Vertical land motion of Pacific Northwest

Abstract:

We use GPS measurements from 400 stations located throughout the Pacific Northwest to estimate steady-state vertical land motion for the purpose of constraining relative sea level rise projections. Vertical motions are typically only a few percent of horizontal rates and the same order of magnitude as current sea level rise rates, so may either ameliorate or exacerbate future climate impacts. We use data from receivers operating from 1994 through 2015, each with at least three years of continuous daily measurements. Furthermore, daily position time series resulting from the processing of two GPS centers, namely the EarthScope Plate Boundary Observatory (PBO) and the Pacific Northwest Geodetic Array (PANGA), are considered throughout this study. The goal is two fold: the dissemination into the scientific community of the difference in processing between these two centers, and the level of agreement between the estimated crustal for future sea-level studies in the Pacific Northwest.

We model both target and reference frame receiver trajectories as a superposition of discrete processes comprising steady-state tectonic motion, annual and bi-annual sinusoids exhibiting stationary phase and amplitude that reflect both local hydrology as well as artifacts introduced through satellite clock and orbit corrections, and discrete offsets due to known earthquakes (with Mw > 6) and hardware changes.

Qualitatively, Vancouver Island shows long-term uplift of ~2 mm/year, consistent with both interseismic strain accumulation from the Juan de Fuca subduction along the coast and post-glacial rebound inland, and consistent with earlier reports based on few stations and shorter time series. Further south, coastal uplift rates transition to near-zero south of Pacific Beach, and remain low southward to Cape Blanco. Vertical motion is more heterogeneous throughout Puget Sound, but most regions show subsidence of ~0.5 – 1 mm/yr. The predominant subsidence throughout Puget Sound, where the major population centers lie, is likely to increase the apparent rate of sea level rise and exacerbate the long-term climatic impact to the cosmopolitan centers.