Free-surface controls on the stability of shallow rate-and-state faults

Ari G. Silver, & Federico Ciardo

Submitted August 30, 2026, SCEC Contribution #15426, 2026 SCEC Annual Meeting Poster #010

Slow slip events are increasingly recognized at shallow depths, yet the mechanical conditions that permit their nucleation remain uncertain. Most existing stability criteria are derived for faults embedded in unbounded elastic media and therefore neglect the influence of the Earth’s free surface. Here, we investigate how free-surface effects modify the onset of instability on finite faults governed by rate-and-state friction.

We consider a planar slipping patch in a two-dimensional elastic half-space, arbitrarily oriented beneath a traction-free surface and subjected to either in-plane or anti-plane shear. The corresponding boundary integral formulation accounts for free-surface modifications to elastic stress transfer and, for in-plane slip, the coupling between shear and normal tractions. We examine both creeping and locked updip boundary conditions, together with spatial variations in effective normal stress, and determine the critical fault length at the first Hopf bifurcation.

Across the range of frictional properties, fault orientations, shear modes, and boundary conditions considered, the normalized critical length decreases nonlinearly as the fault approaches the free surface. Thus, relative to predictions for an unbounded medium, shallow faults may become unstable over shorter normalized dimensions. The corresponding dimensional nucleation length, however, also depends on the depth variation of effective normal stress and pore pressure. These results show that free-surface effects can substantially alter shallow fault stability and should be included when assessing the conditions that favor aseismic transients and slow slip near the surface.

Key Words
Slip Stability, Slow Slip Events

Citation
Silver, A. G., & Ciardo, F. (2026, 08). Free-surface controls on the stability of shallow rate-and-state faults. Poster Presentation at 2026 SCEC Annual Meeting.

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