Preparatory and precursory processes leading up to the 2014 phreatic eruption of Mount Ontake, Japan

Abstract

We analyzed seismicity linked to the 2014 phreatic eruption of Mount Ontake, Japan, on 27 September 2014. We first relocated shallow volcano tectonic (VT) earthquakes and long-period (LP) events from August to September 2014. By applying a matched-filter technique to continuous waveforms using these relocated earthquakes, we detected numerous additional micro-earthquakes beneath the craters. 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. b-values gradually increased from 1.2 to 1.7 from 11 to 16 September 2014 then declined gradually and dropped to 0.8 just before the eruption. During the 10-min period immediately preceding the phreatic 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 of pre-eruptive tremor amplitude and with an anomalous tiltmeter signal that indicated summit upheaval. Therefore, the migrating seismicity suggests that the vertical conduit was filled with pressurized fluids, which rapidly propagated to the surface during the final 10 min before the eruption.

Source mechanism of a VLP event immediately before the 2014 eruption of Mt. Ontake, Japan

“…7, we compare the estimated VLP source with other observations. The hypocenters of VT earthquakes relocated by Kato et al (2015) are aligned in an NNW-SSE direction, subparallel to the strike direction of the VLP source crack. The absolute location difference between the VLP source and the densely populated region of the VT hypocenters may be explained by location errors of both events.…”

“…Because of the regional stress field, preexisting seismic (shear) faults may have aligned along the NNW-SSE direction. These faults could have acted as Kato et al (2015). We did not use any magnitude threshold, and the magnitude ranged from −0.5 to +1.5.…”

Volcanic plume measurements using a UAV for the 2014 Mt. Ontake eruption

“…Ontake occurred at around 11:52 (JST) on September 27, 2014, from Jigokudani area. Although precursory increase in the seismicity was detected about 2 weeks before the eruption, there was no significant volcanic tremor or deformation until about 10 min before the onset of the eruption (Kato et al 2015). The eruption was a phreatic one, and the eruption column went up about 9 km and a low-temperature pyroclastic flow went down 2.5 km on the SW flank of the volcano (JMA 2014a).…”

Detection of crustal deformation prior to the 2014 Mt. Ontake eruption by the stacking method

“…For example, in seismic array processing (e.g., Johnson and Dudgeon 1993), seismic wave data are stacked to improve the SNR by assuming the direction of arrival of seismic waves and their apparent velocity and then shifting the time period covered by each data record. The basic principle of how SNR of the stacked data will be improved is described here.…”