Preparatory process preceding the 2014 eruption of Mount Ontake volcano, Japan: insights from precise leveling measurements
- Masayuki Murase1Email author,
- Fumiaki Kimata2,
- Yoshiko Yamanaka3,
- Shinichiro Horikawa3,
- Kenjiro Matsuhiro3,
- Takeshi Matsushima4,
- Hitoshi Mori5,
- Takahiro Ohkura6,
- Shin Yoshikawa6,
- Rikio Miyajima2,
- Hiroyuki Inoue6,
- Taketoshi Mishima6,
- Tadaomi Sonoda7,
- Kazunari Uchida4,
- Keigo Yamamoto7 and
- Harushisa Nakamichi7
© Murase et al. 2016
Received: 6 October 2015
Accepted: 13 January 2016
Published: 22 January 2016
Preparatory activity preceding the 2014 eruption of Mount Ontake volcano was estimated from vertical deformation detected using a precise leveling survey. Notable uplift (2006–2009) and subsidence (2009–2014) were detected on the eastern flank of the volcano. We estimated pressure source models based on the vertical deformation and used these to infer preparatory process preceding the 2014 eruption. Our results suggest that the subsidence experienced between 2009 and 2014 (including the period of the 2014 eruption) occurred as a result of a sill-like tensile crack with a depth of 2.5 km. This tensile crack might inflate prior to the eruption and deflate during the 2014 activity. A two-tensile-crack model was used to explain uplift from 2006 to 2009. The geometry of the shallow crack was assumed to be the same as the sill-like tensile crack. The deep crack was estimated to be 2 km in length, 4.5 km in width, and 3 km in depth. Distinct uplifts began on the volcano flanks in 2006 and were followed by seismic activities and a small phreatic eruption in 2007. From the partially surveyed leveling data in August 2013, uplift might continue until August 2013 without seismic activity in the summit area. Based on the uplift from 2006 to 2013, magma ascended rapidly beneath the summit area in December 2006, and deep and shallow tensile cracks were expanded between 2006 and 2013. The presence of expanded cracks between 2007 and 2013 has not been inferred by previous studies. A phreatic eruption occurred on 27 September 2014, and, following this activity, the shallow crack may have deflated.
KeywordsMount Ontake volcano Preparatory process Precise leveling survey Vertical deformation Pressure source
On 27 September 2014, a sudden phreatic eruption occurred at the summit of Mount Ontake volcano, located at the southern end of the Northern Japan Alps, central Japan. The eruption took 63 lives and represented the worst volcanic disaster in post-World War II Japanese history.
Mount Ontake volcano is the second highest stratovolcano in Japan. Throughout its eruption history, it has produced mainly andesite lavas and pyroclastics, including minor amounts of rhyolite, dacite, and basalt. The volcano was considered to be dormant since its last eruptive activity approximately 23,000 years ago (Yamada and Kobayashi 1988); however, recent studies indicated that phreatic eruptions have occurred during the past 6000 years (Kimura and Yoshida 1999; Matsumoto and Kobayashi 1995).
Volcano research and monitoring are important for the understanding volcanic activity and for the mitigation of volcanic hazards. Geodetic, geophysical, and geochemical monitoring techniques are commonly used for detecting signs of volcanic unrest. Seismic monitoring has recently revealed micro-seismic activity, including volcano-tectonic (VT) earthquakes, tremors, long-period (LP) earthquakes, and very long-period (VLP) earthquakes, in the summit area of Mount Ontake volcano (Nakamichi et al. 2009). In addition, geochemical monitoring (e.g., δ13C of CO2 and 3He/4He ratios) has focused on the hot springs of the eastern flank of the volcano (Sano et al. 1984). However, deformation in the area around Mount Ontake volcano has not been extensively studied.
Continuous global navigation satellite system (GNSS) benchmarks detected deformation on the volcano flanks prior to a small 2007 eruption, and pressure source models were proposed (Geospatial Information Authority of Japan (GSI) 2008; Ishikawa 2008; Meteorological Research Institute, Japan (MRI) and Japan Meteorological Agency (JMA) 2007). Before the 2014 eruption, deformation initiated from early September 2014 and was detected by a tiltmeter at Tanohara (4 km southeast of the summit; JMA 2014). However, the GNSS data did not show clear precursory deformation between 2007 and September 2014. Volcanic-hydrothermal activity is commonly thought to cause phreatic eruptions, but the associated deformation can be difficult to detect using GNSS measurements, as variations are too small and too localized oftentimes (Daita et al. 2009; Murase et al. 2014). While tiltmeters are sensitive enough to detect early signs of volcanic unrest, they are not adequate for monitoring long-term deformation.
