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 2143
Crustal and upper mantle structure beneath central South America
deduced from ScS reverberation waveforms and receiver functions
Dr. Keiko Kuge
Department of Geophysics Kyoto University IASPEI
Yusuke Okaue
We investigated the crust and upper mantle structure beneath the central South America, approximately
at 20 degree south, where the Nazca plate is subducting beneath the western coast and the Brazilian
craton resides in the east. Our approach is to model ScS reverberation waveforms and receiver
functions at GSN stations. ScS reverberation waveforms are sensitive to the path-averaged S-wave
velocities, attenuation, and depths and reflection coefficients of discontinuities in the crust and mantle.
Our procedure to determine these parameters follows that of Kato et al. (2001) utilizing ray-based
theoretical waveform modeling and grid search optimization. Finiteness of source time duration is
additionally taken into consideration in synthesizing waveforms of a great earthquake in this study.
Depths of mantle discontinuities were separately estimated from arrival times of PS conversion phases
on receiver functions. We also modeled receiver functions from early P coda to determine crustal
structure. We found significant differences in the crust and uppermost mantle between the western
coast and eastern continent. The Moho beneath the station NNA on the western coast is deeper than
beneath the station BDFB in the eastern continent. While the Conrad and Moho beneath BDFB were
estimated at depths of 22 and 40 km, respectively, discontinuities at depths of 25, 35, 60, and 70 km
were detected beneath NNA. Some phases on the receiver functions imply an inclination of the
discontinuities at 60-70 km depths, and the discontinuities can be the manifestations of the top of the
subducting Nazca plate. A high value of Q (>400) was obtained for the upper mantle beneath NNA, but
not beneath BDFB. This value is much higher than predicted from the expected mantle transition zone
temperature, and an origin of this high Q might exist in the uppermost mantle. These observed
differences between the east (NNA) and west (BDFB) could be attributed to the subducting Nazca plate.
In the transition zone, the 410-km discontinuity is shallow beneath the western coast, which is
consistently found in both studies. The mantle transition zone is thinner in the western coast (260 km)
than in the eastern continent (270km), but it is thicker in either area than that beneath Japan. On the
receiver functions in the western coast, we were able to identify a phase possibly from the 520-km
discontinuity. Using SS reflection coefficients from the ScS reverberations and PS conversion coefficients
from the receiver functions, we attempted to obtain constraints on density and Vs contrasts at mantle
discontinuities. For the 410-km and 660-km discontinuities, the upper bounds of density contrast are 7-
9 % and 15-20 %, respectively, while the upper bounds of Vs contrast are 6-14 % and 7-20 % or more,
respectively.
Keywords: south america, crust, upper mantle