Tilted Transverse Isotropy in Earth’s Inner Core

Prof. Jeroen Tromp, Princeton University

2024.10.15（星期二）10:00，理科二号楼2807

Abstract: Seismic waves propagating through the inner core of the Earth along the same direction as its rotation axis exhibit faster arrival times compared to waves traveling in the equatorial plane. This phenomenon has been attributed to a transversely isotropic model of the inner core, wherein a fast symmetry axis parallel to the rotation axis explains these observations. However, recent advancements have led to the development of more intricate inner core models that incorporate distinct regional variations, such as hemispheric features, isotropic shallow layers, and an innermost inner core. Many of these models assume a fixed symmetry axis with spatially variable transverse isotropy, resulting in overly complex representations. In this study, we propose an alternative explanation for the travel times of seismic waves sensitive to the inner core by introducing tilted transverse isotropy. Instead of assuming a fixed symmetry axis with spatial variability, we maintain a fixed magnitude of transverse isotropy while allowing the orientation of the symmetry axis to vary spatially. Our proposed model successfully fits travel time data and performs comparably to spatially variable fixed-axis models, but with fewer parameters. Our model incorporates a central inner core characterized by a pronounced alignment of the fast symmetry axis parallel to the Earth's spin axis. Additionally, it includes two shallow caps beneath the Mid-Atlantic and the Indian Ocean/Indonesia regions, where the symmetry axes are tilted towards the equatorial plane. We anticipate that our novel model will impact future research on the dynamics and mineralogy of the inner core.

Jeroen Tromp is the Blair Professor of Geology, Professor of Geosciences and Applied and Computational Mathematics of Princeton University, and the Director of the Princeton Institute for Computational Science and Engineering. He obtained his BSc in geophysics in 1988 from the University of Utrecht in the Netherlands and PhD in geophysics in 1992 from Princeton University. He was a faculty member of the Department of Earth and Planetary Sciences at Harvard University from 1992 to 2000, and a professor at Caltech from 2000 to 2008, where he was the Director of the Seismological Laboratory and McMillan Professor of Geophysics. Before joining Princeton University in 2009, he spent a year as a distinguished visiting professor in the Department of Civil Engi­neering at the National University of Singapore. Prof. Tromp has worked on various topics in geophysics, including but not limited to earth’s free oscillations, surface-wave propagation on a laterally heterogeneous earth, inner-core anisotropy, global 3D density structure, seismic tomography, seismic hazard analysis, planetary seismology, and post-glacial rebound. He is a co-author of Theoretical Global Seismology (Dahlen & Tromp, 1998), a classical textbook for graduate students in seismology.