ARTICLE Received 27 Jul 2015 | Accepted 21 Jan 2016 | Published 19 Feb 2016 DOI: 10.1038/ncomms10797 OPEN Monitoring eruption activity using temporal stress changes at Mount Ontake volcano Toshiko Terakawa1, Aitaro Kato1, Yoshiko Yamanaka1, Yuta Maeda1, Shinichiro Horikawa1, Kenjiro Matsuhiro1 & Takashi Okuda1 Volcanic activity is often accompanied by many small earthquakes. Earthquake focal mechanisms represent the fault orientation and slip direction, which are influenced by the stress field. Focal mechanisms of volcano-tectonic earthquakes provide information on the state of volcanoes via stresses. Here we demonstrate that quantitative evaluation of temporal stress changes beneath Mt. Ontake, Japan, using the misfit angles of focal mechanism solutions to the regional stress field, is effective for eruption monitoring. The moving average of misfit angles indicates that during the precursory period the local stress field beneath Mt. Ontake was deviated from the regional stress field, presumably by stress perturbations caused by the inflation of magmatic/hydrothermal fluids, which was removed immediately after the expulsion of volcanic ejecta. The deviation of the local stress field can be an indicator of increases in volcanic activity. The proposed method may contribute to the mitigation of volcanic hazards. 1 Earthquake and Volcano Research Center, Graduate School of Environmental Studies, Nagoya University, D2-2 (510) Furo-cho Chikusa-ku, Nagoya 464-8601, Japan. Correspondence and requests for materials should be addressed to T.T. (email: [email protected]). NATURE COMMUNICATIONS | 7:10797 | DOI: 10.1038/ncomms10797 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms10797 he local stress field around volcanoes represents the reaching a peak on 11 September 2014 before decaying over time superposition of the regional stress field and stress until the eruption, although the seismicity increased again Tperturbations related to volcanic activity1–4. Temporal between 15 and 21 September 2014 during this period. stress changes over periods of weeks to months are generally Long-period earthquakes with peak frequencies of 1–5 Hz were attributed to volcanic processes. Examining the focal mechanism recorded beginning around 15 September, but less than 10 solutions of volcano-tectonic (VT) earthquakes provides detailed long-period events were detected in real time. Application of a information on the state of a volcano via the local stress field. This matched-filter technique to continuous waveforms (23 August to has the potential to contribute to prediction of eruptions over the 30 September 2014) resulted in detection of thousands of micro- short and medium term (weeks to months). seismic events, indicating that an increase in VT events was Mt. Ontake volcano is the second highest stratovolcano in followed by an increase in the number of long-period events 5 Japan and is located at the southern end of the Northern Japan days later6. This detailed analysis also revealed that VT events Alps. On 27 September 2014, around 11:52 hours JST (UTC þ 9), beneath the craters migrated upwards as well as laterally in the Mt. Ontake produced a hydrothermal (steam-type) eruption with NNW–SSE direction for the final 10 min preceding the eruption6. a volcanic explosivity index (VEI) value of 2 (ref. 5). Mt. Ontake The phenomenon synchronized with a rapid increase of pre- had been considered dormant until its first historical eruption in eruptive tremor amplitudes and with an unusual tiltmeter signal October 1979. The 2014 eruption struck on a sunny Saturday in indicating summit upheaval6. After the eruption, the magnitudes the autumn foliage season, resulting in the worst volcanic disaster of events became larger (Mo1); in contrast, most pre-eruption in Japan in the last 90 years: 58 people died and 5 are still missing. events had a magnitude of less than 0. Mt. Ontake is continuously monitored by several relatively The time history of volcanic earthquakes during the pre- dense networks of permanent seismic stations operated by eruption period was explained by the generic volcanic earthquake Nagoya University, Japan Meteorological Agency (JMA), swarm model7. In this respect, the history was similar to that National Research Institute for Earth Science and Disaster associated with a previous minor eruption in 2007 (VEI ¼ 0)8. Prevention (NIED), and Gifu and Nagano prefectures However, unlike the 2007 eruption, the 2014 eruption was not (Fig. 1a,b). The summit region of Mt. Ontake (Fig. 1a,b; latitude preceded by active volcanic tremors and inflation of the volcanic 35.875° N–35.900° N, longitude 137.44° E–137.50° E) is