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

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. We find that effective normal stress primarily controls SSE recurrence interval, whereas inter-SSE coupling heterogeneity governs along-strike moment variability. Our independent inversion of inter-SSE GNSS velocities yields mean coupling of ~60% in northern Cascadia, ~48% in central Cascadia, and ~34% in southern Cascadia, with lower long-term coupling. Comparing inter-SSE and long-term coupling suggests that transient SSEs recover ~1/3 of the slip deficit in the north and south but only $\sim$16\% in central Cascadia, implying persistent slip-deficit accumulation in the central margin. Together, these results suggest that coupling heterogeneity provides a unified geodetic and physics-based explanation for Cascadia SSE segmentation.