The Esan volcanic complex (EVC), northern Japan, is an active volcanic complex. The EVC represents a potential threat owing to its close proximity to inhabited areas, and abundant phreatic deposits occur around its volcanic aprons. Eleven samples of black paleosol and charcoal were collected from trenches dug at two sites (A and B) for the purpose of ¹⁴C dating. Sites A and B are located 2.6 km and 2.2 km from the source crater, respectively, and the deposits at these sites represent a proximal facies. At least 11 (at site A) and 12 (at site B) volcaniclastic units are identified. A newly identified Holocene phreatic unit (Es-0) is dated at ca 11 ka and is older than the EsMP unit that is a Holocene lava-dome building episode at ca 9 ka. Holocene phreatic episodes Es-1 and Es-3 can be divided into three and five subunits, respectively, and include a pyroclastic surge deposit that would have posed a threat to current local communities. The sequence of Holocene eruptions includes at least 11 tephra events over 11,000 years. The material ejected during phreatic explosions is characterised by highly heterogeneous grain size. Therefore, grain size analysis of erupted phreatic material might not effectively discriminate the processes of deposition, in contrast to such analysis of magmatic ejecta. The topography of the EVC largely constrains the area of deposition of phreatic ejecta.

1. [1] J. Stix and J. M. de Moor, “Understanding and forecasting phreatic eruptions driven by magmatic degassing,” Earth, Planets and Space, Vol.70, No.1, Article: 83, doi:10.1186/s40623-018-0855-z, 2018.
2. [2] K. Mayer, B. Scheu, H. A. Gilg, M. J. Heap, B. M. Kennedy, Y. Lavallée, M. Letham-Brake, and D. B. Dingwell, “Experimental constraints on phreatic eruption processes at Whakaari (White Island volcano),” J. Volcanol. Geotherm. Res., Vol.302, pp. 150-162, doi:10.1016/j.jvolgeores.2015.06.014, 2015.
3. [3] C. G. Newhall and R. P. Hoblitt, “Constructing event trees for volcanic crises,” Bull. Volcanol., Vol.64, No.1, pp. 3-20, doi:10.1007/s004450100173, 2002.
4. [4] W. Marzocchi, L. Sandri, and J. Selva, “BET_EF: a probabilistic tool for long- and short-term eruption forecasting,” Bull. Volcanol., Vol.70, No.5, pp. 623-632, doi:10.1007/s00445-007-0157-y, 2008.
5. [5] A. Neri, W. P. Aspinall, R. Cioni, A. Bertagnini, P. J. Baxter, G. Zuccaro, D. Andronico, S. Barsotti, P. D. Cole, T. Esposti Ongaro, T. K. Hincks, G. Macedonio, P. Papale, M. Rosi, R. Santacroce, and G. Woo, “Developing an event tree for probabilistic hazard and risk assessment at Vesuvius,” J. Volcanol. Geotherm. Res., Vol.178, No.3, pp. 397-415, doi:10.1016/j.jvolgeores.2008.05.014, 2008.
6. [6] J. S. Pallister, D. J. Schneider, J. P. Griswold, R. H. Keeler, W. C. Burton, C. Noyles, C. G. Newhall, and A. Ratdomopurbo, “Merapi 2010 eruption—Chronology and extrusion rates monitored with satellite radar and used in eruption forecasting,” J. Volcanol. Geotherm. Res., Vol.261, pp. 144-152, doi:10.1016/j.jvolgeores.2012.07.012, 2013.
7. [7] C. G. Newhall and J. S. Pallister, “Using multiple data sets to populate probabilistic volcanic event trees,” J. F. Shroder and P. Papale (Eds.), “Volcanic Hazards, Risks, and Disasters,” pp. 203-232, doi:10.1016/B978-0-12-396453-3.00008-3, Elsevier, 2015.
8. [8] C. G. Newhall and S. Self, “The volcanic explosivity index (VEI): An estimate of explosive magnitude for historical volcanism,” J. Geophys. Res., Vol.87, No.C2, pp. 1231-1238, 1982.
9. [9] D. M. Pyle, “Sizes of volcanic eruptions,” H. Sigurdsson et al. (eds.), “Encylopedia of Volcanoes,” pp. 263-269, Academic Press, 2000.
10. [10] S. Nakada, A. Zaennudin, F. Maeno, M. Yoshimoto, and N. Hokanishi, “Credibility of volcanic ash thicknesses reported by the media and local residents following the 2014 Eruption of Kelud volcano, Indonesia,” J. Disaster. Res., Vol.11, No.1, pp. 53-59, 2016.
11. [11] Japan Meteorological Agency (Ed.), “19. Esan,” National Catalogue of the Active Volcanoes in Japan, The 4th edition, English version, pp. 1-11, 2013.
12. [12] D. Miura, K. Arai, K. Toshida, T. Ochiai, M. Tanaka, and T. Iida, “Eruption history, conduit migration, and steady discharge of magma for the past 50,000 yr at Esan volcanic complex, northern Japan,” Geol. Soc. Am. Bull., Vol.125, No.9-10, pp. 1503-1519, doi:10.1130/B30732.1, 2013.
13. [13] Y. Katsui, I. Yokoyama, H. Okada, and T. Tsubo, “Esan, volcano geology, eruption history, current activity and hazard mitigation,” The committee of hazard mitigation in Hokkaido, 99pp., 1983 (in Japanese).
14. [14] Committee of Disaster Prevention Council at Esan Volcano (CDPCEV) (Ed.), “The booklet of hazard prevention at Esan Volcano: Former Esan-town and Todohokke village, Hokkaido, Japan,” 18pp., 2001 (in Japanese).
15. [15] Y. Katsui, T. Suzuki, T. Soya, and Y. Yoshihisa, “Geological map of Hokkaido-Komagatake Volcano,” Geological map of volcanoes No.5, Geological Survey of Japan, 1989 (in Japanese with English abstract).
16. [16] K. Arai, “Eruptive history and volcanic hazard assessment for Esan volcano, southwestern Hokkaido: Particularly based on the last 10,000 years activities,” M.Sc. Thesis for Hokkaido University, 1998 (in Japanese with English abstract).
17. [17] M. Okuno, M. Yoshimoto, K. Arai, T. Nakamura, T. Ui, and K. Wada, “AMS radiocarbon age of the Ko-f tephra from Hokkaido–Komagatake volcano, southwestern Hokkaido, Japan,” J. Geol. Soc. Jpn., Vol.105, No.5, pp. 364-369, 1999 (in Japanese with English abstract).
18. [18] H. Machida and F. Arai, “Atlas of Tephra in and around Japan,” Revised edition, University of Tokyo Press, 2003 (in Japanese).
19. [19] G. P. L. Walker, “Grain-size characteristics of pyroclastic deposits,” J. Geol., Vol.79, No.6, pp. 696-714, doi:10.1086/627699, 1971.
20. [20] F. Maeno, S. Nakada, T. Oikawa, M. Yoshimoto, J. Komori, Y. Ishizuka, Y. Takeshita, T. Shimano, T. Kaneko, and M. Nagai, “Reconstruction of a phreatic eruption on 27 September 2014 at Ontake volcano, central Japan, based on proximal pyroclastic density current and fallout deposits,” Earth, Planets and Space, Vol.68, No,1, Article: 82, doi:10.1186/s40623-016-0449-6, 2016.