Vertical Land Motion in Western Washington: Separating Cascadia Locking From Other Sources

Date: 4/24/2019

Time: 06:00 PM

Room: Grand Ballroom

To help forecast local relative sea level rise in coastal Washington, we have compiled a new vertical land motion (VLM) dataset for Western Washington that combines GPS, tide gauges, and differential leveling data. We find two dominant signals in the data that are most-evident in north-central Washington, an east-west gradient consistent with locking along the Cascadia subduction zone (CSZ), and a smaller magnitude north-south gradient that is apparent east of the region affected by subduction zone locking. The two gradients are superimposed in the coastal region, although VLM data are routinely used to inform subduction zone models that assume the data are absent of independent sources of motion. To assess the contribution of each component, we generate simple elastic dislocation models for CSZ locking with and without the north-south gradient by subtracting the observed gradient east of the Puget Sound from all of Washington, including the CSZ locking-dominated gradient in coastal Washington, west of the Cascades. We compare the predicted horizontal strain from each model to the observed horizontal strain, measured from horizontal GPS motion. Preliminary results show that the observed vertical and horizontal strain best fit our model with the north-south regional uplift gradient removed. Therefore we hypothesize that this gradient is superimposed on the coastal region, and likely unrelated to the CSZ. We suggest that the observed north-south gradient is GIA, however other possible mechanisms are possible, including a 3D viscoelastic response to past CSZ rupture, and local subsidence associated with the broader Puget Sound forearc basin region.

Presenting Author: Tyler J. Newton

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