Several recent studies have used stacking or other techniques to
image secondary seismic phases resulting from reflections and phase
conversions at the transition zone discontinuities. These phases permit more
detailed mapping of the depth and sharpness of the 410- and 660-km
discontinuities than has been possible in traditional modeling of refraction data.
Revenaugh and Jordan [1991a] examined long-period ScS (the core-reflected
shear wave) reverberations from deep earthquakes in the western Pacific and
identified both top- and bottom-side discontinuity reflections. They
were able to constrain the impedance contrasts across both the 410- and 660-km
interfaces and found 
20 km variations in their average depths between
different source-receiver corridors.
A more global picture of discontinuity topography can be provided
by precursors to SS that result from underside discontinuity reflections. The SS
bouncepoints are sufficiently widespread that global maps of discontinuity
topography can be produced [ Shearer and Masters, 1992; Shearer, 1993].
These maps show large scale patterns of topography on the 410- and 660-km
discontinuities with about 30 to 40 km of relief. A 
20-km depression in the
660-km discontinuity is seen in the northwest Pacific, apparently associated
with the subduction zones in this region. The relatively small depth
variations observed for the transition zone discontinuities are consistent with phase
changes causing most of the velocity and density increases [ Revenaugh and
Jordan, 1991a; Shearer, 1991], although a small compositional change cannot be
ruled out.
To obtain higher-resolution images of discontinuity structure, records
from deep events can resolve interfaces in the immediate vicinity of the
subducting slabs. Vidale and Benz [1992] combined data from 881 short-period
stations in the western United States to image discontinuity structure near several
circum-Pacific subduction zones. They found that the 410-km discontinuity was
locally elevated, while the 660-km interface was depressed, a result consistent with
the response of the appropriate phase changes to the colder temperatures within the
subducting slab. Apparent elevation of the 410-km discontinuity near
subduction zones was also noted in observations of precursors to sS [ Zhang and Lay,
1993]. In another short-period study, Wicks and Richards [1993] used
data from an Australian array to produce a detailed map of a depression in the
660-km discontinuity beneath the Izu-Bonin subduction zone.
High-frequency precursors to 
were used by Benz and Vidale [1993] to show that the
410- and 660-km discontinuities beneath the Indian Ocean are locally sharp,
with a significant fraction of the jump in properties occuring within 4 km or
less.