Gondwana Research 16 (2009) 368–369
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Editorial
A tribute to Professor Akira Hasegawa Dr. Akira Hasegawa, now Professor Emeritus of Tohoku University, Japan, is a leading seismologist and a world leader in the study of subduction zones. He has made many fundamental contributions to our understanding of seismicity, structure, magmatism, and dynamics of subduction zones. Because of his important contributions to Earth Sciences, he was elected as a Fellow of the American Geophysical Union (AGU) in January 2008. Prof. Hasegawa was born in Gunma Prefecture, Japan on March 30, 1945. He entered the Department of Geophysics, Faculty of Science, Tohoku University in April 1963, and graduated with Bachelor of Science Degree in March 1967. Subsequently he entered the Graduate School of Science, Tohoku University in April 1967 and obtained Master of Science Degree in March 1969. He joined the faculty of the Department of Geophysics, Tohoku University as an Assistant Professor in April 1971 while being a Ph.D. student in the department. He was promoted to Associate Professor in July 1977 and Full Professor in April 1989 at Research Center for Prediction of Earthquakes and Volcanic Eruptions, Tohoku University. During June 1978 to December 1979, he was a visiting research fellow at the Department of Terrestrial Magnetism, Carnegie Institution of Washington, USA. Although in April 2008 he officially retired from the faculty of Research Center for Prediction of Earthquakes and Volcanic Eruptions, he has been actively continuing his scientific research as Professor Emeritus of Tohoku University.
Prof. Hasegawa's most important and famous contribution is the discovery of the double-planed structure of the Wadati–Benioff deep seismic zone, now widely called the double seismic zone, in the mid 1970s with his long-term coworker, Professor Norihito Umino. Although in 1973 and 1974 two researchers suggested the existence of double seismic zone beneath Kuril and Kanto, Japan, it is Prof. Hasegawa and his coworkers who first provided unambiguous pieces of evidence to demonstrate the existence of the double seismic zone beneath Northeast Japan using the high-quality data recorded by the dense and high-sensitivity seismograph network covering NE Japan, which was installed by the staff of Tohoku University from the late 1960s to early 1970s. They first published their findings in 1975 in Journal of Seismological Society of Japan by Umino and Hasegawa, and then in Tectonophysics and Geophysical Journal of the Royal Astronomical Society in 1978 by Hasegawa, Umino and Takagi. Their discovery made a great impact on many fields of Earth sciences. Soon after their discovery, a great number of researchers followed their path that Prof. Hasegawa opened to search for the double seismic zone in other subduction regions and to study the mechanical, physical and petrologic geneses of the double seismic zone. Such studies have lasted for over three decades and are still being conducted today. To unravel the detailed structure of the double seismic zone and the subducting oceanic lithosphere, from the late 1970s to the mid 1980s Prof. Hasegawa and his coworkers detected ScSp, PS and SP
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Editorial
converted phases at the upper boundary of the subducting Pacific slab beneath Japan, and used the converted phases to locate and map the upper boundary of the slab. They found a thin low-velocity layer existing on top of the slab, which corresponds to the subducted oceanic crust. Since late 1980s, Prof. Hasegawa and his coworkers have made detailed studies of the three-dimensional velocity, attenuation and anisotropy structures of the Japan subduction zone, which resulted in a revolutionary new view of the arc magmatism and volcanism. They developed a new tomography method and imaged clearly the subducting Pacific slab and magma chambers in the mantle wedge beneath the active arc volcanoes under NE Japan, which provided sound seismological evidence for the new view that the arc magmas are produced by a combination of mantle-wedge corner flow (convection) and fluids from the dehydration of the subducting oceanic lithosphere. Their new tomography method has been widely used by the seismologists in the world for studying the seismic structure and dynamics in various regions and tectonic environments. In addition, Prof. Hasegawa and his coworkers have studied the generation process of earthquakes that occurred in and around NE Japan. This includes the detection of an M 4.8 earthquake sequence occurring regularly at the same location of the plate boundary off Kamaishi, NE Japan, and the successful prediction of the next earthquake in this sequence. They also devised an efficient and robust seismic data recording system to conduct portable observations of large-earthquake aftershocks and volcanic tremors. Thanks to the extraordinary efforts by Prof. Hasegawa and his coworkers, now NE Japan is the best studied convergent margin in the world, and is a brilliant prototype of seismological research of subduction zones. We invite our readers to refer to the GR Focus paper by Hasegawa et al. (2009-this issue) and several other papers on related themes by other workers appearing in this special issue of Gondwana Research. In his long scientific career, Prof. Hasegawa has supervised over 40 master and Ph.D. students and post-doctoral fellows in Japan and from China, Russia, Argentina, Sri Lanka, and Kyrgyzstan. Most of his students are active members, some of them are leading figures, in national universities, research institutions or oil industry in Japan and other countries. He also collaborated with many researchers in USA,
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China, South America and Turkey to conduct seismological studies in those countries and regions. In addition, he has been serving numerous committees on the research and prevention of seismic, volcanic and other natural hazards in Japan. Now he is a leading seismologist in Japan to advise the Japanese government on making national policies on the earthquake research, disaster prevention and prediction. Prof. Hasegawa is not only an outstanding scientist and educator, but is also an excellent person. To his colleagues and students, he is a trustworthy mentor and friend, rather than a strict boss. This is also the reason why he could build, keep and lead a very strong research group for nearly four decades. We dedicate this special issue of Gondwana Research to Professor Akira Hasegawa: our mentor, colleague and friend. We sincerely wish him all success and good health to actively continue his brilliant and productive scientific career. Reference Hasegawa, A., Nakajima, J., Uchida, N., Okada, T., Zhao, D., Matsuzawa, T., Umino, N., 2009. Plate subduction and generation of earthquakes and magmas in Japan inferred from seismic observations: An overview. Gondwana Research 16, 370–400 (this issue).
D. Zhao Tohoku University, Japan Corresponding author. E-mail address:
[email protected]. M. Santosh Kochi University, Japan S. Maruyama Tokyo Institute of Technology, Japan 3 May 2009
