Descent of tremor source locations before the 2014 phreatic eruption of Ontake volcano, Japan
© Ogiso et al. 2015
Received: 12 August 2015
Accepted: 16 December 2015
Published: 29 December 2015
On 27 September 2014, Ontake volcano, in central Japan, suddenly erupted without precursory activity. We estimated and tracked the source locations of volcanic tremor associated with the eruption at high temporal resolution, using a method based on the spatial distribution of tremor amplitudes. Although the tremor source locations were not well constrained in depth, their epicenters were well located beneath the erupted crater and the summit. Tremor sources were seen to descend approximately 2 km over a period of several minutes prior to the beginning of the eruption. Detailed analysis of the time series of tremor amplitudes suggests that this descent is a robust feature. Our finding may be an important constraint for modeling the 2014 eruption of Ontake volcano as well as for monitoring activities on this and other volcanoes.
Ontake volcano, in central Japan, is the nation’s second highest volcano with an elevation of 3067 m. According to the Japan Meteorological Agency and the Volcanological Society of Japan (2013), magmatic eruptions have occurred four times at Ontake during the last 100,000 years in addition to occasional phreatic eruptions. Only three eruptions were recorded in human history before the 2014 eruption. The first eruption occurred in the early morning of 28 October 1979 when there were only two seismic stations around Ontake, one located on 12 km north and the other on 13 km southeast. Although hypocenter determination of its associate earthquakes was impossible at that time, these two stations recorded precursory seismicity that started in the previous evening (Aoki et al. 1980). Approximately 200,000 tonnes of ash was produced during the 1979 eruption (Kobayashi 1980). The second eruption occurred on the middle of May, 1991. Aerial photographs by the press detected fumarolic activity from one of the craters formed during the 1979 eruption. Ash was distributed in the area of about 200 m long by 100 m width just in the east of the craters. The amount of ash was not more than dozens of tonnes. On-site survey estimated that the eruption occurred between 13 and 18 May. Preceding the eruption, seismicity at Ontake had increased since late April and many tremors were observed at the same time (Matsumoto Weather Station 1991; Nagoya University 1991). The third eruption occurred in 2007. Volcanic earthquake swarm began on the end of December 2006 (Otsuka and Fujimatsu 2009). On 25 January, the largest volcanic tremor during the 2007 activity occurred. This tremor contains a component of very long period (about 20–100 s) (Nakamichi et al. 2009). At almost the same time of the beginning of volcanic earthquake swarm, inflation of the volcanic edifice was detected by continuous GPS observations (Takagi et al. 2007). Although significant precursors were observed in both seismicity and crustal deformation, the 2007 eruption was only recognized afterward when ash was seen on top of snow during an on-site survey. The ash was too little to estimate its amount. The date of the eruption was not specified in the 2007 activity (Japan Meteorological Agency 2008).
The Tokyo Volcano Observation and Information Center, Japan Meteorological Agency (hereafter Tokyo VOIC) monitors active volcanoes in the Kanto and Chubu regions of Japan, including Ontake, using seismic data, ground deformation measurements, video observations, and on-site surveys. Around 11:52 on 27 September 2014 (dates and times are Japan Standard Time, 9 h ahead of UTC), an eruption suddenly began, causing pyroclastic flows that were recorded by a video camera placed on the southern flank of Ontake (Tokyo Volcano Observation and Information Center 2014). Clouds obstructed any views of active craters from video observations at that time, but new craters were later identified by aerial observations (e.g., Kaneko et al. 2014). Kaneko et al. (2014) also reported that the impact craters of volcanic bombs were distributed mainly in the northeast of erupted craters by careful analysis of aerial photographs. The density of volcanic bombs was extremely high (more than 10 craters per 16 m2) within 500 m from craters. Volcanic bombs were not found over 1 km away from the erupted craters in the photographs. Diameters of the impact craters were between 10 cm and 1 m. The type of eruptions was determined to be phreatic, and the amount of ashfall was estimated to be about 600,000 to 1,500,000 tonnes (Takarada et al. 2014). According to the official report from Tokyo Volcano Observation and Information Center (2014), the eruption continued until at least 10 October. Because 27 September was a holiday and the weather was fine in this area, there were many hikers near the summit when the eruption began. As a result, there were 57 confirmed fatalities and 6 people remain missing (Nagano Prefectural Government 2015). This eruptive event left us many issues in not only volcanological but also social aspects. Even if an eruption is small and there are no casualties, the eruptive event sometimes leaves a large social impact, leading to difficult challenges of scientific communication (e.g., Leonard et al. 2014). Similar to the case of Tongariro volcano (Leonard et al. 2014), the 2014 eruption of Ontake volcano revealed some challenges to the mitigation of volcanic disaster, and many investigations have been conducted by both national and local governments (Yamaoka 2015).
Observations of the volcanic tremor and ground deformation at Ontake before the 2014 eruption contain some important information about eruption-related activities. In this paper, we report estimates of the source locations of volcanic tremor at the time of the eruption and discuss what this information reveals its volcanic processes prior to the eruption.
