NEW_Accomplishments.indd - IRIS

CORE–MANTLE BOUNDARY
2006 IRIS 5-YEAR PROPOSAL
A Step in the D” Discontinuity Imaged by Kirchoff Migration Through
3D Tomographic Models
Alex Hutko, Thorne Lay • University of California, Santa Cruz
Justin Revenaugh • University of Minnesota
Ed Garnero • Arizona State University
A simple migration scheme is used to image topography of between 50 and 100 km over less than 200 km laterally of the
D” discontinuity. Correcting for first-order travel-time perturbations based on one-dimensional ray tracing through S-wave tomography
models does not change our interpretation, but does affect the absolute depth of the reflector. The abrupt change in
its height is unlikely to be due to a phase change and temperature differences alone, so chemical heterogeneities are required
to be consistent with the observations.
We also observe
a low-velocity scatterer that
is approximately 2 degrees
in diameter. This feature is
more model dependant but
agrees with finite frequency
tomography results from
Hung et al. (2004). Though
this ray-based method does
not account for bending of
the ray path due to three-dimensional
structure nor does it include scattered energy after the
ray
has reached its turning point, we find it works remarkably well
where data density is sufficient, while having significant gains is
ef-
ficiency over wave equation based methods. In its initial phase, US
Array will have an aperture approximately three times greater than
currently available to study the lower mantle beneath the Americas.
The greater number of crossing rays, which is critical for lower
mantle studies, will undoubtedly lead to further exciting discoveries
and new constraints for other members of the Earth science community.
Map showing earthquakes (black stars), seismographs (red triangles), ScS bounce
points on the CMB (blue dots), and the great circle paths along which the migrated
data stack (bottom figure) was made (green lines). Our data set consists
of 273 seismograms from 61 receivers recording ground displacement from 15
deep earthquakes. The black rectangle shows the area where our resolution is
high enough to map D” discontinuity topography and detect features as small
as 100 km. The grey rectangle shows the approximate area that we will be able
to image at High-Resolution with data from US Array. Imaging small features in
the lowermost mantle across large scale lengths is key to understanding their
relationship with the overlying tectonics.
Hung, S.-H.,, Garnero, E. J., Chiao, L.-Y., Kuo,, B.-Y. , Lay, T., Finite-frequency tomography of Dʼʼ shear velocity heterogeneity beneath the Caribbean, J.
Geophys. Res., in review, 2005.
Lay, T., E.J. Garnero, and S.A. Russell, Lateral Variation of the D” Discontinuity Beneath the Cocos Plate, Geophys. Res. Lett., 31, doi:10.1029/
2004GL020300, 2004.
Thomas, C., Garnero, E.J., and Lay, T., High-resolution imaging of lowermost mantle structure under the Cocos Plate, J. Geophys. Res., 109, doi:10.1029/
2004JB003013, 2004.
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