[S-VC30] Mitigation of volcanic disasters - basic and applied researches

[SVC30-P06] New publication series of large-volume ignimbrites derived from large-scale eruptions in Japan

*Shinji Takarada¹, Ayumu Nishihara¹, Hideo Hoshizumi¹, Tadashi Yamasaki¹, Yasuaki Kaneda¹, Nobuo Geshi¹ (1.Geological Survey of Japan, AIST)

Keywords:Large-volume, Large Eruption, Ignimbrite, Ash fall, Distribution

Geological Survey of Japan (GSJ), AIST has started to publish a series of maps that display the distributions of large-volume ignimbrites, which were erupted during the late Pleistocene in Japan. The "Distribution Map of Large-volume Ignimbrite in Japan" series provides information on the distribution and characteristics of volcanic deposits from large-scale explosive eruptions, based on the compiled results of geological field surveys carried out by GSJ and borehole data. After the Tohoku earthquake in 2011, society has focused on geological events which can cause an infrequent but severe disaster. Mainly, massive eruptions with large-scale pyroclastic flows can generate disastrous impacts not only in the vicinity of the volcano but also over a wide area. Geological records indicate that such massive eruptions have occurred in the past in Japan. Moreover, the distributions of products ejected during massive eruptions provide information about the impacts of future eruptions of a similar scale. The distribution of volcanic products such as ash and pumice from a large-scale prehistoric eruption is not easy to recognize on currently available geological maps. The geological record of massive explosions has often been eroded and covered by younger sediments. Therefore, this publication series aims to present the distribution and affected area of ignimbrites just after large-scale eruptions based on the reconstruction of ignimbrite distribution where the deposits are unexposed on the surface and eroded. The maps for this series also show point data of the distribution of the altitude and the thickness of the ignimbrite, the maximum size of pumice and lithic fragments in the ignimbrite, the preferred orientation of the pumice fragments, and the distribution of associated co-ignimbrite ash fall. Large-scale explosive eruptions and the related emplacement of ignimbrite deposits over vast areas have the potential to cause massive disasters, although such events are infrequent. The aim of publishing the distribution maps of large-volume ignimbrites in Japan is to place fundamental constraints on the impacts of large-scale eruptions that will help mitigate potential disasters in the future.

"Distribution Map of Ito Ignimbrite and associated deposits, Aira Caldera, Japan," which is the first publication of this series, contains a 1:250,000 scale map of the distribution of the Ito ignimbrite in the area within ~100 km from Kagoshima Bay. In addition to the distribution of the Ito ignimbrite on the ground surface, this map also shows the subsurface distribution of the ignimbrite based on borehole data. The map also indicates point data of the distribution of the altitude and the thickness of the ignimbrite, the maximum size of pumice and lithic fragments in the ignimbrite, and the preferred orientation of the pumice fragments (Figure). The distribution of the Aira-Tn ash fall deposit, the co-ignimbrite ash of the Ito ignimbrite, is also shown. The reconstructed distribution of the Ito ignimbrite based on numerical simulation is also shown. The distribution map contains an explanation booklet that provides background geological information about the Aira caldera and sequence of the Ito eruption, as well as descriptions of the Ito ignimbrite and associated deposits. The explanation booklet provides a pictorial of representative field occurrences of the Ito ignimbrite and associated deposits. All contents in PDF and GIS files can be downloaded from the website: https://www.gsj.jp/Map/JP/lvi.html.

GSJ plans to publish 12 distribution maps for the series on large-volume ignimbrites in Japan: Ito, Shikotsu, Aso-4, Aso-3, Toya, Ata, Kutcharo IV, Kutcharo I, Hachinohe, Ofudo, Koya, and Hakone-Tokyo ignimbrites. The "Distribution Map of Large-volume Ignimbrite in Japan" series will use a standard format to present the distribution of large-volume ignimbrites and associated ash fall deposits. These publications are expected to be used by researchers in universities and research institutes and will provide information about the possible extent of affected areas during future large-volume eruptions. This information could play a crucial role in disaster mitigation plans operated by national and regional governments and the construction of resilient infrastructure.