the NAF, SHF, and SSAF. Our results suggest that fluid-related, dynamic frictional processes may play a critical role in controlling the seismic potential of these shallow fault zones.

Key Words
fault creep, rate-and-state friction, dilatancy, crustal faults

Citation
Jiang, J., & Fialko, Y. (2018, 08). Probing mechanisms of unsteady shallow creep on major crustal faults. Poster Presentation at 2018 SCEC Annual Meeting.


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