Tetsumaru Itaya Earth & Environment︱ The Kibi Plateau is a peneplain, an area of land that is mostly level and unaffected by crustal into the Pacific Ocean. As they flow movement. At 777 m, Mount Tenjin is the highest point within the Kibi Plateau (arrow). across older geological formations, they deposit sediments on top which became layers of fluvial (river) deposits. Dr Itaya and his team have found that on the Kibi Plateau, despite being in such a geologically active area where movement of the Earth’s surface occurs frequently through faulting and folding, the fluvial deposits show no record of AKibi stable location Plateauwithin active Japan any vertical or horizontal displacement. The coastal, shallow marine sediments also found in this area show no significant The Japan Islands experience he Earth is a geologically dynamic infrastructure in Japan, including the inclination (tilting), which would have some of the most extreme planet. The rigid tectonic plates Fukushima Daiichi Nuclear Power Plant. been evidence of uplift or tilting caused earthquakes and volcanoes Tcovering its surface may be moving by movement of the crust. This suggests in the world. Located on the far slower than we can feel, but we Earthquakes are one of the most violent boundaries (where one rigid crustal The Kibi Plateau is a peneplain, an area that the Kibi Plateau has been a stable subduction junction between certainly feel the effects. The occurrence natural hazards on Earth, and despite plate is slowly sliding beneath another). of land that is mostly level and is not unit experiencing a very minimal rate of four different continental of earthquakes caused by tectonic plates years of work, geologists still cannot The Japanese Islands are situated affected by crustal movement. They uplift since the Eocene, even though the and oceanic plates, the scraping past each other are the most accurately predict when they will occur. across four tectonic plates: the Pacific, are usually formed by long periods of movements of tectonic plates in this area islands are at risk from large obvious and dramatic proof that the It is possible to put some protections in Eurasian, North American and Philippine erosion, in this case by the action of have been so great that they drove the earthquakes and volcanoes Earth’s surface is constantly shifting. place that can save lives, such as tsunami Sea plates. There is active, ongoing rivers. It was discovered in the early creation of the Japan Sea. as the crust moves below. Dr early warning systems and designing subduction between these plates, 1900s, but it is only now fully understood Tetsumaru Itaya is a geologist The Japanese Islands, situated on the buildings that are ‘earthquake proof’, resulting in the intense seismic and amid the wider geological context of CLUES FROM SEISMIC at the Institute of GeoHistory Pacific Ring of Fire, certainly feels the but it is very difficult to protect people volcanic activity experienced across the Japan. The Kibi Plateau’s stability may TOMOGRAPHY of Japan Geochronology effects of plate tectonics and is one of and infrastructure completely from the islands of Japan. play an important role in Japan’s future. Dr Itaya and his team also used seismic Network, and his research the most dangerous locations for natural most violent events. Dr Tetsumaru Itaya tomography to research the geology focuses on the Kibi Plateau. disasters in the world. Regular, low is a geologist working at the Institute of Japan’s location at this complex plate ANCIENT RIVER SYSTEMS of the Kibi Plateau. This is a method of This unusual area of stable land intensity earthquakes are common as GeoHistory of Japan Geochronology boundary area means that its geological The team discovered a particular type imaging the subsurface of Earth with may provide an ideal location well as major volcanism. Occasionally, Network (NPO). His research is focused history is challenging to study. Where of rock deposit on the Kibi Plateau seismic waves and tells geologists about for some of Japan’s important Japan experiences large earthquake on using suites of geological analyses there are so many different influences on that indicates just how stable the the rock structures below the ground. services, as it appears to have events that are a major threat to life and to stitch together a picture of Japan’s its development, as Seismic waves are been unaffected by major crustal movements for 34 cause extensive damage to infrastructure. geological history in order to identify well as the complex generated during million years and continues to The most recent and powerful Tōhoku areas of stable land within geologically nature of its position The Kibi Plateau is four times larger than earthquakes, but be stable in the future. earthquake in 2011 was a megathrust active Japan. today, makes it scientists can also earthquake, reaching magnitude a real challenge the area of Tokyo, and has been stable generate them on 9.0, making it one of the strongest THE GEOLOGY OF JAPAN for geologists over a significant geological timescale. a small local scale earthquakes ever recorded. Its offshore So why is Japan so geologically active? to understand. with explosions or epicentre meant that it additionally The islands sit on the Pacific Ring of Fire, However, by better understanding plateau is. Since the late Eocene Epoch impacts on the ground surface to image triggered a large tsunami that caused an area within the Pacific Basin where how the islands developed and what (around 34 million years ago), rivers the subsurface geology below. They look extensive loss of life and damage to there are multiple subduction plate forces are acting on them improves the have flowed across the eastern Asian at how the seismic waves travel through likelihood of being able to better predict continent (including the Kibi Plateau) the Earth and build an image, as seismic where and when earthquakes in Japan may occur and how we can limit the damage they cause.

SOLID FOUNDATIONS Dr Tetsumaru Itaya and his team use suites of geochemical (analysing the mineralogy and chemistry of rocks) and geophysical (studying the structure of the rocks below the Earth’s surface) analyses to study the geology of Japan. They are particularly interested in an area within the Okayama Prefecture of southwest Japan known as the Kibi Plateau. They have been able to confirm it is a stable Kibi Plateau. This unusual area of stable land geological unit through the use of appears to have been unaffected by major The U-shaped valley of the Yoshii river (above). crustal movements for 34 million years. systematic geological analyses to build a The cross-section of the Yoshi river is accurately complete geological picture. approximated by a parabola (right).

