CSEDI: Testing Resolution of Deep Earth Seismic Structure Under the Pacific Using Geodynamic Models

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The temperature and composition structure in the Earth's lower mantle provides important clues to the chemical differentiation and dynamics of the planet, especially in regard to how heat is transported from the core to the mantle. A large-scale, slow seismic-velocity anomaly in the lower mantle beneath the Pacific has been imaged in tomographic studies. The thermal and compositional structure of this anomaly remains poorly understood. This research integrates seismological and geodynamical efforts to investigate important issues in understanding the structure of this deep seismic anomaly. We will use thermochemical convection models to develop candidate mantle structures that can be investigated with seismic wave propagation simulations using realistic earthquake and receiver geometries. This will allow us to identify seismic phases that are sensitive to differences between the candidate mantle structures and to investigate whether finite-frequency theory provides additional insight beyond ray-theory for the geometry of these structures, and how 3-D attenuation structure affects our interpretation of tomographic images. This research addresses several challenging problems in understanding the thermal and chemical structure in the Pacific lower mantle through seismic wave propagation in thermo-chemical plume models with focuses on (1) the sensitivity of seismic waves to 3-D variations in temperature and compositional in the lower mantle; (2) the effects of different scaling parameters in translation between temperature and compositional anomalies to seismic wave speed and anelastic attenuation and (3) the resolution limits of current seismic data (including USArray data) and seismic tomographic methods (ray theory and finite-frequency theory) in imaging the structure of the Pacific lower mantle and distinguishing between different plume models including 1) the 'standard' isochemical whole mantle model, with a cluster of narrow plumes under the central Pacific, 2) a thermochemical 'dome' (or pile) under the central Pacific with plumes arising from the interface, and 3) an isolated, sluggish lower mantle with upper mantle derived plumes. Seismological challenges, including quantifying the relative importance of anelastic attenuation and focusing-defocusing effects, and tradeoffs between elastic and anelastic structure will be addressed with full wave propagation simulations in a variety of thermochemical plume models.