Some studies have reported ground deformation detected using precise leveling surveys on active volcanoes, with the results allowing for the prediction of eruptive activity. On Sakurajima Island, Japan, uplift was detected on Sakurajima volcano, while around the Aira Caldera, deformation in the center of the caldera can be explained by the inflation of a spherical source (Yamamoto et al. 2013). Yamamoto et al. (2013) used this data to suggest the imminent potential of new and intensive eruptive activity at Sakurajima volcano. In the Campi Flegrei caldera, Italy, the geometry and density of a deformation source has been estimated from leveling, electronic distance measurement (EDM), and gravity surveys. The estimated density of the source was considered to be almost the same as that of water, unambiguously suggesting the presence of geothermal fluid (Battaglia et al. 2006). At the Tatun Volcano Group, Taiwan, a shallow spherical source was proposed as a plausible model to explain small uplift in a fumarolic area, and the imminent potential of phreatic activity was also suggested (Murase et al. 2014).
In this study, notable uplift (2006–2009) and subsidence (2009–2014) on the eastern flank of Mount Ontake volcano were detected using precise leveling surveys. Based on this deformation pattern, we suggest that magma intruded the shallow plumbing system between 2006 and 2014.
Previous geodetic and seismic studies of Mount Ontake volcano (2006–2014)
In data from the continuous GNSS survey, a rapid change was first detected on the baseline between both flanks of Mount Ontake volcano between December 2006 and January 2007, while a minor change was detected from January 2007 to February 2007 (GSI 2008). A small phreatic eruption was identified by ash-fall deposits in the fumarolic region of the southern part of the summit in late March 2007 (JMA 2008b). However, from March 2007 to September 2014, no clear deformation associated with volcanic-hydrothermal activity was detected by the continuous GNSS survey. However, MRI and JMA (2007) reported GNSS data that showed a minor inflation (~5 mm) in a very small region of the summit area from February 2007 to April 2007. At the beginning of September 2014, a slight increase was detected on some baselines between both flanks of Mount Ontake volcano. This continued into mid-September, after which slight contraction was observed between mid-September 2014 and December 2014 (GSI 2015).
To explain the deformation signal observed from far-field observations between 2006 and 2007, GSI (2008) proposed a model consisting of a dike at a 5.4-km depth and a spherical source at a 10-km depth. In contrast, MRI and JMA (2007) and Ishikawa (2008) used data including near-field observations for their modeling. MRI and JMA (2007) proposed a spherical source at a 0-km depth and a dike at a 2.5-km depth. Ishikawa (2008) proposed a two-dike model with depths of 0 and 4 km.
At the end of December 2006, seismic activity increased beneath the summit area (JMA 2008a). VT earthquakes occurred beneath the summit at depths between 1 km above sea level and 3 km below sea level (Nakamichi et al. 2009). A VLP earthquake occurred on 25 January 2007, when VT activity had almost ceased; thus, VT activity was followed by the occurrence of LP earthquakes and tremors.
Seismic activity peaked in March 2007 when the small phreatic eruption occurred but subsided to almost no activity by mid-2007. Seismicity continued at a low level until September 2014. From 10 to 11 September 2014, VT earthquakes increased rapidly. Subsequently, LP earthquakes occurred five times from 14 to 24 September. Finally, the phreatic eruption occurred on 27 September 2014. Pre-eruptive seismic activity was low and of short duration, as compared with that preceding the 2007 eruption.
On the other hand, earthquake swarms have been observed continuously on the south-southeastern flank of Mount Ontake volcano since 1978. The epicenter of the swarm migrated from the southern flank to the eastern flank in 1993 (Nagoya University 1999), and swarm activity still continues today. Our repeated precise leveling survey, conducted since 1999, revealed uplift of 3–6 mm/year in and around the swarm region from 2002 to 2006 (Kimata et al. 2003, 2004, 2011). In this study, we mainly focused on data of the precise leveling surveys from 2006 to 2014 in order to highlight the pre-eruptive activity at Mount Ontake volcano prior to the 2014 eruption.
