Along‐Strike Coupling Heterogeneity in Cascadia's Slow‐Slip Zone Constrained by GNSS and Reduced‐Order Rate‐And‐State Friction Modeling

Abstract 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.