Estimation of total discharged mass from the phreatic eruption of Ontake Volcano, central Japan, on September 27, 2014
© The Author(s) 2016
Received: 4 January 2016
Accepted: 11 July 2016
Published: 2 August 2016
The total mass discharged by the phreatic eruption of Ontake Volcano, central Japan, on September 27, 2014, was estimated using several methods. The estimated discharged mass was 1.2 × 106 t (segment integration method), 8.9 × 105 t (Pyle’s exponential method), and varied from 8.6 × 103 to 2.5 × 106 t (Hayakawa’s single isopach method). The segment integration and Pyle’s exponential methods gave similar values. The single isopach method, however, gave a wide range of results depending on which contour was used. Therefore, the total discharged mass of the 2014 eruption is estimated at between 8.9 × 105 and 1.2 × 106 t. More than 90 % of the total mass accumulated within the proximal area. This shows how important it is to include a proximal area field survey for the total mass estimation of phreatic eruptions. A detailed isopleth mass distribution map was prepared covering as far as 85 km from the source. The main ash-fall dispersal was ENE in the proximal and medial areas and E in the distal area. The secondary distribution lobes also extended to the S and NW proximally, reflecting the effects of elutriation ash and surge deposits from pyroclastic density currents during the phreatic eruption. The total discharged mass of the 1979 phreatic eruption was also calculated for comparison. The resulting volume of 1.9 × 106 t (using the segment integration method) indicates that it was about 1.6–2.1 times larger than the 2014 eruption. The estimated average discharged mass flux rate of the 2014 eruption was 1.7 × 108 kg/h and for the 1979 eruption was 1.0 × 108 kg/h. One of the possible reasons for the higher flux rate of the 2014 eruption is the occurrence of pyroclastic density currents at the summit area.
Over the last several decades, there has been much debate about how best to use the ash-fall isopleth and isopach distribution maps. Walker (1980, 1981) proposed a crystal concentration method. Hayakawa (1985) proposed a method that required only a single isopach (V = 12.2TA; T: thickness, A: selected isopach area). The constant of 12.2 was obtained from plinian tephra fall volumes using the crystal concentration method. Pyle (1999), however, proposed that this value was variable rather than constant. Pyle (1989, 1995) and Fierstein and Nathenson (1992) proposed an exponential decay method with [T = T 0exp(−kA 1/2); T: thickness, T 0: extrapolated maximum thickness, A: isopach area, k: slope on a lnT–A plot]. Takarada et al. (2001) proposed a segment integration method. Bonadonna et al. (1998) and Bonadonna and Houghton (2005) proposed a power law method. Bonadonna and Costa (2012) proposed a Weibull method, Tajima et al. (2013) proposed an ellipse-approximated isopach method, and Green et al. (2016) proposed a Bayesian statistical method to estimate tephra volumes from a limited number of sparsely distributed observation points. The main reasons that so many methods have been proposed are as follows: The fact that many studies have a limited number of sampling data, different fitting curves have been used for various styles of eruptions, and there are inaccuracies involved in the extrapolation of fitted curves to the proximal and distal regions. In this study, we use the segment integration, exponential, and single isopach methods to estimate the total discharge mass of the September 27, 2014, Ontake phreatic eruption. We also estimate the total discharge mass of the 1979 Ontake volcano phreatic eruption, in order to compare the two eruptions using the segment integration method.
The Geological Survey of Japan conducted a 3-day field survey of the ash-fall deposit as a part of the Joint Research Team for ash fall in Ontake 2014 eruption, from September 28 to 30, 2014 (AIST 2014; The Joint Research Team for ash fall in Ontake 2014 eruption 2015). The Ontake Volcano Proximal Area Survey Joint Research Team also conducted a field survey at the summit proximal area on November 8, 2014.
The ash-fall deposits were collected from relatively flat surfaces (e.g., the top of a fire hose box; Fig. 1a, d, f), and the area of the collected samples was measured. The samples were dried and weighed, and weight/m2 (g/m2) was calculated for each sampling point.
