In the model, horizontal velocity directions are highly controlled by the modulating factor of north-south shear transmitted by the Pacific plate. Changing fault friction on the Pacific-North America plate boundary radically affects this shear component, as does changing the coupling friction on the Cascadia subduction thrust. The model velocities shown are a compromise which allow a best fit to the stress indicators. It is also important to remember that our model is a creep model which represents integration over several earthquake cycles and the GPS data represents much less than one earthquake cycle, reflecting instantaneous elastic strain rates. The arrows in this figure are the calculated fault slip vectors, again related to creep over several earthquake cycles.
Red arrows are horizontal shortening and black arrows are right-lateral rates.
They should simulate the long term slip components of the main faults in western Washington. The numbers are fault segments as specified in the model grid. OWL is the Olympic-Wallowa lineament. The most shortening and dextrak slip occurs on the northeast-dipping Devils Mt. fault zone. Low values of horizontal shortening occur on the OLMT and SFZ west end.