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The outcrop of Paleogene shallow marine E: [email protected] T: +81-86-201-3131 W: https://tetsumaruitaya.wixsite.com/mysite sediments without any significant inclination observed in the small island (Maejima) of Seto Inland Sea. Inset: The sediments consist mainly of sandstone and conglomerates containing Research Objectives References trace fossils. Tetsumaru Itaya’s research explores K-Ar phengite Sonehara, T., Yagi, K., Takeshita, H., Aoki, K., Aoki, S., THE PAST AND FUTURE geochronology of HP-UHP metamorphic rocks and Otofuji, Y-I. & Itaya, T. (2020). Kibi Plateau: A stable- OF THE KIBI PLATEAU K-Ar precise dating of young volcanic rocks, fault coherent tectonic unit in the active Japanese Islands. The Kibi Plateau is a peneplain, an area The Kibi Plateau is an unusual gouges and others. Scientific Reports Nature Research, 10:3786 https://doi. peneplain, due to its large size and org/10.1038/s41598-020-60448-x of land that is mostly level and is not location within an area of considerable affected by crustal movement. geological unrest. It is around four Detail Liu, X. & Zhao, Z. (2015). Seismic attenuation tomography times larger than the area of Tokyo, and of the Southwest Japan arc: New insight into subduction waves are reflected or refracted when kilometres, suggesting that the geology appears to be stable over a significant Tetsumaru Itaya dynamics. Geophys. J. Int. 201, 135–156. https://doi. they encounter a change in surface, such was consistent up to this depth and geological timescale. When the Japan Japan Geochronology Network -jGnet org/10.1093/gji/ggv007 as different rock types or fault structures. supporting the idea that the Plateau is Sea was formed, between 20 and 15 2-5 Nakashima, Naka-ku, Okayama 703-8252, Japan a very stable tectonic unit. The team million years ago (the Early Miocene Otofuji,Y-I., Itaya, T., & Matsuda, T. (1991). Rapid rotation The team looked at how seismic waves were able to identify the extent of the epoch), this stable tectonic unit drifted Bio of southwest Japan-palaeomagnetism and K-Ar ages of moved through the crust below the area covered by this stable tectonic south and a series of major tectonic Tetsumaru Itaya is a geochronologist with an interest in Miocene volcanic rocks of southwest Japan. Geophys. Kibi Plateau and they maintained high unit, which reaches a volume of 140,000 events occurred. These included the petrology. He worked at Okayama University of Science J. Int. 105, 397-405. https://academic.oup.com/gji/ velocities with little loss of intensity km3, and there are no active faults in the formation of the Median Tectonic Line, until 2017. Since 2014 he has held the role of President article/105/2/397/705554 through the upper crust to a depth of 20 central Kibi Plateau. Japan’s longest fault system which for the Japan Geochronology Network (NPO). is still active today. These tectonic events appear to have protected the Funding Personal Response Kibi Plateau, which shows no evidence This research has been partially funded by Okayama of being affected by volcanism or Prefecture in the frame of the Regional Problem deformation, and preserved it as a Resolution Support Project. What will the next steps in your research be on the single stable unit. geology of the Islands of Japan? Collaborators We plan to investigate the western part of the Kibi So what does the future hold for the • Takafumi Sonehara (Hiruzen Institute for Geology Plateau (belonging to the eastern Hiroshima Prefecture), Kibi Plateau? The team’s geological and Chronology) where geological information for the Paleogene fluvial findings support the idea that the Kibi • Koshi Yagi (Hiruzen Institute for Geology deposits is scarce. We will clarify the formation history and origin of the entire Kibi Plateau, and accurately Plateau region will continue to be a and Chronology) reveal the distribution range and stable period of stable stable, coherent tectonic unit for a • Hiroyuki Takeshita (Hiruzen Institute for Geology land mass. With help from Korean geologists, our aim considerable time in the future despite and Chronology) is to look for the source of the paleo Susai River in the the surrounding Islands of Japan • Kazumasa Aoki (Okayama University of Science) Eocene, where we assume the source of river is in the experiencing extensive seismic and • Shogo Aoki (Okayama University of Science) northern part of the Korean Peninsula. volcanic activity. Dr Itaya suggests that • Yo-ichiro Otofuji (Kobe University) this could be incredibly important to the Japanese people. Their capital city, Tokyo, is vulnerable to severe natural disasters. He suggests that the stable land of the Kibi Plateau should be used as a location for Japan’s capital city, or as a relocation area for important capital functions. This could also include a location for precision machine The profile C-C’ cross a central part of the Kibi plateau region. High-resolutionS- wave attenuation (Qs) manufacturing and Japanese branches tomography of the Nankai subduction zone along the profiles as shown on the inset map. Surface topography of important international companies. and land area (black horizontal bar) are shown at the top of each cross-section. The red and open triangles denote active and quaternary volcanoes, respectively, within a 20 km width of each profile. The background Taking advantage of the stability the seismicity and low-frequency earthquakes that occurred within a 20 km width of each profile are shown in Kibi Plateau has to offer provides white crosses and red dots, respectively. From: Figure 13, Liu, X. & Zhao, Z. (2015). Seismic attenuation tomography of the Southwest Japan arc: New another form of protection from major insight into subduction dynamics. Geophys. J. Int. 201, 135–156. https://doi.org/10.1093/gji/ggv007 earthquakes for Japan and its people. Yummyphotos/Shutterstock.com

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