Evidence for Stress Diffusion and Along-Strike Segmentation in Cascadia Tremor Migrations Lasting Minutes to Weeks

The properties and rheology of subduction zones have been intensively studied to forecast potential megathrust earthquake scenarios. However, in Cascadia, the absence of recent megathrust events limits available seismic evidence. Tectonic tremors occurring downdip of the megathrust provide valuable insights into stress accumulation and propagation and can help constrain stress states and rheology. We build upon a previous method to extract tremor migrations in a large catalog of ∼330,000 tremors, addressing location errors and temporal resolution of the catalog, which are particularly crucial for this region. The spatiotemporally dense tremor activity identified by our improved method, along with the greatly increased number of extracted migrations (∼13,700), facilitates a more quantitative analysis. Our findings suggest that tremor migration is primarily controlled by stress diffusion in a medium exhibiting viscous behavior rather than fluid diffusion. The observed relationship between migration speed and duration implies a diffusivity of 10³–10⁵ m²/s, and aligns with a simple model indicating an approximately 30-km-wide zone of slow slip and tremor propagation. Additionally, we identify three along-strike barriers to tremor migrations, consistent with previously identified segments persisting from shallow to deep. Notably, the barrier near 48.5°N consistently decelerates, terminates, or initiates large tremor episodes, likely due to geometric distortions, including flattening and bending of the slab. In contrast, a barrier near 42.5°N abruptly halts migrations and accumulates stress, but can be breached by sufficient stress perturbation. Thus, tremor migrations can constrain geometric segmentation and diffusive behavior of tremorgenic regions from a dynamic perspective.