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 2166
3D models of the upper mantle developed using the coupled spectral
element method
Mr. Vedran Lekic
Department of Earth and Planetary Science University of California
Seismic waveforms contain information on the elastic and anelastic structure of the earth. High quality
data from global seismic networks combined with approximate waveform modeling techniques that rely
on first-order perturbation theory have made possible the development of high resolution global models
of shear wave velocity and, recently, radial anisotropy (e.g. Panning and Romanowicz, 2006), with
reliable details of wavelength 1000 km or less. However, mapping the 3D distribution of seismic
attenuation has lagged behind due to difficulties in accounting for purely elastic effects of scattering and
(de)focusing that result from often poorly constrained gradients of elastic structure. Furthermore, even
when elastic structure is known, inaccuracies in forward modeling can obscure the anelastic signal.
Therefore, anelastic 3D models only exist presently for theupper mantle and only resolve wavelengths
larger than 2000 km (degree12 in spherical harmonics). The models agree qualitatively, and,
inparticular, show correlation with velocity structure in the uppermost ~250 km of the mantle. However,
the quantitative agreement is not as good, and the amplitudes of lateral variations, which are critical for
interpretation in terms of physical processes, are not well constrained. Therefore, development of high
resolution global models of attenuation is predicated upon both better retrieval of gradients of elastic
structure and implementation of a forward modeling theory that accurately predicts the effects of elastic
structure on seismic waveforms. In the first step of the development of a new generation 3D Q model
of the upper mantle, we have applied the coupled Spectral Element Method (cSEM, Capdeville et al.,
2003) which allows a complete description of the seismic wavefield to the forward modeling of long
period (60s) waveforms. We present preliminary models of elastic structure developed from a waveform
dataset of 3 component surface waves and overtones recorded at more than 100 stations of the
IRIS/GSN, GEOSCOPE, GEOFON and various regional networks. In particular, cSEM allows accurate
modeling of crustal effects, especially beneath continental shields, where conventional methods that rely
on first order perturbation theory are inadequate. We explore the contamination of elastic models of the
mantle that can result from inadequate forward modeling theory.
Keywords: mantle, tomography, attenuation