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2009-07-31 – SEMINARS by Matthew Agius and Joanne Adam

31 July, 2009 (16:00 GMT), 5 Merrion Square, Dublin 2.

Speaker: Matthew Aguis (DIAS).
Title: Surface-Wave Phase-Velocity Analysis across the Tibetan Plateau using Broadband Interstation Dispersion Measurements.


In order to constrain variations in the crustal and lithospheric structure across Tibet, we measure phase velocities of seismic surface waves. The data are seismograms recorded by broadband instruments of permanent and temporary networks within and around the plateau. Phase-velocity measurements are performed in broad period ranges using a recent implementation of the 2-station method. A combination of the cross-correlation and multimode-waveform-inversion measurements using tens to hundreds of seismograms per station pair produces robust, accurate phase velocity curves for Rayleigh and Love waves. We use our new measurements to infer phase-velocity variations and to constrain shear-velocity profiles in different parts of the plateau, including radial anisotropy and depths of lithospheric discontinuities. We observe the crust to be uniformly thick beneath the central part of the plateau, thinning to 50-60 km in the Northeast and to 40-50 km in Yunnan, south of the Sichuan Basin. A distinctive crustal low-velocity zone in the 25-50 km depth range is required by the dispersion data in the southern part of the plateau. The low-velocity zone is characterised by strong radial anisotropy, likely to indicate pervasive horizontal flow. Towards the north and southeast of the plateau, the crustal shear-velocity profile changes and does not display a distinct low-velocity layer bounded by high-velocity ones above and below. The data indicate thick, seismically fast mantle lithosphere beneath the Himalaya and the southern part of the plateau, in contrast to the thinner and seismically slower
mantle lithosphere in the north.


Speaker: Joanne Adam (DIAS).
Title: Southern Africa contains two of the oldest lithospheric blocks on Earth: the Kaapvaal and Zimbabwe Cratons.


Understanding the physical properties of southern African lithosphere can help us learn more of the formation and evolution of continents. The Kaapvaal Craton and the neighboring Limpopo Belt have been the focus of numerous geophysical and geological studies. Questions remain, however, regarding lateral variations in the structure and thickness of the lithosphere. In this study, we aim to constrain accurate S-wave velocity profiles in different parts of the region. In order to do that, we measured phase velocities of surface waves in broad period ranges using a combination of cross-correlation and multimode waveform inversion approaches. With both methods, we computed dispersion curves of surface waves between pairs of stations. We then computed robust, accurate average dispersion curves
constraining the Lithospheric structure across the Kaapvaal craton and the Limpopo belt. The period ranges is at least 5-200 s for Rayleigh and 5-50 s for Love waves. The results confirm the thinning of the crust from the Limpopo Belt and northern Kaapvaal Craton towards the southern part of the craton, observed previously. A reduction in S-wave velocity near the bottom of the lithosphere at 150-200 km is required by the data both from the Kaapvaal Craton and the Limpopo Belt. This reduction is less pronounced, however, in the Limpopo region.