LATERAL VARIATIONS IN MANTLE VELOCITY STRUCTURE AND DISCONTINUITIES DETERMINED FROM P, PP, S, SS, AND SS-SDS TRAVEL-TIME RESIDUALS

On the basis of P, PP, S, SS arrival times and SS - S410S, SS - S660S differential times, we construct models of mantle P and S velocity str ucture and boundary topography of the 410-km and 660-km discontinuitie s. Events from the catalog of the International Seismological Centre ( ISC) are relocated relative to the International Association of Seismo logy and Physics of the Earth's Interior 1991 (IASP91) velocity model using both P and S arrival times. The arrival times are corrected for ellipticity and the PP and SS residuals are corrected for the topograp hy at the bounce point. The cap-averaged PP - P and SS - S differentia l time residuals, plotted at the PP and SS surface reflection points, form broad coherent patterns. The geographic distribution of the cap a veraged residuals agrees quite well with PP - P and SS - S differentia l time residuals derived from long period Global Digital Seismograph N etwork (GDSN) data. A robust l(p) inversion scheme is used to infer gl obal mantle structure. Synthetic tests indicate that for regions well sampled by SS - S410S and SS - S660S differential times, the velocity estimates are not seriously contaminated by the topography of the 410- and BGO-km discontinuities. However, estimates of boundary deflection s may be influenced by extensive P and S velocity variations of 3 % or greater. We find the 410-km discontinuity to be depressed by as much as 24 km beneath North America. Conversely, the discontinuity is defle cted upward underneath Eurasia. In some regions the topography of the 660-km discontinuity is quite distinct from that of the 410-km discont inuity, but the two appear to be positively correlated. A series of de pressions are found at several intersections of the BGO-km discontinui ty with known subduction zones. The elevated topography in the 410-km discontinuity beneath Europe is underlain by a trough in the 660-km di scontinuity. A number of subduction zones are characterized by a thinn ing of the transition zone. Negative P and S velocity anomalies, under lying back-are basins and tectonically active continental regions, enc ircle the Pacific. Where they are resolved, the stable continental cra tons are systematically positive velocity features that extend below 2 00 km. With the inclusion of PP and SS travel time residuals we are be tter able to constrain midmantle structure. Most notably, in the depth range 35-660 km beneath the Northwest Pacific we observe high P veloc ity. Where they are resolved, mid-ocean ridges are most clearly imaged as low velocity features in the S model. The northern portion of the Mid-Atlantic Ridge is underlain by negative S velocity anomalies. In t he Pacific, the East Pacific Rise is an extensive low S velocity anoma ly.