## Blocks

We define our plates as rigid blocks on a spherical earth bounded by dislocations in an elastic halfspace and invert for poles and rates of rotation that minimize the misfit to the GPS velocities using an extension of the block modeling code by Meade and Hager (2005). The segments that bound the blocks represent uniformly slipping elastic dislocations locked to some specified depth. Because our inversion combines rigid block rotation with elastic strain accumulation effects, the parameterization of the block boundary geometry is critical. Geometry of the block boundaries is based heavily on seismicity and adopted from prior analyses (Bürgmann et al., 2005) or adjusted as indicated by the geodetic data (Kogan et al., 2003).

We invert the horizontal GPS velocities for poles of rotation constrained by the prescribed block geometry defined above. Systematic patterns in the residual velocities (observed minus predicted) are used as an indicator of where and how the model matches the observed surface velocities. Misfit statistics are used to formally evaluate the statistical significance of the plate kinematic scenarios we test.

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