A REGIONALIZED UPPER-MANTLE (RUM) SEISMIC MODEL

Seismic velocity heterogeneity in the Earth's mantle is strongly conce ntrated near its top. The shallow heterogeneity of the mantle correlat es strongly with surface tectonics. We use these observations as const raints of a tomographic experiment aimed at building a regionalized up per mantle (RUM) reference model. We use a select set of teleseismic t ravel times to minimize the mapping of mislocation into structure. The data selection emphasizes the robustness of individual picks. The for m of the RUM model is a set of velocity profiles as functions of depth through the upper mantle for each of the different tectonic provinces of Earth. Together the profiles constitute a three-dimensional model which incorporates considerable structural detail but is described by only 90 parameters and has only about 22 degrees of freedom. This is a chieved by irregularly sampling a detailed regionalization of the glob e, by detailed mapping of subducted lithosphere in the mantle as defin ed by seismicity, and by combining these structures in an irregular gr id in which bookkeeping is efficiently handled. The resulting RUM mode l includes subducting slabs as sharp fast features in the upper mantle . Old continents are fast; young oceans are slow. Models have been der ived for both compressional and shear velocity. The RUM model is desig ned to represent as much of upper mantle heterogeneity as seen by body wave travel times as possible with a simple model. It can be useful a s a reference model for individual tectonic regions. Travel times are efficiently generated for the RUM model. Mislocations of explosions of known location are significantly reduced when corrections for the RUM model are applied to travel time residuals for a spherically symmetri cal Earth model.