SCRIPPS INSTITUTION OF OCEANOGRAPHY FACULTY CANDIDATE SEMINAR - Seismology
DATE: March 9th, Monday, 3:15 p.m.
LOCATION: Eckart 227
SPEAKER: Daniel Frost
TITLE:The dynamic history of the inner core constrained by seismic anisotropy
The crystallisation of the Earth's inner core over geological times releases light elements and heat into the overlying outer core, driving outer core convection and the generation of the Earth’s magnetic field. Resolving the rate and pattern of inner core growth is thus crucial to understanding the evolution of the geodynamo. The growth history of Earth’s inner core is likely recorded in the distribution and strength of seismic anisotropy - the dependence of seismic wavespeed on the direction of wave propagation. The anisotropy is proposed to result from alignment of intrinsically anisotropic iron crystals by a flow field constrained by the conditions at the inner core boundary. Therefore, patterns of inner core anisotropy inform our understanding of the evolution of outer core convection and of the magnetic field since the inner core nucleation.
I use measurements of seismic body waves sampling deep into the inner core to calculate the strength and distribution of anisotropy. Using seismic tomography and array methods I demonstrate that some inner core data are strongly contaminated by seismic structure in the upper mantle, specifically the Alaskan subduction zone. I use the remaining data to constrain the growth pattern and composition of the inner core. Using geodynamic growth models and mineral physics calculations, I simulate development of inner core texture to match the seismic observations. I propose a model of an inner core composed of an iron-nickel alloy that grows preferentially at the equator and on its eastern hemisphere.
Our model has implications for the evolution of structure throughout the Earth. It lends support to the idea of flow in the outer core organized in Taylor columns with a small asymmetry in heat flux in the equatorial region near the inner-core boundary. The modeling implies a relatively young inner core (1.0-1.4 Ga), and suggests that a stable pattern of long wavelength heterogeneity may have persisted at the base of the mantle since it started crystallizing.