Closing the gap between regional and global travel time tomography

Recent global travel time tomography studies by Zhou [1996] and van der Hil st ct al. [1997] have been performed with cell parameterizations of the ord er of those frequently used in regional tomography studies (i.e., with cell sizes of 1 degrees-2 degrees). These new global models constitute a consid erable improvement over previous results that were obtained with rather coa rse parameterizations (5 degrees cells). The inferred structures are, howev er, of larger scale than is usually obtained in regional models, and it is not clear where and if individual cells are actually resolved. This study a ims at resolving lateral heterogeneity on scales as small as 0.6 degrees in the upper mantle and 1.2 degrees-3 degrees in the lower mantle. This allow s for the adequate mapping of expected small-scale structures induced by, f or example, lithosphere subduction, deep mantle upwellings, and mid-ocean r idges. There are three major contributions that allow for this advancement. First, we employ an irregular grid of nonoverlapping cells adapted to the heterogeneous sampling of the Earth's mantle by seismic waves [Spakman and Bijwaard, 1998]. Second, we exploit the global data set of Engdahl et al. [ 1998], which is a reprocessed version of the global data set of the Interna tional Seismological Centre. Their reprocessing included hypocenter redeter mination and phase reidentification. Finally, we combine all data used (P, pP, and pwP phases) into nearly 5 million ray bundles with a limited spatia l extent such that averaging over large mantle volumes is prevented while t he signal-to-noise ratio is improved. In the approximate solution of the hu ge inverse problem we obtain a variance reduction of 57.1%. Synthetic sensi tivity tests indicate horizontal resolution on the scale of the smallest ce lls (0.6 degrees or 1.2 degrees) in the shallow parts of subduction zones d ecreasing to approximately 2 degrees-3 degrees resolution in well-sampled r egions in the lower mantle. Vertical resolution can be worse (up to several hundreds of kilometers) in subduction zones with rays predominantly pointi ng along dip. Important features of the solution are as follows: 100-200 km thick high-velocity slabs beneath all major subduction zones, sometimes fl attening in the transition zone and sometimes directly penetrating into the lower mantle; large high-velocity anomalies in the lower mantle that have been attributed to subduction of the Tethys ocean and the Farallon plate; a nd low-velocity anomalies continuing across the 660 km discontinuity to hot spots at the surface under Iceland, east Africa, the Canary Islands, Yellow stone, and the Society Islands. Our findings corroborate that the 660 km bo undary may resist but not prevent (present day) large-scale mass transfer f rom upper to lower mantle or vice versa. This observation confirms the resu lts of previous, global mantle studies that employed coarser parameterizati ons.