Isopleth mass distribution map and estimation of total discharged mass
The field survey results were, for example, 148.2 g/m2 at 8 km NE from the source (Fig. 1a), 114.5 g/m2 at 10.3 km ENE (Fig. 1b), 109 g/m2 at 10.4 km ENE, 52.0 g/m2 at 11.1 km ENE (Fig. 1d), 23.1 g/m2 at 15.3 km ENE (Fig. 1e), and 1.3 g/m2 at 19.0 km E (Fig. 1f). The dispersal axis for the ash-fall deposit is ENE in the proximal and medial areas and toward the E in distal areas (Fig. 2).
The total discharged mass of the September 27, 2014, phreatic eruption was calculated to be 1.18 × 106 t (1.18 × 109 kg; Fig. 3). The proximal deposit, within the 1-cm (1.5 × 104 g/m2) isopach, consists of 95 % of the total discharged mass (1.12 × 106 t). However, the mass of the ash in the distal region (<4 g/m2) was only 0.9 % (1.0 × 104 t) of the total discharged mass.
Total discharged mass estimation using the single isopach method (Hayakawa 1985) for each isopleth contour line
Discharged mass (t)
6.7 × 10−7
1.2 × 109
1.0 × 104
1.3 × 10−6
5.3 × 108
8.6 × 10 3 (min)
2.7 × 10−6
3.1 × 108
1.0 × 104
5.3 × 10−6
2.0 × 108
1.3 × 104
1.1 × 10−5
1.3 × 108
1.7 × 104
2.1 × 10−5
8.6 × 107
2.2 × 104
4.3 × 10−5
6.5 × 107
3.4 × 104
8.5 × 10−5
5.2 × 107
5.4 × 104
1.7 × 10−4
4.1 × 107
8.5 × 104
3.4 × 10−4
2.9 × 107
1.2 × 105
6.8 × 10−4
1.9 × 107
1.6 × 105
1.0 × 10−2
1.0 × 107
1.2 × 106
5.0 × 10−2
3.5 × 106
2.2 × 106
1.8 × 106
2.2 × 106
6.7 × 105
2.5 × 10 6 (max)
3.0 × 105
1.8 × 106
4.7 × 104
5.7 × 105
Comparison with the 1979 Ontake eruption
Discharged mass estimation methods
Takarada et al. (2014) previously calculated the total discharged mass for the September 27, 2014, eruption as between 6.2 × 105 and 9.9 × 105 t, but no proximal field survey data were available at that time. The estimations including proximal field data are 8.9 × 105 t (exponential method; Fig. 4) and 1.2 × 106 t (segment integration method; Fig. 3). Therefore, detailed proximal area data are relatively important for obtaining a more accurate estimation of the total discharged mass of phreatic eruptions. More than 90 % of the total mass (1.1 × 106 t) of ash-fall accumulated in 2014 was within the proximal area (>1 cm in thickness; Fig. 3), showing that, for small eruptions, field measurements in the proximal area are necessary in order to make reliable estimates of discharged mass. By contrast, the mass of fallout in the distal region (<4 g/m2) was only 0.9 % (1.0 × 104 t) of the total discharged mass. Therefore, detailed field surveying of ash-fall distributions in the distal region is less important than in the proximal and medial area for the estimation of the total discharged mass of a small-scale phreatic eruption. Sparse sampling points are sufficient.
Even when well-constrained (i.e., with many sampling points) contour lines were used (such as 32 and 64 g/m2) using the single isopach method, the resulting volumes were much smaller (2.2 × 104 and 3.4 × 104 t; Table 1) than the results obtained using the segment integration and exponential methods (8.9 × 105 and 1.2 × 106 t). The equation V = 12.2TA was originally obtained empirically from relatively large-scale volcanic eruptions (>2 km3). Therefore, Hayakawa’s single isopach method is not suitable for small-scale eruptions. We would like to suggest that error values should be shown when the single isopach method is used for middle and large-scale eruptions.
