International Workshop on the mechanism of Phreatic Eruption

Phreatic eruption history at the latest stage (since ca. 3ka)
of Hakone Volcano
Makoto Kobayashi1, Kazutaka Mannen2, Tamami Yamaguchi and Masashi Nagai4
1 Mt.Fuji World Heritage Centre, Shizuoka, 2 Hot Springs Research Institute of Kanagawa Prefecture, 3 Hakone Geo
Museum, 4 National Research Institute for Earth Science and Disaster ResilienceE-mail: makoto1_kobayashi(at)pref. shizuoka.lg.jp

The increase of fumarolic and geothermal activities has been observed frequently in theOwakudani fumarolic zone at the foot of Kanmurigatake, the latest lava dome (ca.3ka) ofHakone volcano. This study has been focusing on clarifying history of the phreatic eruption
after the Kanmurigatake event in order to understand the hazard of the eruption of Hakone
volcano before the 2015 eruption. We discovered eruption deposits (Owakudani tephra group(Hk-Ow tephras): Hk-Ow1 to Ow5) and studied the eruption mechanism based on thosecomponent material, through investigating their ages, distributions, and deposition facies(Kobayashi et al., 2006).
Currently, we are conducting research to extract and describe the vent-like terrain thatwere traces of past eruptions in detail using the Red Relief Image Map (RRIM: Chiba et al.,2007) acquired from airborne high-precision LIDER DEM. As a result, it was found that there
are several clear fissure vents in the NW-SE direction on the central cones around Kamiyamaand Owakudani (Fig. 1). In particular, it was found that the vent terrains are concentrated
near the ridge extending northeast from the summit of Kamiyama (B and C in Fig. 1).
Furthermore, it was found that there is a fissure vent about 500m long on the north side fromdense area of vent terrain, and this fissure vent crosses the prefectural road connecting thefoot from Owakudani Park (A in Fig. 1). In the geological survey, we correlated eruption
events based on the stratigraphy and 14C age, and considered the source vents based on
the distribution of each eruption deposits (Fig. 2).
As a result, it is considered that most of the fissure vents around Kamiyama andOwakudani were related to the Kamiyama sector collapse, the Kanmurigatake event or
earlier Kamiyama eruption. On the other hand, the fissure vents of Hk-Ow tephras are locatedonly in the NW-SE region centering on the headwall of the Owakudani from the NE ridge ofKamiyama. These fissure vents are in good agreement with the location and direction of thehydrothermal intrusion at the 2015 eruption (Doke et al., 2018), so the 2015 eruption should
be interpreted as reactivation of the hydrothermal crack that caused the Hk-Ow tephras

International Workshop on the mechanism of Phreatic Eruption

Fig. 1 Map of fissure vents on central cones around Kamiyama and Owakudani.
The RRIM was created based on 1m DEM obtained from LP data provided by Kanagawa
Prefecture. The RRIM creation method (Patent 3670274, Patent 4272146) of Asia
Navigation Co., Ltd. was employed (produced by Mr. Chiba T.).

Fig. 2 Space-Time diagram of eruption deposits around Kamiyama and Owakudani.
Chiba T, Suzuki Y and Hiramatsu T (2007) Digital Terrain Representation Methods and Red Relief
Image Map, A New Visualization Approach, “MAP”, Journal of the Japan Cartographic
Association 45-1 2007
Kobayashi M, Mannen K and Okuno M et al. (2006) The Owakidani Tephra Group: A Newly
Discovered Post-magmatic Eruption Product of Hakone Volcano, Japan, Bulletin of theVolcanological Society of Japan, doi: 10.18940/kazan.51.4_245
Doke R, Harada M, Mannen K, Itadera K, Takenaka J (2018) InSAR analysis for detecting theroute of hydrothermal fluid to the surface during the 2015 phreatic eruption of Hakone Volcano,Japan. Earth, Planets Sp 70:63. doi: 10.1186/s40623-018-0834-4