Abstract. We stack long-period, transverse-component seismograms recorded by the Global Digital Seismograph Network (GDSN) , Incorporated Research Institutions for Seismology-International Deployment of Accelerometers (IRIS-IDA) (1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996), and Geoscope (1988-1996) networks to map large-scale topography on the 410-and 660-km seismic velocity discontinuities. Underside reflections from these discontinuities arrive as precursors to the SS phase, and their timing can be used to obtain global variations of the depth to the reflectors. We analyze over 13,000 records from events rnb >5.5, focal depth < 75 km, and range 110 ø to 180 ø by picking and aligning on SS, then stacking the records along the theoretical travel time curves for the discontinuity reflections. Separate stacks are obtained for 416 equally spaced caps of 10 ø radius; clear 410-and 660-km reflections are visible for almost all of the caps while 520-km reflections are seen in about half of the caps. The differential travel times between the precursors and the SS arrival are measured on each stack, with uncertainty estimates obtained using a bootstrap resampling method. We then compute discontinuity depths relative to the isotropic Preliminary Reference Earth Model (PREM) at 40-s period, correcting for surface topography and crustal thickness variations using the CRUST5.0 model of Mooney et al. [1995], and for upper mantle S velocity heterogeneity using model S 16B30 of Masters et al. [1996]. The resulting maps of discontinuity topography have more complete coverage than previous studies; observed depths are highly correlated between adjacent caps and appear dominated by large-scale topography variations. The 660-km discontinuity exhibits peak-to-peak topography of about 38 km, with regional depressions that correlate with areas of current and past subduction around the Pacific Ocean. Large-scale topography on the 410-km discontinuity is lower in amplitude and largely uncorrelated with the topography on the 660-km interface. The width of the transition zone, Wxz, as measured by the separation between the 410-and 660-km discontinuities, appears thickest in areas of active sulxluction (e.g., Kurils, Philippines, and Tonga) and thins beneath Antarctica and much of the central Pacific Ocean. Spatial variations in Wxz appear unrelated to ocean-continent differences but do roughly correlate with the S 16B30 velocities in the transition zone, consistent with a common thermal origin for both patterns. The lower-amplitude 520-kin reflector is more difficult to resolve but appears to be a global feature as it is observed preferentially for those bounce point caps with the most data.

The average seismic velocity and density jumps across the 410- and 660-kilometer discontinuities in the upper mantle were determined by modeling the observed range dependence in long-period seismic wave arrivals that reflect off of these interfaces. The preliminary reference Earth model (PREM) is within the computed 95 percent confidence ellipse for the 410-km discontinuity but outside the allowed jumps across the 660-kilometer discontinuity. Current pyrolite mantle models appear consistent with the constraints for the 410-kilometer discontinuity but overpredict amplitudes for the 660-kilometer reflections. The density jump across the 660-kilometer discontinuity is between 4 and 6 percent, below the PREM value of 9.3 percent commonly used in mantle convection calculations.

S U M M A R YGlobal mapping of 410 and 660 km discontinuity topography and transition zone thickness has proven to be a powerful tool for constraining mantle chemistry, dynamics and mineralogy. Numerous seismic and mineral physics studies suggest that the 410 km discontinuity results from the phase change of olivine to wadsleyite and the 660 km discontinuity results from the phase change of ringwoodite to perovskite and magnesiowustite. Underside reflections of the 410 and 660 km discontinuities arrive as precursors to SS. With the recent development of a semi-automated method of determining SS arrivals, we have more than tripled the Flanagan and Shearer (1998a) data set of handpicked SS waveforms. We are able to increase resolution by stacking waveforms in 5 • rather than 10 • radius bins as well as increasing data coverage significantly in the southern hemisphere. The resulting SS-S410S and SS-S660S times are heavily influenced by upper-mantle velocity structure. We perform a joint inversion for discontinuity topography and velocity heterogeneity as well as performing a simple velocity correction to the precursor differential times and find little difference between the two methods. The 660 km discontinuity topography and transition zone thickness are correlated with velocities in the transition zone whereas the 410 km discontinuity topography is not. In addition, the 410 km discontinuity topography is not correlated with the 660 km discontinuity topography, rather anticorrelated, as expected due to the opposite signs of the Clapeyron slopes of their respective phase changes. These results suggest that, whereas the topography of 660 km discontinuity could be dominated by thermal effects, the topography of the 410 km discontinuity is likely dominated by compositional effects. In addition, unlike previous studies which find less topography on the 410 km discontinuity than on the 660 km discontinuity, our 410 and 660 km topography have similar amplitudes.

Abstract.We derive a new map of global topography on the 410-km discontinuity from observations of precursors to PP obtained by stacking almost 25,000 long-period seismograms. The inferred '410' topography exhibits average peak-to-peak amplitude of about 30 km, has a strong degree-one component, and is highly correlated with previous results obtained from SS precursors [Flanagan and Shearer, 1998]. Spatial variations in '410' topography appear unrelated to ocean-continent differences, suggesting that continental roots are not a significant factor in observed global temperature variations at 410 km depth.