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Analyzing the continuous volcanic tremors detected during the 2015 phreatic eruption of the Hakone volcano
© The Author(s) 2017
- Received: 12 July 2017
- Accepted: 27 November 2017
- Published: 6 December 2017
- Volcanic tremor
- Phreatic eruption
- Duration-amplitude distribution
- Hakone volcano
A volcanic tremor is a continuous seismic signal that lasts minutes to days in duration and is observed during volcanic eruptions or sometimes independently. Most volcanic tremors are represented in a restricted frequency range of 1–9 Hz and with a wide variety of emerging patterns (McNutt 1992). For example, the volcanic tremor prior to the magma eruption at Sakurajima, western Japan (Kamo et al. 1977), represented a clear series of spectral peaks within 1–10 Hz that changed with time, suggesting a resonance of the conduit and temporal changes of the properties of the material within it. On the other hand, volcanic tremors with a period near 15 s were also observed at the Aso volcano; this was related to the resonance of a shallow crack (e.g., Kawakatsu et al. 2000). Aki and Koyanagi (1981) reported that a continuous harmonic tremor under Kilauea, Hawaii, varied with time in relation to the flow of magmatic fluid in a deep magma source. Volcanic tremors were also observed during a phreatic eruption (e.g., Ogiso et al. 2015). The duration-amplitude distribution of volcanic tremors is generally described well by an exponential function (e.g., Benoit et al. 2003; Chardot et al. 2015) rather than the power law scaling reported for an ordinal earthquake.
Several generation mechanisms have been proposed for volcanic tremors, although a physical understanding of their origins has been elusive. Julian (1994) proposed a model in which oscillations of the channel were excited by a nonlinear process that occurred when magmatic fluid flowed through it. Chouet (1988) demonstrated that resonance induced in a fluid-filled crack by an impulsive pressure transient could explain many of the observed characteristics associated with long-period events and harmonic tremors. A magma-wagging oscillation against the restoring gas-spring force of the annulus around the magma column was modeled for volcanic tremors during a magma eruption (Jellinek and Bercovici 2011). The growth and collapse of bubbles as groundwater boils is thought to be a reasonable mechanism for harmonic tremors at geysers (Leet 1988). However, volcanic tremors accompanied by a phreatic eruption remain poorly understood. The installation of a dense network of seismic stations near the source of the signal is essential. A small phreatic eruption of the Hakone volcano provided an opportunity to address this issue, because seismic data were measured through such a dense network of stations that were installed near the eruption vents.
An abrupt increase in the number of VT earthquakes occurred in the morning of June 29, 2015 (07:32 on June 29, 2015, JST) (Fig. 2). From 07:32 to 07:34, tilt meters and broadband seismometers around the Owakudani geothermal region detected tilt changes of approximately 10 μ rad (Fig. 1) (Honda et al. 2015). Honda et al. (2015) concluded that these tilt changes could be explained, assuming that a shallow open-crack source approximately 5 cm in dilation and oriented NW–SE had expanded to an elevation of approximately 850 m (i.e., several hundred meters below the surface). The open-crack source was also estimated from surface deformations detected by InSAR (Doke et al. 2015). Mannen et al. (2015) reported ash fall in the Owakudani geothermal region around 12:00 on June 29, and several vents were formed there from the afternoon of June 29 until the early morning of July 1. During this phreatic eruption, only 100 tons of altered material that developed near the surface of the steaming area was released (Nagai et al. 2015). The total mass discharged during the eruption was less than 1% of that discharged from the 2014 phreatic eruption of the Ontake volcano, Japan, as estimated by Takarada et al. (2016).
In the present study, we conducted a detailed investigation of the characteristics of the continuous volcanic tremor observed from around 11:00 on June 29 until the early morning of July 1, 2015, including its frequency content, temporal variations in its amplitude, duration-amplitude distribution, and source locations. By comparing these results with other observations, such as the infrasonic waves and vent formation timings, we discuss the generation mechanism of volcanic tremors.