normally edifice until 7 min before the eruption6,9. The time scale for the almost aseismic, with an average of less than one event (Mo0) precursor period was also different from that in 2007. In general, per month, whereas the surrounding region is characterized by a the various observations of precursor processes make eruption high level of background seismicity (see Methods). The onset of forecasting difficult, although changes in volcanic seismicity have VT earthquakes was observed on 31 August 2014 (Fig. 1b,c), been successfully used empirically for predicting eruptions10,11. ab137.4° 137.5° 35.95° M 2 1,000 1,000 M 1 M 0 1,000 2,000 M −1 1,000 M −2 2,000 1,000 36.2° 1,000 2,000 2,000 2,000 35.90° 1,000 2,000 1,000 1,000 2,000 1,000 (km) 1,000 1,000 2,000 1,000 36.0° 1,000 1,000 0 1 2 1,000 1,000 35.85° 1,000 1,000 2,000 1,000 –2 0 2 4 6 1,000 2,000 1,000 Depth (km) 1,000 1,000 2,000 1,000 1,000 c 35.8° Eruption 1,000 400 1,000 2,000 1,000 1,000 2,000 300 1,000 1,000 M3 (km) 1,000 M2 M1 01020 M0 200 35.6° 137.2° 137.6° 138.0° 100 Number of events 0 0152053010 25 9/1 11 21 10/1 11 21 31 Depth (km) Year 2014 Figure 1 | Seismicity around Mount Ontake. (a) Earthquake hypocentres and seismic stations in the Ontake region and (b) summit region. The circles denote hypocentres, and the colour scale corresponds to focal depth. The light blue, grey and black squares denote seismic stations operated by the Nagoya University, the national institutes and prefectural institutes, respectively. The blue rectangle in the inset in a shows the location of the Ontake region. The triangle, star, red rectangles and pink rectangle denote Mt. Ontake, the hypocentre of the 1984 Western Nagano Prefecture Earthquake, the summit region and the region depicted in Fig. 3b, respectively. The black dashed lines in a,b indicate the boundary between Nagano and Gifu prefectures. (c) Histogram of VT events per day in the summit region. 2 NATURE COMMUNICATIONS | 7:10797 | DOI: 10.1038/ncomms10797 | www.nature.com/naturecommunications NATURE COMMUNICATIONS | DOI: 10.1038/ncomms10797 ARTICLE The local stress field beneath volcanoes is directly influenced a Eruption by the inflation of the ascending magma and magmatic/ 16 Before the eruption After the eruption Normal 1–4 6 hydrothermal fluids . As the focal mechanism solution of VT 14 % Strike-slip 23 % Reverse 29 % 24 % events contains information on the local stress field, the 52 % Odd 12 10% 41 % relationships between stresses and volcanic processes have been 10 14% studied by examining focal mechanisms and using theoretical 8 models with the aim to successfully predict eruptions1–4,12–17. However, systematic spatiotemporal changes in focal mechanisms 6 4 do not necessarily indicate changes in the stress field18, because Number of events earthquakes can be triggered at faults that are misoriented to the 2 stress field by a decrease in fault strength caused by an increase in 0 pore fluid pressure19,20. This effect cannot be ignored when 9/1 10/1 11/1 12/1 1/1 2/1 3/1 Year 2015 evaluating the local stress fields beneath active volcanoes because Year 2014 the existence of over-pressurized fluids is certain. b 60 35.90° 0 Here we propose a method to detect true temporal changes in (km) 31/08/2014−27/09/2014:11:502,400 the local stress field beneath Mt. Ontake by examining the focal 0 1 3,000 2,200 mechanism solutions of VT events relative to the regional stress 3,000 pattern. Enhanced volcanic activity causes significant stress 35.89° perturbation with E–W tension, using the crustal structure 2,600 2,800 controlled by the regional stress field. This results in a large M 1.0 35.88° 2,400 deviation of the local stress field from the regional stress field 2,600 M 0.0 before the 2014 eruption. We demonstrate that quantitative M −0.5 evaluation of temporal stress changes is an effective tool for c 60 eruption monitoring. 0 35.90° (km) 27/09/2014:11:50−31/10/20142,400 0 1 Misfit angles 3,000 2,200 180° Results 3,000 Focal mechanism solutions of VT earthquakes. We estimated 35.89° the focal mechanism solutions of 94 VT earthquakes in the 2,600 90° summit region (August 2014 to March 2015) from S:P amplitude 2,800 35.88° 2,400 ratios and P wave polarity data obtained through the dense 2,600 21 seismic networks (Fig. 1a) using the HASH software package .In 0° the analysis, we assumed that the source was double couple (DC) 137.44° 137.46° 137.48° 137.50° and considered three velocity models (Supplementary Fig. 1) to Figure 2 | Temporal changes in focal mechanism solutions. account for possible errors in the hypocentres and take-off angles (a) Classification of focal mechanism solutions22.