Along-strike coupling heterogeneity in Cascadia’s slow-slip zone constrained by GNSS and reduced-order rate-and-state friction modeling

Yohai Magen , Alice-Agnes Gabriel, Dave A May

Slow slip events (SSEs) in the Cascadia subduction zone exhibit along-strike segmentation, where the central segment has longer recurrence intervals but smaller moments. We quantify the controls on this variability by combining geodetic inter-SSE coupling inversion with Bayesian inference of a quasi-dynamic rate-and-state friction SSE-cycle model accelerated by reduced-order modeling. Our simulations show that effective normal stress controls the SSE recurrence interval, while subduction coupling controls the SSE moment. Our inversion of inter-SSE GNSS velocities yields comparable inter-SSE coupling along strike (∼60–70%), whereas long-term coupling ranges from near zero in the south to ∼42% in central Cascadia. Transient SSEs recover the full slip deficit in the south but leave persistent deficits of ∼30% and ∼42% of plate convergence in the north and central segments. These results indicate that effective normal stress and inter-SSE coupling provide a unified geodetic and physics-based explanation for Cascadia SSE segmentation.