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Lahar Characteristics As a Function of Triggering Mechanism at A Kataoka et al. Earth, Planets and Space (2018) 70:113 https://doi.org/10.1186/s40623-018-0873-x FULL PAPER Open Access Lahar characteristics as a function of triggering mechanism at a seasonally snow‑clad volcano: contrasting lahars following the 2014 phreatic eruption of Ontake Volcano, Japan Kyoko S. Kataoka1*, Takane Matsumoto1, Takeshi Saito2, Katsuhisa Kawashima1, Yoshitaka Nagahashi3, Tsutomu Iyobe4, Akihiko Sasaki5,6 and Keisuke Suzuki5 Abstract In association with the September 2014 phreatic eruption (VEI 1–2) at Ontake Volcano, a syn-eruptive and two post-eruptive lahars occurred in the Akagawa–Nigorigawa River, southern fank of the volcano. The present contribu- tion describes and discusses the contrasting features of the two post-eruptive lahars, which caused a major impact on downstream river morphology, and re-examines the description of the syn-eruptive lahar in the previous study. The frst post-eruptive lahar occurred 8 days after the eruption by the rainstorm (October 5, 2014, before the snowy season), and the second lahar was associated with the rain-on-snow (ROS) event on April 20, 2015, in the early spring of the snowmelt season. The October rain-triggered lahar, which can be interpreted as a cohesive debris fow, reached at least ~ 11 km downstream and left muddy matrix-rich sediments with high clay content (10–20 wt% of clay in matrix). The lahar deposits contain hydrothermally altered rock fragments, sulfde/sulfate minerals, and clay miner- als and show extremely high total sulfur content (10–14 wt%) in matrix part, indicating source material from the September phreatic eruption deposits. The presence of “rain-triggered” clay-rich lahar and deposits originating from a single small phreatic eruption is important because usually such clay-rich lahars are known to occur in association with large-scale sector collapse and debris avalanches. The April ROS-triggered lahar was caused by the heavy rain and accompanying snow melting. The lahar was dilute and partly erosional and evolved into hyperconcentrated fow, which left fnes-depleted sandy and gravelly deposits. Despite these lahars that originated from the same volcanic source and occurring within a 7-month period, the fow and resulting depositional characteristics are totally diferent. These diferent types of lahars after a single eruptive event need diferent simulations and mitigation of lahar hazards with timing (season) of the lahar onset. In comparison with rainfall intensity, snow-melting rate, and the contrasting lahars occurred in 2014/2015, it is postulated that the generation, size, and types of lahars can vary with the timing of eruption, whether it happens during the pre-snow season, snow season, or rainy season. Keywords: Ontake Volcano, 2014 eruption, Phreatic eruption, Clay-rich lahar, Rain-on-snow, Cohesive debris fow *Correspondence: [email protected]‑u.ac.jp 1 Research Institute for Natural Hazards and Disaster Recovery, Niigata University, Ikarashi 2‑8050, Nishi‑ku, Niigata 950‑2181, Japan Full list of author information is available at the end of the article © The Author(s) 2018. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creat​iveco​mmons​.org/licen​ses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Kataoka et al. Earth, Planets and Space (2018) 70:113 Page 2 of 28 Introduction Background and settings On September 27, 2014, Ontake Volcano in central Japan Historical and prehistorical eruptions of Ontake Volcano confronted mountaineers with an unpredicted small- Ontake Volcano (3067 m a.s.l. (above sea level), at sum- scale phreatic eruption (VEI 1–2: Maeno et al. 2016) mit Kengamine peak; 35°53′ 34″N, 137°28′49″E) is and caused 63 fatalities (including 5 missing) at the sum- located on Honshu Island at the border of Nagano and mit. Studies about the eruption and the tragedy have Gifu Prefectures (Fig. 1). During the Holocene epoch, addressed the processes and mechanism of the eruption before the 2014 eruption, Ontake Volcano has erupted at itself (e.g., Oikawa et al. 2016). Also, further concerns least 21 times; most have been phreatic and three have and interests immediately after the eruption were about occurred within the last 50 years. Te frst recorded future eruptions and their prediction: whether the erup- (witnessed) historical eruption at Ontake was a phre- tion would evolve into magmatic explosive phase. How- atic eruption which occurred on October 28, 1979, from ever, there was another concern for lahar hazards with vents in Jigokudani and Haccho-tarumi (e.g., Soya et al. snow/ice meltwater (Major and Newhall 1989; Pierson 1980). After the 1979 eruption, small-scale phreatic erup- et al. 1990; Manville et al. 2000; Waythomas 2014) such at tions again occurred in May 1991 and March 2007 from the seasonally snow-clad Ontake Volcano. Further