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Mapping the Upper Mantle with the Spectral Element Method
Ved Lekic (Berkeley Seismological Laboratory), Barbara Romanowicz (Berkeley Seismological Laboratory)
Over the past quarter century, ray- and perturbation-theoretical
techniques have enabled tomographers to map the large
scale structure of the Earth’s mantle. However, the approximations
underpinning these techniques prevent them from
accurately predicting the effects of sharp or strong heterogeneities
[e.g. Wang and Dahlen, 1995], as well as those arising
from crustal structures [e.g. Lekic et al., 2010]. As a result, the
images obtained are incomplete and contaminated, and amplitudes
of observed anomalies are poorly constrained. Yet, it is
precisely the amplitude and sharpness of seismic anomalies that
are needed to decipher the temperature and composition (T/C)
variations that underlie them. Mapping T/C variations in the
mantle is crucial for understanding how these control mantle
convection, and is one of the grand challenges facing Earth
Science.
We have replaced traditional forward modeling approximations
by full waveforms computed using the coupled Spectral
Element Method [CSEM: Capdeville et al. 2003], and have
developed a new model of upper mantle 3D shear velocity and
radial anisotropy variations. CSEM is a fully numerical technique
that makes possible very accurate simulations of wave
propagation through complex 3D Earth structures. Our model
– SEMum – confirms the long wavelength structures imaged
by previous efforts, but is characterized by stronger amplitudes,
which implicate composition in explaining the observed heterogeneities.
Furthermore, SEMum reveals structures not clearly
imaged by earlier global studies. Perhaps the most intriguing of
these is the pattern of low velocities beneath the Pacific Ocean,
which might be indicative of ongoing small-scale convection.
Regional comparisons demonstrate that SEMum can match
the resolution of smaller-scale studies, and replicate these
throughout the globe. This is due to both the improved modeling
technique and the wealth of information contained in – and
successfully extracted from – high-fidelity recordings of complete
seismic waveforms provided by the Global Seismographic
Network (GSN), its international partners and regional broadband
networks.
References
Maps of the Voigt average shear wave-speed variations with respect to the average
velocity at each depth. Note that the limits of color scales change with depth
and that the colors saturate in certain regions. Black circles indicate locations
of hotspots.
Capdeville, Y., Chaljub, E., Vilotte, J. P., & Montagner, J. P., 2003. Coupling the spectral element method with a modal solution for elastic wave
propagation in global earth models, Geophys. J. Int., 152, 34–67.
Lekic, V., Panning, M. & B. Romanowicz, 2010. A simple method for improving crustal corrections in waveform tomography, Geophys. J. Int.,
Wang, Z. & Dahlen, F., 1995. Validity of surface-wave ray theory on a laterally heterogeneous earth, Geophys. J. Int., 123(3), 757–773.
Acknowledgements: This study was supported by NSF EAR-0738284 to BR and an NSF Graduate Research Fellowship to VL.
2010 IRIS Core Programs Proposal | Volume II | Upper Mantle Structure and Dynamics | II-199