GPS deformation in a region of high crustal seismicity: N. Cascadia forearc

We estimate the rate of crustal deformation in the central and northern Cascadia forearc based on a combination of existing global positioning system (GPS) velocity data along the Cascadia subduction zone. GPS strain rates and velocities show that the northwestern Washington–southwestern British Columbia region is currently shortening at 3–3.5 mm yr−1 in a N–S direction, in good agreement with inference from crustal earthquake statistics. On the long-term, the shortening rate is 5–6 mm yr−1, providing that the subduction-related interseismic loading of the margin is purely elastic. Compared to the velocity of the Oregon forearc with respect to North America (∼7 mm yr−1), this indicates that most of the forearc motion is accommodated in the Puget–Georgia basin area, corresponding to the main concentration of crustal seismicity. The difference between the current and long-term shortening rates may be taken up during subduction megathrust earthquakes. Thus, these events could produce a sudden increase of N–S compression in the Puget sound region and could trigger major Seattle-fault-type crustal earthquakes.