Twelve permanent seismic stations are operated by the Hot Springs Research Institute, the National Research Institute for Earth Science and Disaster Resilience Hi-net, and the Japan Meteorological Agency (Fig. 1) in and around the caldera of Hakone volcano. Seven of these are borehole-type short-period (1 Hz) seismometers equipped with tilt meters, and the other four are short-period (1 Hz) seismometers on the surface of the ground. In addition to these permanent stations, we set up seven portable seismic stations, including four short-period seismometers (2 Hz) and three broadband (120 s) seismometers, prior to the eruption. All the stations took recordings continuously at a sample rate of 200 Hz for the short-period seismometers and 100 Hz for the broadband seismometers. Consequently, a dense seismic network, including four short-period seismometers and one broadband seismometer (Fig. 1b), developed within 1 km of the vents prior to the eruption. Detailed information on the hypocenter distribution of VT earthquakes (Fig. 2) was obtained based on the seismic data using the double difference (DD) method (Waldhauser and Ellsworth 2000) and the three-dimensional velocity structures estimated by Yukutake et al. (2015).
In the present analysis, the seismic waveform data from 13 stations located within 5 km of the vents, which represent a high signal-to-noise ratio, were used (Fig. 6) after the frequency response of the seismometer was removed. The root-mean-square (RMS) envelope was calculated from the three-component velocity waveforms 1 min in duration. Then, considering the frequency content of the volcanic tremor, we applied a band-pass filter between 2 and 6 Hz and used a 2-s sliding time window to calculate the RMS amplitude. A waveform record with a sample rate of 200 Hz was reduced to 100 Hz. We visually excluded the signals from VT earthquakes if they were contained within a 1-min waveform record. We calculated the cross-correlation coefficients of envelope seismograms across all station pairs by moving the 1-min trace with the lag time of every sampling interval. The lag time with the maximum correlation coefficient was used as the differential arrival time for the coherent signal between two stations. We used only the differential arrival time data with cross-correlations greater than or equal to 0.8.
If differential arrival times meeting the above cross-correlation threshold were obtained for at least 20 station pairs, we then tried to estimate a location for the source of the waveform envelope record. We conducted a grid search at intervals of 100 m, setting the nodes of grids within ± 2 km E–W and N–S and from − 1 to 3 km vertically, centered at the position of the vents. On the vertical axis, 0 km corresponds to sea level and − 1 km corresponds to the elevation at the Owakudani region. We calculated the synthetic arrival time from each grid node to the stations using the three-dimensional velocity structure of the Hakone volcano estimated by Yukutake et al. (2015). The pseudo-bending method (Um and Thurber 1987) was applied to calculate the travel time from a grid node to a station containing its elevation. We assumed that seismic waves detected by the envelopes propagated at S-wave velocities. We calculated the residuals at each grid node as the sum of misfits between observed and synthetic differential arrival times for all available pairs and found the best source for the location of the tremor that produced the minimum residual.
Validity of the location of the source of the tremor
The particle motion of the volcanic tremor at OWD station (Additional file 1: Figure A1) is a predominately transverse component (SH wave). A coherent phase of the volcanic tremor is propagated with apparent velocities of 1.5–2.0 km/s (Additional file 1: Figure A2). The average P-wave velocity above sea level within the Hakone caldera estimated in a seismic experiment using explosive sources is 2.76 km/s (Oda 2008), which corresponds to an S-wave velocity of 1.60 km/s, assuming a Vp/Vs ratio of 1.73. Therefore, it is reasonable to assume that the signal of the volcanic tremor was generated near the surface and propagated at S-wave velocity. This is consistent with the results of grid search (Fig. 7).
