About the Cover

The dense sampling of seismic wave-fields by the USArray Transportable Array is creating unique opportunities for seismic imaging of North America, as described in the article, “Upper Mantle Heterogeneity beneath North America from Travel Time Tomography with Global and USArray Transportable Array Data,” beginning on page 384. On the cover, test inversions with synthetic data demonstrate the gradual eastward improvement of resolution of structure from February to November 2007 (top). Meanwhile, tomography with USArray data has begun to constrain interesting tectonic features in the western United States (bottom). Depth slices through the current MIT P-wave model reveal the contrast in detail resolved beneath the transportable arrays in the west and sparse station coverage in the central part of the continent. Twice yearly the tomographic model will be updated with new USArray travel-time data and made available to the geophysical community.