Characteristics of the syneruptive-spouted type lahar generated by the September 2014 eruption of Mount Ontake, Japan

Abstract

Mount Ontake erupted at 11:52 am on September 27, 2014, which generated pyroclastic density currents, ballistic projectiles, ash falls, and a small-scale lahar that spouted directly from craters formed by the eruption. Because this lahar may have been generated by water released from within these craters, we refer to this lahar as a "syneruptivespouted type lahar" in this study. The lahar of the 2014 eruption was small relative to the other syneruptive type lahars reported in the past that were snowmelt type or crater lake breakout type lahars. Nevertheless, in the 2014 event, the syneruptive-spouted type lahar extended approximately 5 km downstream from the Jigokudani crater via the Akagawa River, with an estimated total volume of ~1.2 × 10 5 m 3. We have reviewed other representative syneruptivespouted type lahars that have been reported in Japan. The syneruptive-spouted type lahar attributed to the September 2014 eruption had the longest runout distance and largest volume of all cases studied. The mineral assemblage identified from samples of the lahar deposits is similar to that of ash-fall deposits from the same eruption. Previous workers deduced that the ash was derived mainly from shallow depths (within 2 km of the surface). The syneruptivespouted type lahar deposits are therefore also considered to have originated from shallow depths. A syneruptivespouted type lahar is a small-scale phenomenon that causes little direct damage to infrastructure, but has long-term influence on water quality. Increases in turbidity and decreases in pH are expected to occur in the Mount Ontake area downstream of Nigorisawa after heavy rainfall events in the future. Therefore, the potential indirect (but long term) damage of syneruptive-spouted type lahars should be considered for hazard mapping and planning volcanic disaster prevention measures.

Heat source of the 2014 phreatic eruption of Mount Ontake, Japan

“…A pyroclastic density current flowed ~3 km to the south from the vent Maeno et al, 2016;Nakano et al, 2014;Oikawa et al, 2016;Yamamoto, 2014). A small (~1.2 × 10 5 m 3 ) lahar and a stream of hot water were expelled from Jigoku-dani crater and extend-ed~5 km downstream (Sasaki et al, 2016). The eruption column reached a height of 7000 m above the craters and drifted towards the east (Sato et al, 2017;Takeshita et al, 2017).…”

Special issue “The phreatic eruption of Mt. Ontake volcano in 2014”

“…The temperature of the PDC in this specific case was 30-100 °C ), but could potentially have been higher because the temperature at the hydrothermal source is estimated to have been 270-280 °C (Ikehata and Maruoka 2016). Whereas the fatalities caused by ballistic impact occurred within 1 km of the vent, the vent-sourced lahar traveled about 5 km, sufficient distance to threaten nearby communities and affect the pH and turbidity of water supplies (Sasaki et al 2016); thus, there is also potential for more distal damage.…”

“…The discharged mass of the 2014 eruption is about half that of the 1979 eruption. Sasaki et al (2016) describe the syneruptive-spouted type lahar generated by water directly emitted from the craters. The lahar flowed downvalley south of the crater to a distance of 5 km.…”

“…Ontake eruption tephra was ejected as PDC, ballistics, and air fall. Takarada et al (2016) and Maeno et al (2016) document the total volume of tephra and the size of the eruption and Sasaki et al (2016) describe the resulting lahar. and Tsunematsu et al (2016) analyze the distribution of ballistic ejecta and Tsunematsu et al (2016) simulate ballistic trajectory and calculate a landing energy of 10 4 J, essential data for future shelter designs.…”

Geological study of phreatic eruptions

“…, 20012006, 2006, 2007c, Kaneko et al, 2016Oikawa et al, 2016;Sasaki et al, 2016Sasaki et al, 2015Sasaki et al, , 2015Sasaki et al, 1962 Table 4 debris flow part muddy water part Sasaki et al 2016, Yamada, 1962, 2007c;Procter et al, 2014;Sasaki et al, 2016Procter et al, 2014Sasaski et al, 2016, 2007c, , 2006, 1966, 2001;, 2007c;Procter et al, 2014;Sasaki et al, 2016Sasaki et al, , 1966, 2007c15 60% 5 15% , 1966, 2001;, 2007c;Sasaki et al, 2016 cohesive debris flow Scott et al, 1995;Vallance and Scott, 1997 Hirose, W., Nakagawa, M., Takarada, S., Yoshida, M., Okazaki, N., Ishimaru, S., Ui, T. , 2007a 8 11 1998 10 11 November 1996 & November 1998 erptions at Meakandake Volcano, eastern Hokkaiso, Japan…”