Joseph Alan Henton

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 31 in a N^S direction, in good agreement with inference from crustal earthquake statistics. On the longterm, the shortening rate is 5^6 mm yr 31 , 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 (V 7m m yr 31 ), 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. Published by Elsevier Science B.V.