Leveling survey errors
We also considered the scale error, which is considered to be the main factor involved in systematic errors from leveling surveys. A significant cause of such errors is the shift in scale due to thermal expansion of the leveling rod. However, in our survey, we used spear invar rods to minimize thermal expansion. In addition, we made a correction for thermal expansion using the linear expansion coefficient of spear invar. We also took temperature measurements at the time of the leveling surveys in order to estimate the correction value. We additionally checked the influence using a case where the linear expansion coefficient had an uncertainty of 5.0 × 10−7/°C, where the temperature difference was 10 °C and where the maximum relative height from benchmark BM34 was approximately 350 m. A scale error of only 1.75 mm was estimated, and this value was considered sufficiently small as to not significantly affect the results. Therefore, we concluded that the results were accurate enough to detect a deformation of greater than a few millimeters.
While a number of models have been proposed to explain the deformation observed in GNSS data from 2006 to 2007, a unified model has not been developed. Based on previous studies and leveling results, we hypothesized that the magma intruded the shallow system from 2006 to 2009 but that the 2014 eruption and aftermath were characterized by deflation. In this study, we considered four models (spherical source, tensile crack, spherical source with tensile crack, and two tensile cracks) as candidates for the source model.
We began by modeling the period between 2009 and 2014 because the geometry of the 2014 eruptive fissure provided a useful reference for modeling the shallow source during this period. Our leveling data were insufficient to estimate all parameters of the above models because they were acquired only on the eastern flank of the volcano. In particular, it was difficult to determine an exact geometry of the shallow source. Therefore, the location of the spherical source model was assumed to be beneath the center of the 2014 eruptive fissure, while the location, length, and strike angle for the tensile crack model were assumed from the geometry of the eruptive fissure.
For modeling the period between 2006 and 2014, the geometry of a shallow crack and spherical source was assumed to be the same as that of the 2009–2014 models. For the other models (spherical source with tensile crack, and two tensile cracks), the shallow spherical source or shallow tensile crack assumed the same geometry as in the 2009–2014 models, and all parameters of the deep tensile crack were estimated.
Model parameters were optimized using a genetic algorithm (GA) in order to conform to the deformation observed from 2009 to 2014 and 2006 to 2009. The GA method employs a random walk, and, unlike the conventional Monte Carlo method, it uses a probability rule for searching (Goldberg 1989). The GA method is an effective approach to the volcano deformation modeling (e.g., Irwan et al. 2006). Optimal parameters are selected on the basis of minimizing the fitness function, which is defined as the weighted residual sum of squares. The deformation at each benchmark was calculated using an elevation-modified Mogi model (Fukui et al. 2003; Mogi 1958) and dislocation model (Okada 1992). Details of the GA method were described by Winter et al. (1996).
Results and discussion
From April 2009 to October 2014, a time period that included the 2014 eruption, ~8 mm of subsidence was detected on the Yashikino and Kiso-Onsen routes (Figs. 2 and 5), with deformation concentrated in the area to the west of BM205. It is notable that the uplift from 2006 to 2009 was larger than the subsidence from 2009 to 2014. Furthermore, the spatial distribution of deformation from 2009 to 2014 and 2006 to 2009 differed from that of previous periods of deformation (Fig. 5c; Kimata et al. 2011).
Pressure source models and pre-eruptive processes before the 2014 eruption
Akaike’s information criteria (AIC) values for each model (2009–2014)
Tensile crack model
Spherical source model
Model of spherical source with tensile crack
Tensile crack model (2009–2014)
−7.0 × 106
−3.9 × 106/+1.7 × 106
The depth of the top of the deflated crack is just beneath precursory activity of VT events (Kato et al. 2015). Yamamoto (2014) reported that thick white fumes and pyroclastic density currents were produced from the 2014 eruptive fissure. GSI (2015) reported that a slight contraction was detected on the GNSS’s baselines between both the flanks of Mount Ontake volcano from mid-September 2014 to December 2014. Although the timing of deflation cannot be identified from the leveling data, the deflation of a crack could be caused by the emission of volcanic gases and fluids involving the eruption. VT and LP events might occur by the rising volcanic gases and fluids just above the deflated crack before the eruption. We believe that an increase in magma density was caused by the emission of volcanic volatiles from magma, which was followed by magma drain-back.