Considering the location errors estimated using the bootstrap method, the scattering of the tremor locations in an area with a diameter of approximately 500 m (Fig. 9) may be attributed to errors in the estimation of the location of the source, rather than a true spatial distribution of the source of the tremor. This is supported by the temporal changes seen in the amplitude ratio during the tremor activity (Additional file 2). When the amplitude of volcanic tremor was dominant, the seismic amplitude ratios of E.KMYB and T.OWD1 to OWD station, and E.KMYB to T.OWD1 station, converged to 0.3, 0.2, and 0.7–0.8, respectively. These results suggest that the volcanic tremor originated from a similar physical process occurring at practically the same position.
Duration-amplitude distribution of the volcanic tremor
The result showed that the exponential model appeared to fit the data better than the power law model (Fig. 10), which is a contrast to the frequency-magnitude distribution for an ordinal earthquake since the latter obeys the power law model. The power law model implies self-similarity of the source process. The exponential duration-amplitude distribution has also been observed for volcanic tremors in several other volcanoes and geothermal regions, suggesting that the tremor-generating process is scale bound (Benoit et al. 2003). Benoit et al. (2003) suggest two possibilities for a scaling bound on the amplitude of the tremor: (1) fixed-source geometry with variable forces that drive the tremor or (2) constant force and variable source geometries. The characteristics of the frequency component that are invariant with time (Fig. 5), concentrated distribution of the tremor sources near the vents (Fig. 9), and time-invariant amplitude ratios (Additional file 2) suggest that the first model is plausible for the volcanic tremor in the present study.
In Fig. 10, λ is the slope of the line or scaling parameter. The inverse of this parameter, λ −1, can be considered the characteristic amplitude of the distribution (Benoit et al. 2003). The characteristic amplitude of 0.11 cm2 for the tremor in the Hakone volcano is 100 times that of the geothermal noise in the geyser Old Faithful at Yellowstone National Park, Wyoming, in the USA. It is of the same order of magnitude as the noneruptive tremor at Mt. Spurr, Alaska, in the USA, which is interpreted as being of hydrothermal origin, and is one order of magnitude less than that observed during the sub-Plinian eruption at the same volcano. The estimated characteristic amplitude is also of the same order of magnitude as the volcanic explosion earthquakes at Stromboli (Nishimura et al. 2016, 2017), which were related to the behavior of a gas slug in the magma column (e.g., Ripepe and Gordeev 1999).
Relation to vent formation and infrasonic waves
During the eruptive activity, impulsive infrasonic waves were sometimes observed using a microphone installed at OWD station. The occurrence of these appears to accord with the increment of the tremor amplitude. For example, the volcanic tremor with the largest amplitude was observed from 03:00 to 06:00 on July 1, and pronounced impulsive infrasonic wave activity was observed from 04:00 to 06:00 on the same day (Fig. 3). The timing of the vent formation reported by Mannen et al. (2015) also appeared to coincide with the occurrence of the volcanic tremors and impulsive infrasonic waves. The largest vent [15-1 as defined by Mannen et al. (2015)] during the eruption was formed around 04:00 on July 1, based on observations with a light-sensitive monitoring camera. There is high uncertainty regarding when the vent formed, due to poor visibility around the Owakudani region during the eruption; however, the timings of two other vent formations, in the afternoon of June 29 (15-9) and around 07:00 on June 30 (15-5), are also coincident with increases in tremor amplitudes and impulsive infrasonic waves (Fig. 3). These observations indicate that tremor activity is sometimes accompanied by impulsive infrasonic waves and vent formation.
