Scattered-Wave Imaging We are developing algorithms for mapping sources of scattered energy in the crust and uppermost mantle using regional array recordings of teleseismic and/or local events. and using them to examine three important problems: (1) the contribution of topography and shallow heterogeneity to high-frequency, high-amplitude ground motion; (2) distribution of scatterers in the uppermost mantle; and (3) the influence of crustal heterogeneity on seismogenesis. The last topic likely the most exciting. We have been able to demonstrate extremely strong correlations between the local density of aftershocks, the amount of co-seismic slip and the strength of near-fault scattering. The results offer a convincing argument for structural control of earthquake rupture. What makes this really exciting is the possibility of predicting spatial patterns of slip prior to future events, perhaps identifying areas of particular hazard. There is even the possibility of intermediate term prediction through monitoring of scattering strength along prominent fault zones.

CANOE: CAnadian NOrthwest Experiment We are conducting a broad-band seismic experiment in northwestern Canada, a setting in which fundamental questions regarding the structure and dynamics of the mantle's upper and lower boundary layers can be uniquely addressed. Characterized by a series of crustal terranes welded together in order of increasing age from west to east and spanning ~4.0 Ga of Earth's history, the region represents the most nearly complete and continuous age transect of continental lithosphere on the planet and is the ideal location for investigation of the modes and temporal variation of lithospheric assembly and craton stabilization. Our goal is to produce a systemic, 3D determination of volumetric heterogeneity, anisotropy, and discontinuity structure in the upper mantle. In addition, northwestern Canada is at ideal distances from several well-populated earthquake zones, permitting detailed analyses of deep mantle and core-mantle boundary structure beneath the US, the circum-Pacific subduction zones and the central Pacific (including Hawaii). The array consists of approximately 48 3- component broadband (PASSCAL-type) seismic stations, and is anchored by Canadian Seismic Network Stations in Edmonton, Whitehorse, Yellowknife, and Ft. Nelson. The array will be deployed for a 16-month period.

D" and the Core-Mantle Boundary We are studying structural heterogeneity within the D" layer (lowermost 200 to 300 km of the mantle) using high-frequency scattered waves. Of primary interest is the detection and characterization of an ultra-low velocity layers or zones atop the core-mantle boundary. Playing a role very similar to the crust, the ultra-low velocity layer occupies a spatial and temporal middle ground between the silicate mantle and iron core. The likely presence of partial melt ensures low viscosity and formation of a strong convective boundary layer. Disruption and expulsion of boundary layer material is a logical source for mantle plumes. The notion that a thin (< 20 km) layer at the base of the mantle dictates the breakup of continents (through hotspot induced rifting), secular cooling of the lower mantle, and the creation of flood basalts is fascinating me. We are working on several seismic detection and mapping schemes to extend the present seismic sampling of this important structural feature

Kellogg and van der Hilst

Discontinuities in the Mid-Mantle Whether or not the Earth's mantle is chemically layered is one of the oldest questions in geology. Geochemical arguments in favor of chemical layering are countered by seismic images of deep slab penetration and the absence of any consistently observed discontinuity beneath the transition zone. To help answer this important question, we are adapting tools from adaptive optics in the search for mid-mantle discontinuities that might be associated with chemical layering. Using data from regional arrays and hundreds of earthquakes, our methods search for the small seismic arrivals expected from minor changes in composition. Although the targets are subtle, the implications are not. The dynamics of a chemically segregated mantle are very different from a well-mixed mantle: plumes, secular cooling, the fate of slabs, and other aspects of mantle circulation are very different in a layered mantle.

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