S U M M A R YWe interpret Global Positioning System (GPS) measurements in the northwestern United States and adjacent parts of western Canada to describe relative motions of crustal blocks, locking on faults and permanent deformation associated with convergence between the Juan de Fuca and North American plates. To estimate angular velocities of the oceanic Juan de Fuca and Explorer plates and several continental crustal blocks, we invert the GPS velocities together with seafloor spreading rates, earthquake slip vector azimuths and fault slip azimuths and rates. We also determine the degree to which faults are either creeping aseismically or, alternatively, locked on the block-bounding faults. The Cascadia subduction thrust is locked mainly offshore, except in central Oregon, where locking extends inland. Most of Oregon and southwest Washington rotate clockwise relative to North America at rates of 0.4-1.0 • Myr -1 . No shear or extension along the Cascades volcanic arc has occurred at the mm/yr level during the past decade, suggesting that the shear deformation extending northward from the Walker Lane and eastern California shear zone south of Oregon is largely accommodated by block rotation in Oregon. The general agreement of vertical axis rotation rates derived from GPS velocities with those estimated from palaeomagnetic declination anomalies suggests that the rotations have been relatively steady for 10-15 Ma. Additional permanent dextral shear is indicated within the Oregon Coast Range near the coast. Block rotations in the Pacific Northwest do not result in net westward flux of crustal material-the crust is simply spinning and not escaping. On Vancouver Island, where the convergence obliquity is less than in Oregon and Washington, the contractional strain at the coast is more aligned with Juan de Fuca-North America motion. GPS velocities are fit significantly better when Vancouver Island and the southern Coast Mountains move relative to North America in a block-like fashion. The relative motions of the Oregon, western Washington and Vancouver Island crustal blocks indicate that the rate of permanent shortening, the type that causes upper plate earthquakes, across the Puget Sound region is 4.4 ± 0.3 mm yr -1 . This shortening is likely distributed over several faults but GPS data alone cannot determine the partitioning of slip on them. The transition from predominantly shear deformation within the continent south of the Mendocino Triple Junction to predominantly block rotations north of it is similar to changes in tectonic style at other transitions from shear to subduction. This similarity suggests that crustal block rotations are enhanced in the vicinity of subduction zones possibly due to lower resisting stress.

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.[1] We document a sequence of simultaneous short-term and long-term slow slip events (SSEs) at the Hikurangi subduction zone during the 2010/2011 period. The sequence of short-term events (each 2-3 weeks in duration) ruptured much of the shallow plate interface (<15 km) at central and northern Hikurangi over a 6-month period, was accompanied by microseismicity and involved patchy, irregular migration of SSE slip. We suggest that the patchy migration of the short-term SSE is due to large-scale (100-3500 km 2 ) heterogeneities on the plate interface related to seamount subduction and sediment subduction and/or underplating. This is in contrast to a 2010/2011 long-term SSE at the central Hikurangi margin, which evolved steadily over 1.5 years and ruptured much of the plate interface between 20 and 70 km depth. We suggest that the occurrence of long-term versus short-term SSEs at Hikurangi is related to differences in effective normal stresses and relative heterogeneity of the subduction interface. The long-term SSE sequence began 1 year before the short-term sequence. Coulomb stress change models suggest that the long-term SSE may have triggered initiation of the subsequent short-term SSE sequence. Initiation of the short-term sequence occurred in a region just updip of or within an interseismically locked portion of the plate interface and may be located within the updip transition from seismic to aseismic behavior. Alternatively, it could be characteristic of a region undergoing partial interseismic coupling. This is in contrast to SSEs observed elsewhere in the world that typically occur within the downdip transition from seismic to aseismic behavior.Citation: Wallace, L. M., J. Beavan, S. Bannister, and C. Williams (2012), Simultaneous long-term and short-term slow slip events at the Hikurangi subduction margin, New Zealand: Implications for processes that control slow slip event occurrence, duration, and migration,

[1] We employ a domain decomposition approach with Lagrange multipliers to implement fault slip in a finite-element code, PyLith, for use in both quasi-static and dynamic crustal deformation applications. This integrated approach to solving both quasi-static and dynamic simulations leverages common finite-element data structures and implementations of various boundary conditions, discretization schemes, and bulk and fault rheologies. We have developed a custom preconditioner for the Lagrange multiplier portion of the system of equations that provides excellent scalability with problem size compared to conventional additive Schwarz methods. We demonstrate application of this approach using benchmarks for both quasi-static viscoelastic deformation and dynamic spontaneous rupture propagation that verify the numerical implementation in PyLith.Citation: Aagaard, B. T., M. G. Knepley, and C. A. Williams (2013), A domain decomposition approach to implementing fault slip in finite-element models of quasi-static and dynamic crustal deformation,

ABSTRACT. We have used a three-dimensional elastic finite element model to examine the effects of topography on the surface deformation predicted by models of magma chamber deflation. We used the topography of Mt. Etna to control the geometry of our model, and compared the finite element results to those predicted by an analytical solution for a pressurized sphere in an elastic half-space. Topography has a significant effect on the predicted surface deformation for both displacement profiles and synthetic interferograms. Not only are the predicted displacement magnitudes significantly different, but also the map-view patterns of displacement. It is possible to match the predicted displacement magnitudes fairly well by adjusting the elevation of a reference surface; however, the horizontal pattern of deformation is still significantly different. Thus, inversions based on constantelevation reference surfaces may not properly estimate the horizontal position of a magma chamber. We have investigated an approach where the elevation of the reference surface varies for each computation point, corresponding to topography. For vertical displacements and tilts this method provides a good fit to the finite element results, and thus may form the basis for an inversi6n scheme. For radial displacements, a constant reference elevation provides a better fit to the numerical results.