SEISMICITY AND HORIZONTAL STRAIN RATE IN THE PACIFIC NORTHWEST

GPS data from nearly 300 GPS sites have been used in order to understand seismicity patterns and tectonics in the Pacific Northwest. Guided by computed site velocities, seismicity patterns, and geological structures, we have analyzed a model in which the Oregon block is separated from the Washington block at latitude 46º. 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 computed translation vectors and determine rotation pole and 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 NW. In the Oregon block, plate locking and rigid block rotation are sufficient to explain GPS observations and lower rate of seismicity in Oregon. The Washington block is more complicated after removal of plate locking. 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 NS compression (1.8mm/degree/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. NS compression dominates the Yakima block which is also consistent with principal stress direction derived from fault plane solutions. However the northeast Washington sub-block shows a NE extension, but this is currently poorly constrained by GPS data

When western Washington is pushed against the ''backstop'' of British Columbia, block rotation in Oregon is converted to NS compression in northern Washington. The sub-blocks in southwestern WA act as a transition zone between Oregon and British Columbia. The greater variation of strain rate in WA compared to Oregon may explain the increased crustal seismicity in WA. The driving force of crustal earthquakes in Washington can be interpreted as coming from block rotation in Oregon which changes to NS compression in WA. GPS derived velocities in NE Washington are still too uncertain to determine the details of block rotation there