Yoshio Fukao is a deep thinker with influential ideas who has made outstanding observational and theoretical contributions to solid Earth geophysics using seismological tools.
He has been a leader on imaging of subducted slabs in the mantle transition zone. His interest in processes related to subduction started with the characterization of the sources of large and, in particular, deep earthquakes. Later, he clarified the depth extent of subducted oceanic basaltic crust before its transformation to eclogite, thus providing insights on the physical conditions in the slab down to ~60 kilometer (km) depth. In the 1990s, he showed two classes of behavior of slabs around the world: those that lie horizontally above the 660-km discontinuity, which he coined “stagnant slabs,” and those that penetrate deeper and are not as directly connected to present-day subduction. This showed that the Earth’s mantle is in an intermediate state between two highly debated dynamic extremes: one-layer and two-layer convection. Recently, analysis of the latest high-resolution P wave global model developed with former student Masayuki Obayashi led them to demonstrate that most slabs either stagnate above 660 km or flatten out deeper, around 1,000 km depth. These results have turned the attention of the community to a possible rheological boundary around 1,000 km depth and have inspired further studies in geodynamics and mineral physics, aiming at understanding the nature and significance of this potential redefinition of an extended transition zone.
Yoshio Fukao has also worked extensively on Earth’s free oscillations and contributed to another important discovery: that of the presence, in the absence of earthquakes, of a low-frequency “hum” continuously excited by sources in the oceans and/or the atmosphere, which, when applied to planets without earthquakes, may provide a powerful way to study their internal structure.
He has been an exceptional mentor to younger scientists. He has also been a leader in the establishment of research infrastructure in Japan, such as the “Earth simulator” and the Institute for Frontier Research on Earth Evolution, and has been a driving force for the Japanese contribution to international infrastructure in broadband seismology, notably on the ocean floor with programs such as the Ocean Hemispheres Project.
Even though he had to retire from teaching already in 2004, Yoshio Fukao remains impressively active in research, trying out new directions, as evidenced by his latest papers on ocean bores and fine-scale structure in the ocean column.
—Barbara A. Romanowicz, University of California, Berkeley; also at Collège de France, Paris
It is my great honor to be awarded the 2018 AGU Inge Lehmann Medal in recognition of my seismological contributions to solid Earth geophysics. I am especially grateful to Barbara Romanowicz for her citation and would like to thank all who supported my nomination.
My research career started with an experimental study of thermal properties of mantle minerals at the University of Tokyo about 50 years ago. This was to become the backbone of my seismological studies later on. I thank my colleagues in mineral physics, from whom I learned a lot about the rock physics view of Earth. I am deeply indebted to my thesis advisers, Seiya Uyeda and Hiroo Kanamori, for their continued thoughtful advice and encouragement. I spent a year in the 1970s as a postdoc at the University of Cambridge, where I learned a lot from Dan McKenzie about the plate tectonic view of Earth. I then spent 20 years at the early stage of my career at Nagoya University, where I enjoyed everyday discussions and conversations through which we shared surprise and excitement among the colleagues and students in different fields.
I, as a seismologist, have been interested in subduction dynamics and interaction of the solid Earth with the atmosphere and oceans. My findings in the first category include near-trench occurrence of tsunami earthquakes, subduction of untransformed oceanic crust, and, most importantly, stagnant slabs in the mantle transition region. Findings in the second category include Earth’s background free oscillations (Earth’s hum). Most of these findings were made in collaboration with my colleagues. In particular, I am grateful to Masayuki Obayashi, Kiwamu Nishida, and Hiroko Sugioka for their long-continuing collaborations in seismic tomography, Earth hum analysis, and ocean bottom seismology, respectively.
About 20–30 years ago I was involved in building a broadband seismographic network in the western Pacific in a Japanese initiative as part of the international collaborative effort of covering Earth’s surface with broadband seismic stations. I now belong to the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), where I am excited about deploying geophysical instruments at the ocean bottom in the hope to expand the scope of seismology beyond the solid Earth.
As such, I would have been unable to conduct my research without generous support from many people. I express my sincere gratitude to all of them, including my family.
—Yoshio Fukao, Japan Agency for Marine-Earth Science and Technology, Yokohama