International Workshop on the mechanism of Phreatic Eruption

Preparation processes leading up to the 2014 phreatic eruption ofMount Ontake, JapanAitaro Kato1
1 Earthquake Research Institute, the University of Tokyo
E-mail: akato(at)eri.u-tokyo.ac.jp

On 27 September 2014, at 11:52 a.m. JST (UTC + 9), Mount Ontake produced a phreatic(steam type) eruption, spraying volcanic ash, gas, and debris on surrounding areas. Basedon ejecta volume and the maximum ash column height of ~7000 m, the VEI for this eruption
was categorized to 2. The 2014 eruption resulted in the deaths of 58 climbers, with a further
5 missing and presumed dead. Before the 2014 eruption, a micro-seismicity just beneath thecrater, which might be linked to the eruption, had lasted for around one month. We relocatedshallow volcano tectonic (VT) earthquakes and long-period (LP) events from August to
September 2014, using differential travel times. Then, we further detected numerous
additional micro-earthquakes beneath the craters, by applying a template-matching
technique to continuous waveforms using these relocated earthquakes (Kato et al., 2015).
The relocated VT earthquakes aligned on a near-vertical plane oriented NNW–SSE,
suggesting they occurred around a conduit related to the intrusion of magmatic–hydrothermal
fluids into the craters. The frequency of VT earthquakes gradually increased from 6
September 2014 and reached a peak on 11 September 2014. After the peak, seismicity levels
remained elevated until the eruption. During the 10-min period immediately preceding thephreatic eruption, VT earthquakes migrated in the up-dip direction as well as laterally along
the NNW–SSE feature. The migrating seismicity coincided with an accelerated increase ofpre-eruptive tremor amplitude and with an anomalous tiltmeter signal that indicated summit
upheaval. Therefore, the migrating seismicity suggests that the vertical conduit was filled withpressurized fluids, which rapidly propagated to the surface during the final 10 min before theeruption.
Maeda et al. (2017) revealed that the tilt change observed immediately (450 s) before theeruption was at first a linear function in time that then switched to exponential growth. The tilt
signal can be explained by a vertical tensile crack at a depth of 1100 m from the summit. The
linear function was explained by the boiling of underground water controlled by a constantheat supply from a greater depth. The exponential function was explained by thedecompression-induced boiling of water and the upward Darcy flow of the water vapor

International Workshop on the mechanism of Phreatic Eruption

Fig. 1(left) Precursors of the 2014 phreatic eruption(Kato et al., 2015). a) Horizontal distance of detected
VT earthquakes from 11:30 to 12:00 JST on 27
September 2014, projected onto the strike of thehypocenter alignments (N15W-S15E). Circles are
scaled to magnitude and color-coded to depth. Blackdashed lines approximate the locations of the frontsof horizontal earthquake migrations. b) Depth
variations in VT earthquakes, colored to depth.
Circles are scaled to magnitude. Black dashed lines
approximate the locations of the fronts of verticalearthquake

migrations.

c)Bandpass

filtered

waveform between 4 and 12 Hz (black curve) andenvelope between 1 and 4 Hz (red curve) recorded at
the V.ONTN station nearby the summit. d) Time seriesof tiltmeter records observed at the V.ONTN station.
NS and EW components denote the northward andeastward ground-up components, respectively.

through a permeable region of small cracks. This transition can be explained by a breakage
of a barrier in the conduit, leading up to the eruption.
To examine temporal change in the long-term seismicity rate beneath the crater, we are
now searching for similar events applying template-matching technique to continuous
waveform data from 2011 to Sep. 2014. I will show the preliminary result in my talk.
Main References
Kato A, Terakawa T, Yamanaka Y, Maeda Y, Horikawa S, Matsuhiro K and Okuda T (2015)
Preparatory and precursory processes leading up to the 2014 phreatic eruption ofMount Ontake, Japan. Earth, Planets Sp 67: 111, doi:10.1186/s40623-015-0288-x.
Maeda Y, Kato A and Yamanaka Y (2017) Modeling the dynamics of a phreatic eruption
based on a tilt observation: Barrier breakage leading to the 2014 eruption of MountOntake,Japan.

J.

Geophys.

doi:10.1002/2016JB013739.

Res.

SolidEarth,122,

1007–1024,