Held on the 18th Jun 2019
at 8pm to
More than 75 years ago, the Earth’s core was hypothesized to be solid in the centre as a result of a liquid–solid phase change in iron, which implies that shear waves in its solid part (seismic J phase) should exist. Some claims of such observations have been made, but due to its small amplitudes the J phase has remained elusive. According to some researchers, the compressional body waves that convert to shear waves during the passage through the inner core (a.k.a. PKJKP) were termed “the holy grail of body wave seismology”.
Professor Tkalčić will tell a story on how an 80-year quest to find PKJKP waves took an unexpected turn: we employed recent advances in Earth's correlation wavefield to detect the presence of the J phase. From the observations of the J phase, we confirmed the solidity and obtained new estimates for shear properties of the Earth’s inner core, including shear wave speed, shear modulus and attenuation. The newly determined, shear properties of Earth’s inner core explain the absence of PKJKP waves in the seismic wavefield and have significant implications for geodynamics and mineral physics studies.
Further proliferation of seismic recorders in remote areas of the Earth and at the ocean bottom will improve the clarity of the features in Earth’s correlation wavefield and, in turn, place more constraints on the Earth’s interior.
About the Speaker
Professor Hrvoje Tkalčić is a Head of Seismology and Mathematical Geophysics Group at the Australian National University. He has authored over 80 research papers and an academic book on the Earth’s inner core published by Cambridge University Press. His research interests include the structure and dynamics of the Earth’s interior using observational seismology and mathematical geophysics. His recent projects focus on developing new approaches in Earth imaging and studying seismic sources. He is director of the Warramunga Seismic and Infrasound Facility in the Northern Territory, Australia, and he leads efforts in improving global coverage of seismic data by deployments in remote regions of Australia and Southern Ocean.