Buoyant Asthenosphere Beneath Cascadia Influences Megathrust Segmentation

Great megathrust earthquakes (magnitude 8+) do not typically rupture an entire convergent margin but rather are limited to one or more along-strike segments. A fundamental question of subduction zone dynamics and hazard assessment is what physical properties or dynamic processes govern megathrust segmentation. Here we use onshore-offshore teleseismic delay time data to tomographically image the upper mantle seismic structure of the Cascadia subduction zone. Our results reveal along-strike segmentation in the oceanic asthenosphere beneath the subducting plate with pronounced low-velocity anomalies below regions of increased plate locking and greater occurrence of episodic tremor and slip. We attribute the anomalous asthenospheric velocities to independent mantle upwellings associated with hot spot-derived material in northern Cascadia and fragmentation processes at a diffuse plate boundary in southern Cascadia. Based on these relations, we hypothesize that subslab buoyancy modulates the plate coupling force at the thrust interface, thereby contributing to the localization of subduction zone segmentation.