| The Pacific Northwest has potential for huge megathrust earthquakes. The influence of plate locking in the Cascadia subduction zone dominates crustal deformation off the shores of Washington and Oregon, but does not much affect areas far from the trench. The maximum principal strain rate ϵ 1 is −0.013 ± 0.007 μstrain/yr in the Olympic Peninsula, 0.007 ± 0.005 μstrain/yr in the Puget Sound, −0.005 ± 0.005 μstrain/yr at Mt. Rainier, −0.004 ± 0.005 μstrain/yr along the northern Oregon coast, and 0.011 ± 0.006 μstrain/yr in central Oregon. The minimum principal strain rate ϵ2 is −0.083 ± 0.008 μstrain/yr N56°E in the Olympic Peninsula, −0.034 ± 0.007 μstrain/yr N63°E in the Puget Sound, −0.020 ± 0.006 μstrain/yr N53°E at Mt. Rainier, −0.051 ± 0.014 μstrain/yr N85°E along the northern Oregon coast, and −0.010 ± 0.006 μstrain/yr N71°E in central Oregon. A new model of plate locking on the Cascadia subduction zone is similar to a model (1997). The uncertainty of the widths of the locked and transition zone in the model is about 25km–40km.; Guided by computed site velocities, seismicity patterns, heat flow, volcanic data, and geological structures, we find it is necessary to divide the crust in the Pacific Northwest into separate moving blocks. We have analyzed a model in which the Oregon block is separated from the Washington block at latitude 46°. The Washington block has been further divided into 5-subblocks, three in the forearc and two in eastern Washington. We remove contributions of JDF plate locking from the site velocity field and determine a rotation pole and a strain rate for each sub-block. We conclude that Juan de Fuca plate locking has little direct effect on crustal earthquake occurrence in the Pacific Northwest (except for periodic megathrust earthquakes). In the Oregon block, plate locking and rigid block rotation are sufficient to explain GPS observations and the lower rate of seismicity in Oregon. The Washington block is more. The southwestern Washington sub-blocks have higher rotation rates and smaller residual strain rates than the northern sub-blocks. The Olympic sub-block shows the greatest north-south compression (0.017 μstrain/yr). The Puget Lowland sub-block and Mt. Rainier sub-block are shortening along a NNE direction, roughly consistent with the direction of maximum principal stress from fault plane solutions. The overall north-south shortening across the Puget Sound is 3mm/yr, sufficient to generate M7+ earthquakes in the future. NS compression dominates eastern Washington near Yakima which is also consistent with principal stress directions derived from fault plane solutions. Northeast Washington sub-block is currently poorly constrained by GPS data.; As western Washington is pushed northward against the British Columbia, block rotation in Oregon is slowed down in Washington and the motion produces NS compression in northern Washington. The sub-blocks in southwestern WA act as a transition zone between Oregon and British Columbia. Residual strain rate is proportional to crustal seismicity. The great variation of residual strain rate in WA compared to Oregon may explain their crustal seismicity difference. GPS derived velocities in NE Washington are still too uncertain to determine the details of block rotation and strain rate there. |
