Constraining Interseismic Deformation of the Cascadia Subduction Zone: New Insights from Estimates of Vertical Land Motion over Different Timescales

We determine late Holocene (past 4 kyr) vertical land motion (VLM) rates from relative sea level observations along the coastline of western North America and compare these to contemporary (decadal-scale) rates inferred from Global Positioning System data. The residual rates (contemporary minus late Holocene) indicate uplift at most locations, which likely reflects short-term signals associated with the Cascadia subduction earthquake cycle and processes that were recently activated. Regarding the latter, we model and remove the signals associated with ground water extraction and twentieth-century glacier retreat, which have a significant influence in, respectively, California and southern British Columbia. We interpret the remaining signal as being dominated by interseismic deformation associated with the locking of the Cascadia megathrust. A preliminary comparison of this signal with output from a model of interseismic deformation indicates good agreement at most locations within a range of key model parameter values. This agreement is particularly encouraging as the locking model is based on the inversion of only horizontal land motion observations. Considering the data-model fit at all locations, preference was found for models featuring nearly full locking to a relatively shallow depth (<30 km), although the locking state preference is variable along the coast and not strong. The best fitting parameter set varies considerably from site to site, and no set of parameter values was able to capture the residual VLM rates in northwestern Vancouver Island. Our study indicates that the residual VLM data provide useful constraints for megathrust locking models of Cascadia subduction.