GEOPHYSICS Journey to the centre of the Earth Associate Professor Hrvoje Tkalčić discusses his longstanding research interest in global seismology and highlights the importance of understanding the structure and dynamics of the Earth’s inner core What initially sparked your interest in Barbara Romanowicz. This is where my I would say that the main impact lies in global seismology? romance with the inner core started. expanding knowledge in Earth Sciences. For example, seismological results on the Ever since I can remember, I wanted to be Could you explain how earthquakes act as inner core advance overall understanding an astronaut. I was fascinated by nature and ‘internal telescopes’ that provide information of the planetary formation and the the universe, and this led me to enrol on a concerning the Earth’s inner structure? geomagnetic field, without which life on specialised high school degree programme Earth would be impossible. Moreover, the in Mathematics and Computer Sciences Seismic waves generated by large earthquakes innovative techniques applied to analysing and subsequently to study Engineering in and explosions reverberate throughout the and interpreting the inner core sensitive Physics at the University of Zagreb, Croatia. Earth, reflecting from and refracting through seismological data (such as the Bayesian During my time as a student, I realised that its internal boundaries. They bring critical inversion or multi-array seismology) developed an understanding of other planets – how information about the Earth’s internal through this line of work can readily be applied they form and evolve, their internal structure structure to its surface. The seismic wavefield to a range of other geophysical problems. and workings – was predicated on an is recorded either by individual instruments understanding of the Earth. My university had (seismometers) or by arrays of seismometers, Another major impact of our work is that it one of the oldest geophysics programmes a technique that allows weak signals to be keeps Australia at the forefront of this active in Europe, originally funded by Andrija amplified in a similar fashion to how radio area of research. This research attracts the Mohorovičić, who discovered the crust mantle telescopes capture distant signals from space. brightest young scholars from around the boundary in 1910. The breadth of knowledge globe and contributes to the reputation of the I was exposed to in my pre-university years The ground displacement or velocity data ANU as a leading research-driven educational and my interactions at university played a are recorded in a digital format and are institution. My group of students and postdocs great role in my decision to specialise in solid subsequently analysed by seismologists. consist of individuals from the US, UK, earth physics. Seismic imaging techniques are based on an Germany, Croatia, Iran, India, Nepal, Korea understanding of how the seismic wavefield and Vietnam; they therefore bring their unique Why did you become inspired to pursue a propagates through different Earth material experiences to bear and are well equipped to career within the field? (affecting travel times, amplitudes and the serve as ambassadors of Australian science. entire waveform shape) in a similar way to how I made a decision halfway through my studies medical imaging works. For example, we can How would you like to see your research that I would pursue a PhD in geophysics or focus either on imaging the subsurface of the develop in the future? planetary sciences in the US. I could have Australian continent or, from the recordings become a geodynamicist or a planetary of the seismic waves that penetrate deeper I see my future research in global seismology scientist, but I selected a graduate programme within the Earth, we can infer properties of the closely tied to advances in mathematical in seismology at the Department of Earth lowermost mantle and the core using forward geophysics, mineral physics and geodynamics. and Planetary Science at the University of modelling or geophysical inverse theory. We live in times when science can no California, Berkeley. Thus, only two weeks longer thrive if it continues to be confined to after defending my physics degree, I found What is the real-world impact of your group subdisciplines. Therefore, multidisciplinary myself in Berkeley, working on the Earth’s at the Australian National University (ANU)’s collaboration is key to advancement. inner core with one of the most prominent research on the structure and dynamics of global seismologists in the world, Professor Earth’s inner core? www.internationalinnovation.com 1 Seismological studies In the Research School of Earth Sciences at the Australian National University, the Seismology and Mathematical Geophysics Group WITH THE NUMBER of major these rotations from the past 50 years – an is conducting pioneering work in science stories in the news relating output of utmost importance when it comes to geophysical inference to improve to space – from the success of the understanding the geodynamo. Rosetta mission to the exploration scientists’ capacity to study seismic data of Mars – it can be easy to forget MORE COVERAGE, MORE TOOLS that humankind’s knowledge At present, many of Tkalčić’s activities revolve of its own planet is still in its infancy. Many around improving global coverage of seismic fundamental questions remain unanswered, data. As large earthquakes tend to originate particularly in relation to the Earth’s more from some areas of the inner Earth than inaccessible deep mantle and core. others, seismic data is unevenly distributed geographically. This makes the task of At the Australian National University (ANU), tomographic imaging extremely challenging. Associate Professor Hrvoje Tkalčić and his “As such, I have increasingly directed my research team of eight graduate students and research toward finding innovative ways to postdoctoral fellows employ a combination of sample the inner core by means of the existing seismology and mathematical geophysics to data and searching for ‘exotic’ seismic phases,” analyse the seismic wave field that propagates Tkalčić elucidates. through the Earth. This allows them to elucidate its internal structure and dynamics. One way Tkalčić and his ANU colleagues are doing this is through the widespread UNDERGROUND INSIGHTS deployment of seismic sensors in the To date, this approach has enabled the global Australian outback. In recent years, Tkalčić and seismologists at ANU to make a number of his colleagues have deployed two transportable important contributions to understanding spiral arrays of seismic instruments in rural Earth’s internal structure. For example, a locations in Queensland and Western Australia recent collaboration with mineral physicists, under the auspices of an Australian Research involving the large-scale analysis of a Council (ARC) Discovery Grant aimed at substantial global dataset of PKP wave travel facilitating the multi-array, multi-frequency times, led Tkalčić and his colleagues to probing of the Earth’s heterogeneity in hypothesise that the inner core constitutes an its mantle. extremely complex mineralogical structure involving several different phases of iron. The Another area of interest to Tkalčić’s research resulting anisotropy model of the inner core team - also funded by the ARC Discovery was named the ‘Candy Wrapper’. Grant - is the development of new, improved global seismology tools and techniques. “At Tkalčić and his colleagues were also behind present, we are developing tools to improve our a recent discovery relating to the dynamics of understanding of the Australian subsurface the inner core; specifically, the team pursued and advance automated determination of a technique known as ‘earthquake doublets’, characteristics of earthquakes,” Tkalčić whereby the inner core’s rotational dynamics explains. To achieve this goal, the researchers are revealed via the comparison of earthquakes have developed a 3D Earth model that can that occur at different times but originate provide a more accurate insight into wave in the same location. “If you can take two propagation and the orientation of fault snapshots of the inner core at two different planes than its 1D counterparts – data that times – say, 30 years apart – you can infer the can facilitate the rapid and automated differential rotation rate of the inner core with determination of earthquake sources in respect to the mantle from small differences the Australasian region, with important in the recorded waveforms of the doublet,” implications for disaster mitigation. Tkalčić elaborates. This technique showed the scientists that the inner core rotates in a NEXT STEPS fluctuating fashion in respect to the Earth’s Tkalčić has ambitious plans for the future: mantle, enabling them to create a record of “I want to initiate a large multi-institutional, 2 INTERNATIONAL INNOVATION ‘DISMANTLING’ THE EARTH’S STRUCTURE THROUGH EARTHQUAKES THE EARTH’S INNER CORE: REVEALED BY OBSERVATIONAL SEISMOLOGY OBJECTIVES To advance fundamental knowledge about the structure International Innovation provides a preview of Associate Professor Hrvoje Tkalčić’s and dynamics of the Earth’s interior through developing upcoming book innovative techniques for analysing seismological data and improving spatial sampling of deep Earth Next year, Tkalčić will be submitting a manuscript to Cambridge University Press by deploying seismic instruments in remote areas of for his book, The Earth’s inner