Discharged mass flux rate estimations
The 2014 eruption began at 11:52 and ended by 18:00 on September 27 (Sato et al. 2015). The duration of the phreatic eruption was about 6 h, and the total discharged mass was about 1.0 × 106 t (average of 8.9 × 105 and 1.2 × 106 t). Therefore, the average discharged mass flux rate of the 2014 eruption is about 1.7 × 108 kg/h (4.2 × 104 kg/s). The 1979 eruption began at 5:20 and ended by midnight on October 28, 1979 (Kobayashi 1980). The duration of the 1979 eruption was about 18–20 h, and the total discharged mass was 1.9 × 106 m3. Therefore, the average discharged mass flux rate of the 1979 eruption was about 1.0 × 108 kg/h (2.5 × 104 kg/s). The average discharged mass flux rate of the 2014 Ontake eruption was about 1.7 times higher than the 1979 eruption. One of the possible reasons for the higher mass flux rate of the 2014 eruption than the 1979 eruption is the deposition of elutriation ash and surge deposits from pyroclastic density currents in the summit region. The greater discharged mass (1.9 × 106 t) and lower flux rate (1.0 × 108 kg/h) of the 1979 eruption compared to the 2014 eruption can be explained by the 3–3.3 times longer duration of the eruption (18–20 h against 6 h).
The detailed isopleth mass distribution map for the September 27, 2014, phreatic eruption of Ontake Volcano extends as far as 85 km E of the source (Fig. 2) and was made based on the field survey of the Joint Research Team for ash fall in Ontake 2014 eruption. The main dispersal axis was toward the ENE in the proximal and medial region and toward the E in the distal region. Total discharged mass of the 2014 phreatic eruption of Ontake Volcano is estimated at 1.2 × 106 t using the segment integration method and 8.9 × 105 t using Pyle’s exponential method. The large variations in discharged mass calculated using Hayakawa’s single isopach method show clearly that this method is not suitable for the estimation of small-scale eruption volumes.
The October 28, 1979, ash-fall deposit in the medial region covers a wider area than the 2014 deposit. The 2014 deposit in the proximal region, however, covers a wider area than the 1979 deposit, mainly due to the additional deposition of elutriation ash and surge deposits from pyroclastic density currents. The total discharged mass of the 1979 phreatic eruption was estimated at 1.9 × 106 t, 1.6–2.1 times larger than that in 2014.
The estimated average discharged mass flux rate for the 2014 eruption was 1.7 × 108 kg/h and for the 1979 eruption was 1.0 × 108 kg/h. One possible reason for the higher flux rate of the 2014 eruption is the occurrence of pyroclastic density currents in the summit area.
All authors contributed to field surveying and ash sampling, data processing, and drawing the distribution map. TO made the distribution maps of the proximal area. ST calculated the total discharged mass using the segment integration, exponential, and single isopach methods. All authors read and approved the final manuscript.
We are indebted to the members of the Joint Research Team for ash fall in Ontake 2014 eruption: Setsuya Nakada (ERI), Fukashi Maeno (ERI), Mitsuhiro Yoshimoto (MFRI), Wataru Hirose (GSH), Akihiko Terada (Tokyo Institute of Technology), Yusuke Suzuki (Izu Peninsula Geopark), Yasuyuki Miyake (Shinshu University), Yasushi Takahashi (Shinshu University), Yoshihiro Takeshita (Shinshu University), Takahiro Miwa (NIED), Masashi Nagai (NIED), Masaaki Tsutsui (Dia Consultant), and Yasuhisa Tajima (Nihon Koei); and also members of the Ontake Volcano Proximal Area Survey Joint Research Team: Jiro Komori (Teikyo Heisei University), Yoshimoto M, Takeshita Y, Maeno F, Nakada S, and Taketo Shimano (Tokoha University) for valuable data sampling and analysis. Comments from J Fierstein and an anonymous reviewer were quite helpful to improve the manuscript. We appreciate all the support from members of the Research Institute of Earthquake and Volcano Geology, Geological Survey of Japan, AIST.
The authors declare that they have no competing interests.
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