Akaike’s information criteria (AIC) values for each model (2006–2009)
Tensile crack model
Spherical source model
Model of spherical source with tensile crack
Two-tensile-crack model (2006–2009)
Shallow tensile crack
0.9 × 106
−2.9 × 106/+3.2 × 106
Deep tensile crack
6.3 × 106
−1.7 × 106/+9.4 × 106
The occurrence of a shallow crack was supported by its inferred depth, which overlapped with that of the VT events associated with the 2007 eruption (JMA 2008a). Therefore, the shallow crack may have existed from at least 2006. As a possible objection of the crack model, volume change of the crack may be difficult to continue for a long period from 2006 to 2014 because a crack shape is easy to cool and become solidified. In order to avoid the solidification of the crack, it may be necessary to supply hot magma continuously from the deeper part and/or to have a thick body of the crack. Within the shallow crack, there is a mismatch between the increasing volume change (0.9 × 106 m3 from 2006 to 2009) and the decreasing volume change (−7.0 × 106 m3 from 2009 to 2014). It may be difficult to estimate the accurate volume changes of the two-crack model because of insufficient data in the mountain area. We believe that the increasing volume change is underestimated in our model. On the basis of our results, we believe that a pre-eruptive magma intrusion mainly fed into the shallow crack from 2006 to 2009, representing the main preparatory process of the 2014 eruption.
From the above discussion, more dense and precise deformation data is prerequisite for detailed discussion. Therefore, we installed new leveling routes on the mountain area in October 2014 and April 2015 (Murase et al. 2015). Based on the improved data, the remaining issues will be performed in future studies.
Based on the GNSS data, MRI and JMA (2007) proposed a spherical source at a 0-km depth and a dike at a 2.5-km depth. In contrast, Ishikawa (2008) proposed a model of two dikes at depths of 0 and 4 km. Since the observation period for each model was different, they produced different results. Our preferred model was estimated using deformation data from the longest period; thus, we believe that our modeling represents a suitable estimate for the stable source. However, deformations from short-lived shallow sources, as detected by MRI and JMA (2007) and Ishikawa (2008), could not be detected. The sill-like crack may have existed in a stable configuration in the shallow crust from 2006 to 2014. For this period, all models are consistent with the necessary deep dike.
A tensile crack model has already been estimated as a magma source at Mount Asama volcano, Japan (Murakami 2005). A tensile crack with a depth of about 2 km below the surface was estimated by continuous GNSS measurements around Mount Asama volcano from 1996 to 2004. Murakami (2005) proposed that buoyant bubble-rich magma surges episodically into the crack-shaped magma reservoir, then it loses the buoyancy through degassing process, and it finally drains back to the further depth. Such activity has a strong resemblance to that occurring at Mount Ontake volcano.
In addition, a spherical source with a depth of about 8 km below the surface was estimated based on the precise leveling surveys in Mount Asama volcano (Murase et al. 2007). The buoyant bubble-rich magma may supply episodically from the deep spherical source to the shallow tensile crack in Mount Asama volcano. In Mount Ontake volcano, a spherical source with a depth of about 10 km and a shallow dike were estimated based on GNSS in the period from November 2006 to January 2007 (GSI 2008). The magma may supply from the deep spherical source to the shallow tensile cracks in Mount Ontake volcano.
Temporal changes in pre-eruptive processes before the 2014 eruption
From the results between August 2013 and October 2014, the subsidence occurred after August 2013 (Fig. 10a). We, however, found no evidence to decide the exact occurrence time of subsidence from the leveling result because the leveling surveys were only conducted in August 2013 and October 2014. We believe that the subsidence mainly occurred during the 2014 eruption because the record of the tiltmeter located in about 3 km southeast part of the eruptive fissures suddenly showed a decrease in the slope angle or deflation in the 2014 eruption (JMA 2014).
