Title: 3D seismic Imaging of the earth's mantle

Barbara Romanowicz
Berkeley Seismological Laboratory, Berkeley, CA, 94705, USA

Abstract: With recent progress in the simulation of elastic wave propagation in realistic 3D earth models combined with significant increases in computational power, seismologists are well positioned to start fully exploiting the wealth of information about earth structure and dynamics contained in a rapidly expanding database of high quality broadband seismic waveforms, accumulated globally and regionally over the last twenty five years.

Global seismic tomography of the earth's mantle - and particularly the lower mantle - has traditionally focused on the inversion of travel times from a small number of seismic phases, well isolated in time domain seismograms. The resulting uneven sampling of the earth, dictated by the available distribution of earthquake sources and receivers, greatly limits the resolution of 3D structure that can be attained. Waveform tomography, on the other hand, allows one to include information from the entire wavefield generated by an earthquake, in a wide frequency band, provided adequate finite frequency kernels are employed.

I will first introduce some key unresolved issues in global dynamics of the earth's interior, that seismic tomography can help address. I will review the current status of global mantle tomography, contrasting images obtained using different datasets and inversion methodologies, and discussing their various strengths and limitations.

I will point out how features associated with the downgoing convective flow appear better understood at the present time than those associated with upwellings. I will then describe how new waveform inversion and forward modeling approaches can lead to significant progress in seismic imaging of "hotter" regions, and illustrate how the combination of elastic, anelastic and anisotropic forward and inverse modeling provides complementary constraints on the structure and dynamics of the mantle, and in particular on such questions as what is the morphology and the nature of the lower mantle "superplumes", their relation to "hotspots" and their role in the global mantle circulation?