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GLOBAL MANTLE STRUCTURE 2006 <strong>IRIS</strong> 5-YEAR PROPOSAL Global Anisotropy and the Thickness of Continents Yuancheng Gung, Mark Panning, Barbara Romanowicz • Berkeley Seismological Laboratory Maximum depth for which the velocity anomaly with respect to the reference model PREM (Dziewonski and Anderson, 1981) is greater than 2%, for different S-velocity models. Left: “SH” type models; right: “SV” type models. Bottom: V SH model SAW24B16 (Mégnin and Romanowicz, 2000) compared to V SV model S20RTS (Ritsema et al., 1999); middle: SH and SV parts of model S20A (Ekström and Dziewonski, 1998) obtained by inverting T component data and Z,L component data separately; top: SH and SV parts of anisotropic model SAW16AN discussed here. While the roots of continents generally extend to depths greater than 300-350 km in SH models, they do not exceed 200-250 km in SV models. Since the concept of “tectosphere” was first proposed, there have been vigorous debates about the depth extent of continental roots. The analysis of heat flow, mantle xenoliths, gravity and glacial rebound data indicate that the coherent, conductive part of continental roots is not much thicker than 200-250 km. Some global seismic tomographic models agree with this estimate but others indicate much thicker lithosphere under old continents, reaching at least 400 km in depth. Although global Vs models differ from each other significantly in the depth range 200-400 km under the main continental shields, these differences are consistent when they are classified into three categories, depending on the type of data used to derive them: ʻSVʼ (mostly vertical or longitudinal component data, dominated by Rayleigh waves in the upper mantle), ʻSHʼ(mostly transverse component data, dominated by Love waves), and ʻhybridʼ (3 component data). ʻSHʼ and ʻhybridʼ models are better correlated with each other than with ʻSVʼ models, and this difference is accentuated when the correlation is computed only across continental areas. Also, ʻSHʼ (and ʻhybridʼ) models exhibit continental roots that exceed those of ʻSVʼ models by 100 km or more. These disagreements can be reconciled when taking into account anisotropy. We have developed a radially anisotropic model of the upper mantle, based on the inversion of waveforms of fundamental and higher mode surface waves, in the framework of Non-Linear Asymptotic Coupling Theory (Li and Romanowicz, 1995), and including truly anisotropic kernels. Significant radial anisotropy with V SH >V SV is present under most cratons in the depth range 250-400 km, similar to that reported earlier at shallower depths (80-250 km) under ocean basins. We propose that in both cases, this anisotropy is related to shear in the asthenospheric channel, located at different depths under continents and oceans. The seismically defined lithosphere is then at most 200-250 km thick under continents. The Lehmann discontinuity, observed mostly under continents around 200-240 km, and the Gutenberg discontinuity, observed under oceans at shallower depths (~ 60-80 km), may both be associated with the bottom of the lithosphere, marking a transition to flow-induced asthenospheric anisotropy (Gung et al., 2003). Gung, Y., M. Panning and B. Romanowicz, Global anisotropy and the thickness of continents, Nature, 422, 707-710, 2003. NSF grant which supported this work: EAR- 9902777 146
2006 <strong>IRIS</strong> 5-YEAR PROPOSAL GLOBAL MANTLE STRUCTURE Imaging the Anisotropic Shear-wave Velocity Structure of the Earth’s Mantle Bogdan Kustowski, Adam M. Dziewónski, Göran Ekström • Harvard University Over the last 10 years, dozens of new stations of the Global Seismographic Network have been deployed in remote and previously uninstrumented regions. The seismograms recorded at these sites provide new constraints on the structure of the Earthʼs mantle. We use seismograms from the years 1994-2003 to build a new set of waveform data to constrain the transversely isotropic shear-wave velocity structure of the mantle. The waveforms are iteratively inverted for earthquake source mechanisms and for the earth structure using the technique of Woodhouse and Dziewónski (1984). Our new data set consists of 219 well-recorded earthquakes and 10 great (M w ≥ 8) earthquakes. The Harvard CMT solutions of these events are shown in red. Several tens of magnitude 6 earthquakes, which we are also planning to include in the inversion, are shown in blue. All the events distributed throughout the earth recorded by the expanding Global Seismographic Network provide unprecedented global ray-path coverage. The number of seismograms used in the waveform inversion exceeds 120,000 and represents a 60-fold increase compared with the work of Woodhouse and Dziewónski (1984). To further improve resolution in the uppermost and lowermost parts of the mantle, we complement the waveform data set with measurements of surface-wave phase velocities (Ekström et al., 1997) and body-wave travel times (Liu, 1997), respectively. In addition to the anisotropic shear-wave velocity structure, we map the topography of the transition zone discontinuities using measurements of longperiod SS precursors (Gu et al., 2003). The new model will improve our knowledge about the structure and dynamics of the Earthʼs mantle and will allow for more accurate determination of the earthquake hypocenters and source mechanisms. Gu, Y.G., A. M. J . Int., 154, 559-583, 2003. Geophys. 147
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SCIENCE SUMMARIES 2006 IRIS 5-YEAR
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