erup- Haccho-tarumi crater and resulted in ashfall deposition tions could trigger lahars through rapid snowmelt by hot nearby the vent only. Oikawa et al. (2014) reported the pyroclastic density currents. Even without a new erup- presence of at least 13 phreatic eruptions during the last tion, lahars remobilizing the primary September erup- 7500 years before the 1979 eruption and 5 magmatic tion deposits in upslope areas could be triggered later by eruptions during the Holocene. Since the traces of small- heavy rain or by warm rainstorm on winter snowpack. scale eruptions including phreatic eruptions in geological In addition, an earthquake-triggered lahar is always a records are limited, the eruptive records and associated potential hazard in this region, as evidenced by the lahar lahar history of Ontake Volcano still need to be examined triggered during the Naganoken-Seibu earthquake on in detail. September 14, 1984 (Matsuda and Ariyama 1985; Endo et al. 1989), which initiated as fank collapse of southern The September 2014 eruption and a syn‑eruptive lahar part of the edifce. Te resulting lahar in the Denjogawa On September 27, 2014 (11:52 a.m. Japan time), Ontake and Nigorigawa Rivers (Fig. 1a) and other landslides Volcano erupted suddenly with a very short precursor of caused 29 fatalities. Lahars and other sediment-laden tremor and edifce tilting that began 11 min before the fows are serious hazards because they can cause rapid eruption (Oikawa et al. 2014; Maeno et al. 2016). Te aggradation and widening of rivers from proximal to active eruptive vents were in Jigokudani valley (Fig. 1a) distal areas of volcanoes, inundate communities, and with the eruptive column reaching ~ 7.8 (or 5) km destroy infrastructure (Smith 1991; Newhall and Punon- high above the vents (Sato et al. 2016). Te 2014 erup- gbayan 1996; Kataoka et al. 2009; Pierson and Major tion included low-temperature pyroclastic density cur- 2014). rents induced by the collapse of the volcanic plume that Tis paper focuses on three lahars generated at Ontake rose up to 300 m above the vents. Te currents mainly Volcano within a 7-month period, each triggered by a dif- fowed on the southern and western slopes of the volcano ferent mechanism: a phreatic eruption, heavy rainfall, and (Yamamoto 2014; Maeno et al. 2016) although the actual rain-on-snow event (hereafter ROS for “rain-on-snow”: thickness of deposits confned to valleys still remains Kattelmann 1985, 1997; Sui and Koehler 2001; Prad- unknown. Te eruption was phreatic without any juvenile hanang et al. 2013). Tis paper further highlights that at materials. Te ejecta consist of fragmented gray andesite Ontake Volcano, the generation, size, and types of lahars and altered andesite with sulfde and sulfate minerals are dependent, in part, on the timing and type of erup- (Maeno et al. 2016; Minami et al. 2016). Te ashfall dis- tion with respect to the presence of seasonal snowpack. tribution is more extensive than that of the 1979 eruption (See fgure on next page.) Fig. 1 a Index map showing Mt. Ontake (Ontake Volcano) and river systems in the southern slope. Isopach of fallout and distribution of pyroclastic density currents of the 2014 eruption are after Oikawa et al. (2014) and Takarada et al. (2016). These isopach maps are based on the survey points mostly situated at the eastern, northern, and western fanks of the volcano, and very few from the southern part of medial-distal areas. Wire sensors were set at two localities in the Nigorigawa River by the Ministry of Agriculture, Forestry and Fisheries (MAFF) and the Ministry of Land, Infrastructure, Transport and Tourism (MLIT). Published topographical map and DEM (original data from the Geospatial Information Authority of Japan) were processed by Kashmir 3D. b Distribution of the October 2014 lahar (cohesive debris fow) deposits and outcrop localities (see also in a). Mapping is based on the authors’ ground survey and captured aerial photo-images taken 2 days after the lahar (Courtesy to Google Earth). c Longitudinal profles of the Akagawa and Nigorigawa Rivers and distribution of the October 2014 lahar deposits and installed wire sensors Kataoka et al. Earth, Planets and Space (2018) 70:113 Page 3 of 28 a b N 1km S118 (2.5) S117 (1.5) S119 (2.0) S116 (2.0) Akagawa Distal endofthe River eruption-triggered Jigokudani lahar (ravine, craters) S114 (2.0) 1192(0.7) (Sasakietal.,2016) 1195 (0.5) 1283(1.5) 1202 1194 (>0.5) 1191(0.5) 1009 1227(1.1): Time-lapse camera 1460 (1.5) 1199 (0.16),1200 (0.27) 1004 (0.15) 1231(1.0) Wire sensor 13752'E Site forwater Bridge/ford 35 30'N quality measurement 1235(1.5-2.5) 1234(>3.5) Cohesive debris 1232(3.0) flow facies 1232-2(0.2) Mudslurry(without boulders) facies Denjogaw a Unidentified area River duetoclouds 1228 S116 (2.0) 1229 Locality Thicknessof name debris flow Channeldredging depositsinmeter.
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