Data and methods
Usually, a tremor has no clear onsets of P or S waves; thus, the conventional method of source location using phase arrival times is not applicable. We adopted the amplitude source location (ASL) method, which utilizes the spatial distribution of tremor amplitude (Yamasato 1997, Jolly et al. 2002, Battaglia and Aki 2003, Kumagai et al. 2010). One of the advantages of the ASL is high temporal resolution, making it possible to follow the migration of sources during a tremor sequence (Kumagai et al. 2011, Ogiso and Yomogida 2012).
In this study, we assumed that the source amplitude A(f) depends only on frequency, although A(f) also depends on the azimuthal relationship between the source and station due to the inhomogeneous radiation patterns of seismic source. However, Takemura et al. (2009) showed that scattering due to small-scale heterogeneity in the crust masks the radiation pattern at frequencies higher than 5 Hz. Taking the relatively strong crustal heterogeneities in volcanic areas (e.g., Yamamoto and Sato 2010) into consideration, our assumption that source amplitudes have no azimuthal dependency is reasonable for frequencies higher than 5 Hz. In this study, we used 5–10 Hz filtered waveforms to satisfy this assumption. Signal-to-noise ratio of 5–10 Hz was high to be reliable location estimation (Fig. 4).
We conducted a grid search in the whole area of Fig. 2 with an interval of 0.001° in both latitudinal and longitudinal directions and 0.1 km in the depth direction to find the grid cell with the minimum residual. We used the root-mean-square (RMS) amplitude in a given time window as the observed amplitude A i (f) at each station. This study used the frequency range of 5–10 Hz for RMS amplitude calculations; hence, the frequency f is 7.5 Hz in our equations. We assumed that the observed waveforms at each station were composed of direct S waves in this frequency range and thus adopted 2.31 km/s for the velocity of the medium, which is the S wave velocity that Tokyo VOIC uses for hypocenter determination at Ontake volcano. This velocity was estimated from the seismic-refraction profile derived by Ikami et al. (1986) with some trial-and-error modifications (Koji Kato, personal communication). Because the attenuation structure of Ontake is unclear, we assumed Q to be constant at 50, a commonly used approximation in volcanic areas (Koyanagi et al. 1995, Battaglia and Aki 2003, Ogiso and Yomogida 2012, Kumagai et al. 2013). We used only the vertical component waveform of each station.
Site amplification of seismic stations on Ontake with respect to station V.ONTN, based on the configuration of seismometers and data loggers on 27 September 2014
Standard deviation (log scale)
Number of data
Source locations of volcanic earthquakes
Source locations of volcanic tremor
In estimating the source locations of tremor, we adopted a time window of 30 s for assessing RMS amplitude at each station and shifted the window by 15-s increments from 11:45 to 12:10 on 27 September. The beginning of the time window was set for each station to accommodate its travel time from the assumed tremor source (Kumagai et al. 2010). The definition and calculation of error bars are the same as for the earthquake location explained in the previous section.
Possible model for descending tremor source locations
Because the ASL method assumes a point source for seismic events, we can interpret the descent of tremor sources in two ways: (a) there is a point source of tremor that moved downwards, or (b) there is a finite source area of tremor, and the descent indicates the migration of the portion of the area that was generating strong motion. In this section, we propose a point-source model based on the first interpretation.
Our findings may have further implications for monitoring volcanic activities. Because the ASL method can be applied in near-real time, it may enable us to detect and track the migration of volcanic tremor sources immediately after the beginning of tremor is recognized. Although its mechanism is not yet known, migration of tremor source locations may be a distinctive precursor of phreatic eruptions in general. Thus, early recognition of tremor source locations and their migration may afford an early warning of any future phreatic eruptions at Ontake and other active volcanoes.
We used the ASL method to estimate the source locations of volcanic tremor with high temporal resolution just before and after the 27 September 2014 phreatic eruption of Ontake volcano. Our result showed that the tremor source was located beneath the eruption craters and the summit. This source steadily descended during the period of several minutes just before the beginning of the eruption. Although the precision of absolute locations was limited (especially for depths) owing to the accuracy of site amplification factors and insufficient station distribution, our careful check of the observed amplitude ratios of the tremor data suggests that our finding of a descending source is robust.
We hypothesize that the tremor was excited by the decompression of volcanic fluids that took place on a downward-propagating front. At the same time, ascending volcanic gases caused the inflation of the summit area of Ontake, opened a crack just beneath the craters together with active seismicity, and finally escaped in a phreatic eruption. Descending tremor source locations should be further investigated as an immediate precursor of phreatic eruptions and for possible use in monitoring volcanic activities.
We thank the staff of Nagoya University and the Nagano Prefectural Government for their permission to use seismic waveforms and for their efforts to maintain their seismic stations. Koji Kato kindly helped us to collect seismic waveforms and advised us the velocity structure around Ontake volcano. Yoshiro Masuda and Yutaka Nagaoka also helped us to collect seismic waveforms. Careful reviews and helpful comments from two anonymous reviewers and guest editor Koshun Yamaoka greatly improved the manuscript. We also thank Kiyoshi Yomogida for revising the manuscript. We used digital elevation data compiled by the Geospatial Information Authority of Japan. We also used the unified hypocenter catalogue maintained by the Japan Meteorological Agency in cooperation with the Ministry of Education, Culture, Sports, Science and Technology, Japan. Most of the figures were drawn by Generic Mapping Tools (Wessel and Smith, 1998).
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