Looking Inside the Earth and Planets with Coda-Correlation
Professor Hrvoje Tkalčić
Research School of Earth Sciences (RSES), the Australian National University
15:00, Friday, October 20, 2023; Room 2821, Science Building II
Eighty-six years have passed since the discovery of the Earth’s inner core and more than fifty years since our first journey to the Moon. Meanwhile, global seismology has come a long way in providing insights into Earth's internal structure and dynamics. Recent studies addressed 3D heterogeneity, attenuation, anisotropy and differential rotation of the inner core with an unprecedented level of detail. For example, we recently confirmed that its innermost part contains distinct seismic anisotropy from three different lines of study. However, further progress in imaging the Earth's inner core – a planet within the planet – has been impeded by the lack of geographic coverage of body waves from large earthquakes. In seeking the ways forward, we started experimenting with earthquake coda correlation. This contributed to the rise of a new paradigm – the coda-correlation wavefield.
The new data probes the Earth with periods ranging between 20 and 50 s, thus representing a new class of studies that falls between body-wave and normal-mode methods. The first applications proved the inner core’s solidity by unambiguously detecting shear waves and anisotropy in shear-wave velocity. We then showed that a single seismograph and global-scale waveform cross-correlations between seismic events (inter-source correlation) could be used to scan planetary cores. This technique allowed us to constrain the sizes of the cores of Earth and Mars and confirmed that the Martian core is large. As I hope to demonstrate in this lecture, the coda-correlation studies – apart from further development and the proliferation of seismic sensors – may play a central role in global and planetary seismology in the coming decades.
Hrvoje Tkalčić is a professor and Head of Geophysics at the RSES, the Australian National University. He graduated from the University of Zagreb with a Diploma of Engineering in Physics, specializing in Geophysics with Meteorology. He gained his PhD in geophysics from UC Berkeley in 2001. He was a postdoctoral fellow at Scripps/UCSD and LLNL. His research interests include the structure and dynamics of the Earth’s interior using observational seismology and mathematical geophysics, from the crust to the Earth’s center. Professor Tkalčić is the Director of the Warramunga Seismic and Infrasound Array in the Northern Territory, Australia, operated on behalf of the UN CTBTO and the Australian Government. He participates in improving global coverage of seismic data by deployment in remote regions of the Earth, including oceans. He has authored over 100 peer-reviewed research papers and the first book on the Earth’s inner core, The Earth’s Inner Core Revealed by Observational Seismology, published by Cambridge University Press in 2017. He received the inaugural award from AuScope for Excellence in Research in 2016 and the Price Medal from the Royal Astronomical Society in London in 2022. He was elected a Fellow of the AGU in 2020. Professor Tkalčić is a Distinguished Scientist of the Chinese Academy of Sciences President’s International Fellowship Initiative.