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Lithosphere structure of the Earth from surface wave tomography

Abstract

The lithosphere is commonly defined as the outermost rigid layer of the Earth, including the crust and the uppermost part of the mantle. It forms the rigid 'plate' in plate tectonic theory. To study its large-scale structure globally, I use fundamental mode surface waves with a period range between 25 s and 200 s. A large dataset including all long period seismograms publicly available for earthquakes with magnitudes larger than Mw=5.5 between 1977 and 2007 has been assembled. To analyze such a large dataset, an efficient measurement method has been developed based on a cluster analysis technique to measure the velocity and amplitude variations of the surface wave packages. These measurements are then inverted for a self- consistent global dispersion model for both Rayleigh and Love waves, which provides the basic input data for constraining the lithospheric structure at high resolution. These maps of surface wave phase/group velocities match surface tectonics very well. Slow anomalies are found beneath orogenic zones and other regions with thick crust, e.g. the Himalayas and Andes. The Basin and Range province in North America, mid-ocean ridges, and back-arc basins also show up as slow anomalies. Cratons can be seen in the low frequency maps as regions of anomalously high velocities. I also find that azimuthal anisotropy needs to be included in order to obtain reliable isotropic velocity variations for Rayleigh waves, and the uncertainties in earthquake locations can affect the resulting azimuthal anisotropy. In addition, I apply finite-frequency theory to account for the focusing- defocusing effect when modeling the amplitude data and present a set of 2D global attenuation maps for Rayleigh waves. The products from this thesis are expected to find wide use by the seismological and the broader geophysical community. One particular application would be to use the maps of phase/group velocity and attenuation to investigate the evolution of the oceanic lithosphere. All measurement and model files produced in this thesis have been made available on the Internet

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