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The development and interpretation of seismic mantle tomographic results has usually proceeded under the assumption that fast and slow velocity anomalies reflect a spatially heterogeneous temperature field controlling, or being controlled by, mantle convection. Implicit in this approach is an assumption that the effect of anisotropy on seismic velocities is small in comparison with isotropic thermal or compositional effects, or that the tomographic results represent the average isotropic heterogeneity, even if individual seismic observations are affected by anisotropic structure. Velocity anomalies in the oceanic mantle are commonly interpreted as reflections of the progressive cooling (and localized re-heating) of a mechanical and thermal boundary layer consisting of the rigid oceanic lithosphere and the underlying less viscous asthenosphere. We show here that the interpretation of seismic velocity anomalies is considerably more complicated for the mantle beneath the central Pacific: in a broad area, with its center near Hawaii, seismic data reveal a regional anomaly in elastic anisotropy which produces variations of seismic velocities that are at least as large as those due to thermal effects. Seismic anisotropy is an independent indicator of strain in Earth materials, and our new tomographic results therefore provide constraints on both the buoyancy forces (thermal effects) and flow patterns in the mantle.

A sequence of large earthquakes along the Aleutian arc and Kurile-Kamchatka trench from 1952 to 1965 released interplate stresses accumulated over a much longer period prior to the sequence. The subsequent evolution of postseismic deformation of the Pacific lithosphere has been predicted using a viscoelastic coupling model consisting of a purely elastic oceanic lithosphere overlying a viscoelastic asthenosphere with viscosity = 5 $\times$ 1017 Pa s. Southward propagation of both postseismic strain and velocity fronts is consistent with patterns of (apparently triggered) earthquake occurrence along western North America over the past 30 years, including accelerations in California seismicity from about 1979 to 1994. The model is consistent with observed anomalous velocity of broadly distributed Pacific geodetic sites and suggests that stress redistribution following earthquakes may produce tangible effects over a spatial scale of 1000's of km.

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