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    • Interseismic Strain Accumulation in Western Washington from Campaign-mode and Continuous GPS Measurements

    Giorgi Khazaradze and Anthony Qamar

    Geophysics Program, University of Washington, Seattle, WA 98195;
    gia@geophys.washington.edu; tony@geophys.washington.edu

    Elliot Endo

    U.S. Geological Survey, Cascades Volcano Observatory, 5400 MacArthur Blvd., Vancouver, WA 98661; etendo@usgs.gov
     
    Western Washington state, the focus of our studies, is adjacent to the Cascadia Subduction Zone (CSZ), which stretches from northern California to southern British Columbia. Our results are based on analysis of the data from four annual GPS campaigns (1994-1997) conducted by the University of Washington and the Cascades Volcano Observatory. We have also used some of the data collected by the U.S. Geological Survey (Menlo Park) during the same 4 years. Although the total number of surveyed stations exceeds 100, only 25 stations were surveyed in at least three of the past four years. Our discussion will be limited to the analysis of these 25 stations, combined with the data from a half-dozen continuously recording GPS stations which are currently part of the Pacific Northwest Geodetic Array (PANGA).
     
    We find that the interseismic strain accumulation in western Washington is consistent with a kinematic model in which only the interaction of the Juan de Fuca and the North America plates is considered, while the influence of the Pacific plate is ignored. Most of the observed horizontal station velocities exhibit gradual decrease away from the plate boundary. The orientation of the velocity vectors is approximately parallel to the NUVEL-1a predicted convergence direction of the Juan de Fuca and North America plates. These results are in good agreement with the previous geodetic data, as well as with dislocation and thermal models in which the thrust zone is locked and elastic strain is accumulating towards the next large subduction earthquake.
     
    To better understand the observed deformation rates, we use a 3-D elastic dislocation model originally developed by Flück et al. [1997]. The model is based on a summation of point source solutions, following Okada [1985]. Our preliminary modeling results suggest that the locked and transition zone of the thrust fault along the CSZ in Washington are each wider than the earlier published values of 90 km. This has important implications for the size of the potential megathrust earthquake and associated tsunami along the CSZ. Farther away from the trench, in the Puget lowland, where the highest level of crustal seismicity is observed many stations show a significant northward component in their velocities. his observation, combined with the N-S orientation of the maximum compressive stress direction obtained from earthquake focal mechanisms, may establish an important link between the geodetically determined strain rates and stress fields deduced from seismology. However, due to the relatively short time span of the data, and the low level of observed velocities (< 10 mm/yr), it will take at least a few more years of continuous GPS monitoring before we can address this question with certainty.