Constraints on accumulated strain near the ETS zone along Cascadia

- We explore the assumption that no long-term strain accumulates near the ETS zone in Cascadia.
- Models with and without deep secondary locking are largely indistinguishable for most of the data.
- Uplift data near Newport, OR provides statistically significant evidence for deep locking.
- For Newport, the long-term strain accumulation is positioned on the updip edge of the ETS zone.
- For models incorporating deep locking, up to 20% secondary locking is possible near the ETS zone.

Current national seismic hazard models for Cascadia use the zone of episodic tremor and slip (ETS) to denote the lower boundary of the seismogenic zone. Recent numerical models have suggested that an appreciable amount of long-term strain may accumulate at the depth of ETS and questions this assumption. We use uplift rates from leveling campaigns spanning approximately 50-70 yrs in Washington and Oregon to investigate the amount of potential long-term locking near the ETS zone. We evaluate the potential for deeper locking in Cascadia by exploring a range of locking parameters along the subduction zone, including the ETS zone. Of the four east-west leveling profiles studied, three show a reduction in the misfit when secondary locking near the ETS zone is included; however the reduction in misfit values is only statistically significant for one profile. This would suggest that models including a small amount of secondary locking are broadly indistinguishable from models without any secondary locking. If secondary locking is considered, the leveling data allow for locking up to ∼20% of the plate rate near the updip edge of the ETS zone. These results are consistent with, but less resolved, by GPS observations.