2012 IRIS Workshop

Global Waveform Tomography of the Upper Mantle Using the Spectral Element Method: A Second-Generation Model

S W French: UC Berkeley – Berkeley Seismological Laboratory, Vedran Lekic: University of Maryland, Barbara Romanowicz: UC Berkeley – Berkeley Seismological Laboratory

Vertical cross sections of Voigt-average Vs structure across Africa, showing strong lateral variations between cratons and neighboring low velocity anomalies (>15% peak to peak). The lower panel highlights the Kalahari and Tanzanian cratons, as well as very strong, bifurcated slow anomalies underlying the East African Rift through Afar - consistent with geochemical studies of melt source-signature variation along the EAR. Background: dlnVs; Foreground: (gray) Vs profiles at marked points.

Full-resolution graphics file in original format: 0125.ai

The SEMum2 model [French et al., 2011] was developed following a similar procedure to that of the first-generation SEMum model of Lekic and Romanowicz [2011], with a particular emphasis on improved treatment crustal structure. Both are models of radially-anisotropic shear velocity and are based upon inversion of long-period (60s) teleseismic waveforms, fitting minor and major-arc fundamental and overtone-mode surface waves. Our approach combines the accuracy of the spectral element method for modeling of the global wavefield [SEM: e.g. Komatitsch and Tromp, 2002], with the efficiency of non-linear asymptotic coupling theory for calculation of waveform sensitivity kernels [NACT: Li and Romanowicz, 1995]. The most salient feature of the resulting models is unusually strong heterogeneity in the upper 200km of the mantle compared to previous global models – particularly evident for low velocity anomalies. A manuscript discussing the development of SEMum2, as well as the properties of the model itself, will soon be submitted for publication.

Acknoweldgements: This work was supported by the National Science Foundation (NSF-EAR 0738284) and SWF acknowledges support from the NSF Graduate Research Fellowship Program. SEM computations were performed at the Department of Energy National Energy Research Scientific Computing Center.

For further reading: French, S. W., V. Lekic, & B. Romanowicz (2011), Toward global waveform tomography with the SEM: Improving upper-mantle images, Abstract S13C-02 presented at 2011 Fall Meeting, AGU, San Francisco, Calif., 5-9 Dec.

Keywords: upper-mantle_structure, tomography, computational_seismology

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