IGPP Seminar Series

Deciphering Earth's Dynamic Deep Interior using Seismology

by Dr. Edward J. Garnero
Department of Geological Sciences, Arizona State University

Abstract

The whole of Earth’s interior predominantly remains inaccessible (over 99%), and hence the present day thermal and chemical state of the planet, as well as its dynamical behavior and evolution, must be indirectly determined or inferred. The tools to decipher the interior rely mainly on remote sensing methods, which include investigations of geochemistry, geodesy, the geoid, geodynamics, geomagnetism, mineral physics, and seismology. Seismology currently provides the most detailed description of Earth’s present day internal structure, particularly the distribution of elastic property heterogeneity. This, coupled with the other geo-disciplines, defines our current knowledge of the interior, and how it relates to the surface. Of particular importance is structure near the significant boundary layers, as these provide clues regarding the evolution and dynamics of the whole system. The most significant internal boundary layer is the core-mantle boundary (CMB), which is the home of the largest absolute changes in properties within the interior (e.g., density, temperature, flow velocity, phase, state, seismic velocities, and viscosity). Structure near this boundary is expected to be intimately related to, for example, mantle convection and core cooling, with possible links to the fate of subducted material and the genesis of mantle plumes. The CMB region appears as rich in thermal, dynamical, and chemical complexity as the surface, without consideration of which the whole Earth dynamical system cannot be deciphered. In this presentation I will focus on recent high-resolution seismic experiments aimed at better understanding the possible relationship of slabs and plumes to the deep mantle. Our methods include travel time and waveform analyses of seismic waves, array processing, and tomographic inversion. A multi-disciplinary approach is emphasized where possible. Results emphasize a deep mantle that is rich in diversity and complexity, and include: (a) detection of slab material down to 1000 km depth from high frequency P wave reflections, directly confirming the hypothesis of slab penetration into the lower mantle; (b) a vertical “step” in a first-order D” discontinuity at the base of the mantle beneath the Cocos plate, which is interpreted as the phase boundary between Mg-Si perovskite and the recently discovered post-perovskite phase, with the step possibly due to folding and buckling of relatively cold ancient slab material; (c) a multi-layered deep mantle beneath the central Pacific, consistent with a post-perovskite lens within a large-scale (1000’s km laterally) Fe-rich pile of material, underlain by a thin CMB boundary layer – an ultra-low velocity zone (ULVZ); (d) an isolated ULVZ in the SW Pacific consistent with partial melt of mantle rock, and localized plume genesis; and (e) thinning of the transition zone beneath Hawaii: consistent with plume material originating from the lower mantle. This work aims to better understand the relationship of Earth’s surface to its inaccessible interior, i.e., the dynamical evolution of the interior. More information can be found at http://garnero.asu.edu (Publications and Research Images links).
Tuesday, 09 May 2006
3845 Slichter Hall
Refreshments at 3:45 PM
Lecture at 4:00 PM