We compared the temporal changes in vertical deformation to baseline change detected by GNSS (Fig. 10e). A rapid expansion of the baseline was detected from December 2006 to March 2007. However, a significant change was not detected from March 2007 to September 2014. On the other hand, a minor inflation of the summit area was detected by the campaign GNSS survey from February 2007 to April 2007 (MRI and JMA 2007). The locations of GEONET benchmarks were too far to detect clear deformation, as the latter was localized to a very small region of the study area. Using campaign GNSS data from February 2007 to April 2007, MRI and JMA (2007) proposed a shallow inflation source at a 0.1-km depth. Uplift was only detected by the leveling survey from 2007 to 2013. This result is particularly important for the understanding of the preparatory process of the 2014 eruption.
We also briefly addressed deformation along the Yashikino route from 2002 to 2006. It was challenging to consider the preparatory process during the above period using a quantitative approach because deformation was affected by the seismic swarm activity that occurred close to the leveling routes (Fig. 10d). However, minor uplift can be roughly estimated from 2002 to 2006 (Fig. 10b), suggesting the preparatory process may have initiated prior to 2006.
Possible scenarios of the preparatory process preceding the 2014 eruption
Based on the smaller amount of uplift between 2007 and 2009, we inferred that a minor amount of magma was supplied continuously, allowing tensile cracks to slightly expand. The shallow source may have inflated during this period (MRI and JMA 2007). Based on the Kiso-Onsen route time series (Fig. 10a), we believe that this trend continued until August 2013. After the 2007 eruption, a minor inflation of the summit area was detected, not only by the leveling survey but also by the campaign GNSS (MRI and JMA 2007). However, the presence of expanded cracks from 2007 to 2013 had not been inferred until now.
The phreatic eruption occurred on 27 September 2014, and the shallow tensile crack may have deflated at this activity (Fig. 11c). VT and LP events might occur by the rising volcanic gases and fluids just above the deflated crack (Kato et al. 2015). From the leveling result, clear deformation caused by volume change of the deep crack was not detected. The deep crack may not change during this time.
We could not detect what kind of crustal deformation occurred immediately before, during, or after the 2014 phreatic eruption because the leveling surveys were only conducted in August 2013 and October 2014. Based on the continuous GNSS survey, clear deformations were not detected on GEONET benchmarks in the 2014 phreatic eruption because these locations are too far to detect it (Fig. 10e). In order to detect small deformation localized to a very small region around eruptive fissures, continuous GNSS surveys on the mountain area will be needed.
Significant uplift (~10 mm) and subsidence (~8 mm) were detected from 2006 to 2009 and 2009 to 2014, respectively, on the eastern flank of Mount Ontake volcano using precise leveling surveys. The most suitable model to explain the subsidence (2009–2014) was a sill-like tensile crack estimated to be −4.7 m in opening, 1.5 km in width, 2.5 km in depth, and dipping at −15°. A two-tensile-crack model best accounts for the uplift between 2006 and 2009. The geometry of the shallow crack was assumed to be the same as that of the sill-like tensile crack. The opening of the shallow crack was modeled to be 0.6 m. The deep crack was estimated to be 0.7 m in opening, 2 km in length, 4.5 km in width, 3 km in depth, and with a dip of 70°.
Distinct uplift began in 2006 and was followed by seismic activity and a small phreatic eruption. This uplift trend may have continued until August 2013; however, after the 2007 event, deformation was aseismic. The uplift from 2006 to 2013 suggests that magma ascended rapidly beneath the summit area in December 2006 and that deep and shallow tensile cracks expanded between 2006 and 2013. The results suggest that concurrent with the phreatic eruption of 27 September 2014, the shallow crack may have deflated.
The results of this study demonstrate that minor deformation detected by highly accurate and dense geodetic observations (e.g., precise leveling surveys) in mountainous areas can provide useful information for understanding preparatory activity preceding phreatic eruptions.
We thank two anonymous referees for their careful reading of our manuscript and their numerous constructive suggestions that helped to improve this paper. We would like to thank the staff and students of Nagoya University for their support during the surveys. We gratefully acknowledge the Geospatial Information Authority of Japan for providing the F3 solutions from the GEONET benchmarks. We are also grateful to the Japan Meteorological Agency for providing the hypocenter catalogs. This research was partially supported by JSPS KAKENHI (26900002) and the Collaborative Research Program of Disaster Prevention Research Institute, Kyoto University (27G-03). Some figures were prepared using the generic mapping tools (GMT) package (Wessel and Smith 1995).
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