Using MOMA broadband array ScS-S data to image smaller-scale structures atthe base of the mantle

ScS-S residuals obtained at stations of the Missouri-to-Massachusetts (MOMA ) temporary broadband seismic array are used to delineate variations in sei smic velocity structure above the core-mantle boundary (CMB) at scales smal ler than observable with tomographic models. South American earthquakes rec orded at MOMA reveal a slow-velocity anomaly that is at least as small as t he limit of the resolution of ScS waves, about 300 km across. This is model ed as being within a region of fast velocities in whole-mantle models. The slow ScS-S residuals correlate well with a peak in ScS/S relative amplitude s. The small region of slow shear velocity at the CMB could be a pocket of lower mantle rock trapped beneath the descending Farallon slab, or evidence of chemical boundary layer variations.