IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS011 Poster presentation 2180
Kirchhoff migration imaging of reflectors and scatterers in the lowermost
mantle beneath Central America
Mr. Alexander Hutko
Earth Sciences University of California, Santa Cruz IASPEI
Thorne Lay
We use tens of thousands of seismograms from South and Central American earthquakes recorded by
western North American seismic networks to image the lowermost mantle beneath Central America
using a 3D Kirchhoff migration method. P wave studies of the deep mantle often rely on some form of
stacking of many records in order to enhance the signal-to-noise ratio of weak phases generated by
deep structure, such as reflections off of the D" discontinuity. These methods, however, often assume
one-dimensional structure, which is at odds with the evidence for significant heterogeneity. Kirchhoff
migration is a three-dimensional stacking method that allows interactions with structure outside of the
source-receiver plane, thus illuminating a much larger volume. The D" discontinuity beneath Central
America has been readily observed in S wave studies and may be the result of the shear wave velocity
increase associated with the recently discovered perovskite to post-perovskite phase transition. This
phase transition is expected to have weaker effects on P wave velocities than on S wave velocities and
the sharpness of this transition is unknown. Using data at post-critical distances, we observe structures
consistent with a P velocity discontinuity about 200 km above the core-mantle boundary (CMB).
Observing this using short period data suggests that the boundary must be less than a few 10s of km
thick, while observation with lower frequency broadband data exclude the possibility of it being a thin
layer. Whether this discontinuity is co-located for both P and S waves is difficult to resolve. Both the
broadband and the short period P wave data sets also reveal a sharp out-of-plane scatterer, which may
be located close to the CMB. The short period data also indicate reflectivity about 400 km above the
CMB, well above the D" discontinuity, and similar reflectivity is observed under the Central Pacific. This
feature appears to be more consistent with a discontinuity than a scatterer, is hinted at in the
broadband data set and is not observed with S waves. We also present results using P wave data from
pre-critical distances. While reflection and scattering coefficients are much lower at these distances, the
imaging volume is much larger. Results from these data hint at a complicated lower mantle with
multiple features whose origins may lie anywhere between the core-mantle boundary and many
hundred km above it.
Keywords: seismology, migration, cmb