Characteristics of Slow Slip Events Explained by Rate‐Strengthening Faults Subject to Periodic Pore Fluid Pressure Changes

Abstract

Geophysical observations suggest that temporal changes in pore fluid pressure correlate with slow slip events (SSEs) at some subduction zones, including the Hikurangi and Cascadia subduction zones. These fluctuations in pore fluid pressure are attributed to fluid migration before and during SSEs, which may modulate SSE occurrence. To examine the effect of pore fluid pressure changes on SSE generation, we develop numerical models in which periodic pore‐pressure perturbations are applied to a stably sliding, rate‐strengthening fault. By varying the physical characteristics of the pore‐pressure perturbations (amplitude, characteristic length and period), we find models that reproduce shallow Hikurangi SSE properties (duration, magnitude, slip, recurrence) and SSE moments and durations from different subduction zones. The stress drops of modeled SSEs range from ∼20–120 kPa while the amplitudes of pore‐pressure perturbations are several MPa, broadly consistent with those inferred from observations. Our results indicate that large permeability values of ∼10−14 to 10−10 m2 are needed to reproduce the observed SSE properties. Such high values could be due to transient and localized increases in fault zone permeability in the shear zone where SSEs occur. Our results suggest that SSEs may arise on faults in rate‐strengthening frictional conditions subject to pore‐pressure perturbations.

The Stability Transition From Stable to Unstable Frictional Slip With Finite Pore Pressure

“…Importantly, our study demonstrates that at low P p , despite the fault hosting instances of slightly undrained fluid pressure, there is no resolvable effect on fault slip or stress drop. When fluid effects are important, fault weakening or strengthening can be rate-controlled by fluid diffusion (Paola et al, 2007;Perez-Silva et al, 2023). The fault plane will experience periodic increased or decreased frictional strength based on the sense of porosity change (Segall et al, 2010;Sibson, 1986)-whether compactive or dilational.…”

“…We cannot rule out the role of P p in natural fault systems, but it seems possible that slow-slip events are caused by more than just the presence of elevated pore pressures. To activate the dilational mechanisms invoked for SSEs, we believe specific attention needs to be given to the fault drainage state throughout the seismic cycle (Perez-Silva et al, 2023).…”

“…The opposite can also occur whereby undrained compaction leads to increased P p and weakening via fluid pressurization (Segall et al, 2010). Numerical models suggest that anisotropic permeability associated with pore pressure pulses may be the primary control on SSEs in systems such as Hikurangi or Cascadia (Perez-Silva et al, 2023). Such mechanisms are often invoked, however, few studies of SSEs analogs from the lab exist to test these ideas.…”