Ying Zhou, PhD. - US grants

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.

The dominant tectonic process responsible for earthquakes, volcanoes and landform evolution in the continental United States in the past 200 million years has been the subduction of the Farallon plate beneath the North American Plate. Understanding this subduction process has been a challenge as most of the Farallon plate is now several hundred kilometers below the surface. This research takes advantage of a new technique to analyze seismic data and a rich data set collected by EarthScope USArray stations to image the subducted Farallon slab beneath the continental United States, a process similar to the imaging in medical CT Scan. This research aims to advance understanding of the subduction process with focuses on the following questions: (1) How does the older and deeper subducted Farallon plate connect with the younger plate now subducting beneath the Pacific Northwest? (2) What is the relation between the subduction of Farallon plate and the formation of the Yellowstone volcanic track? (3) How does the subducted Farallon plate interact with the deeper mantle across the US continent? High-resolution images of the Farallon slab, seismic measurements as well as computer codes based on the new seismic theory will be made available to the broader Geosciences community. Hands-on course projects and labs based on this research will be developed and implemented in undergraduate teaching.

Slab anomalies have been imaged in wavespeed models in a broad region in North America mantle transition zone, extending from the west coast to the eastern US. The resolutions of wavespeed models are limited in the mantle transition zone due to data sensitivity and the subduction process in the mid mantle remains unclear. The second unresolved question is the role of a deep mantle plume in the formation of the Yellowstone hotspot track. In this research, we will compute finite-frequency sensitivities for waves generated at the 410-km and 660-km discontinuities to image high-resolution mantle transition zone structure beneath the continental US using seismic data recorded at USArray TA stations. The high-resolution models will provide new insights into the dynamics of subduction, especially the relation between subducted materials in the western and eastern US and the role of slab fragmentation in the formation of the Yellowstone hot-spot track. This research will provide a solid foundation for future imaging of mantle transition zone discontinuities and allow the community to take full advantage of frequency-dependent seismic data to tackle scientific questions on mantle convection dynamics.