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Coupling Heterogeneity in the Cascadia Slow-slip Zone From GNSS Inversions and Physics-based Modeling

Slow slip events (SSEs) in the Cascadia subduction zone (CSZ) exhibit pronounced along-strike segmentation in moment magnitude and recurrence intervals. Notably, the central Cascadia segment has longer SSE recurrence intervals but smaller moments. Here, we combine geodetic inversion and physics-based numerical modeling accelerated by scientific machine learning (SciML) to investigate the underlying physics of this variability.

We first derive and invert inter-SSE velocities for CSZ coupling at the SSE depth, exploiting the spatial and temporal alignment of SSEs and tremors. We then use a SciML-based reduced-order model of rate-and-state friction (Magen et al., 2025) for a Bayesian inversion using observable SSE recurrence intervals and moment magnitudes to infer the kinematic coupling and effective normal stress in Cascadia’s SSE zone. Our Bayesian inversion suggests near-lithostatic pore pressure (99.3%-99.7% of lithostatic) and coupling values comparable to those geodetically derived. The SSE cycle modeling reveals strong coupling (60 ± 23%) in the northern SSE zone and weaker coupling (40 ± 20%) in the central and southern segments. The agreement between the geodetics and the physics-based models validates inter-SSE coupling heterogeneity as a primary control on Cascadia SSE segmentation. While effective normal stress dictates recurrence intervals, coupling variations regulate moment release. Finally, we compare our long-term kinematic coupling model, which averages over transient SSE recurrence, with the inter-SSEs model to assess the amount of slip deficit accumulating in the SSE zone. In northern and southern Cascadia, SSE slip primarily releases the slip deficit accumulated between SSEs. Central Cascadia accumulates a slip deficit (~40% of plate rate) not accounted for by transient events. Whether this slip-deficit will be accommodated by a future large SSE, a change in SSE rate, or during a margin-wide megathrust rupture remains an important question.

Magen, Y., May, D., Gabriel, A.-A., (2025), EarthArxiv, doi: 10.31223/X5QT7V

Session: SSJ-SSOC-SSA Joint Session: From Slow to Fast Earthquakes: Bridging the Spectrum of Fault Slip - I

Type: Oral

Room: Ballroom G

Date: 4/16/2026

Presentation Time: 03:00 PM (local time)

Presenting Author: Yohai Magen

Student Presenter: No

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