Similar seismic and infrasonic records were measured during the explosive events at Stromboli (Ripepe et al. 2001). Here the mean Δt, measured as 2.1 s, could not be explained by assuming a common source on the surface of the magma, considering the 300 m between the vent and the station. To explain the large Δt, Ripepe et al. (2001) assumed that the source processes were different, and that the seismic signal was generated by the sudden collapse of foam into a large gas bubble (a gas slug) in the magma column, whereas the infrasound signal was generated by the explosion of a gas slug at the magma surface. The foam collapse model for the generation mechanism of the tremor at Stromboli was supported by the observation of the very-long period (VLP) signals associated with the explosions (e.g., Chouet et al. 1999). On the other hand, the Δt observed at Hakone (Fig. 11) shows that seismic and infrasonic waves are generated at the same time at the surface of the vent. Moreover, we did not observe significant VLP signals exceeding microseismic noise around the infrasonic onsets at the E.KMYB broadband station. Therefore, it is reasonable to consider that the impulsive infrasonic wave and large amplitude seismic signal observed before the infrasonic onset (Fig. 12) were generated by a gas slug bursting at the surface of the vent. The generation of an impulsive infrasonic wave due to a shallow explosive source has been demonstrated by underwater explosion experiments (Ichihara et al. 2009). A similar model was proposed for the tremor activity at White Island, New Zealand (Jolly et al. 2016), in which the dynamic behavior of a gas slug near a surface vent was related to the occurrence of a tremor and infrasonic signals. The high-frequency (> 6 Hz) signal coincident with the infrasonic onset (Fig. 12c) was thought to be produced by a seismic source coupled to the atmosphere, as indicated in Ripepe et al. (2001).
Candidate mechanism for exciting the tremor
Through the analyses in the present study, we obtained the following results associated with the volcanic tremor: (1) the tremor sources were determined to be near the vents and the surface, (2) the frequency component shows a broad peak within 1–6 Hz that does not vary with time and is independent of the amplitude, (3) the duration-amplitude distribution obeys the exponential scaling law, suggesting a scale-bound source process, (4) the seismic signals prior to the infrasound onsets are related to a gas slug bursting at the surface of the vent, (5) the frequency component with the volcanic tremors during the acoustic quiet period is generally consistent with that of the seismic signal prior to the infrasonic onset, suggesting that the generation mechanism for both seismic signals is basically the same, and (6) the infrasonic waves were generated when the tremor signal increased. Since the source of the volcanic tremor is located around the upper extent of the open crack (Fig. 9) as estimated by Honda et al. (2015), and the signal emerged after the tilt changed (Fig. 3), the tremor activity is also thought to be significantly related to the crack opening.
We investigated, in detail, the characteristics of a volcanic tremor observed 2 days after the phreatic eruption at the Hakone volcano, in the Owakudani geothermal region of central Japan. The frequency component of the volcanic tremor is represented by a broad peak within 1–6 Hz. The characteristics of the frequency component do not vary with time and are independent of the amplitude of the tremor. We estimated the location of the source of the tremor using the envelope correlation method and found that it occurred near eruption vents and at the surface. The duration-amplitude distribution of the volcanic tremor is consistent with the exponential scaling law rather than the power law, suggesting a scale-bound source process. Considering the time-invariant frequency component, concentrated distribution of the tremor source, and time-invariant amplitude ratios, the scaling bound on the amplitude of the tremor suggests a fixed-source geometry with variable forces that drive the tremor. The volcanic tremor likely originated from similar physical processes occurring in practically the same place. The signal of the tremor was sometimes coincident with the occurrence of impulsive infrasonic waves and vent formations. The bursting of a gas slug at the surface of the vent may be a reasonable model with which to explain the generation mechanism of volcanic tremors and the occurrence of impulsive infrasonic signals. The tremor signal is closely correlated to eruptive activity and a useful indicator to assess the status of a hydrothermal system during an eruption.
YY analyzed the data and wrote the manuscript. RH, TU, TS, and SS carried out seismic observations and helped draft the manuscript. RD and MH carried out seismic observations and processed the data. YM assisted with the data interpretation. All authors read and approved the final manuscript.
We used waveform data from the seismic stations of the National Research Institute for Earth Science and Disaster Resilience, the Japan Meteorological Agency, and the University of Tokyo. We used data obtained from the microphone installed by the Japan Meteorological Agency. We would like to thank Dr. Mie Ichihara for her important suggestions relating to tremor and infrasonic source processes. Dr. Akihiko Terada provided useful comments on geothermal activity during a phreatic eruption. We also thank Dr. Yuki Abe for helpful comments. Yoshiko Teguri supported the data processing for the impulsive infrasonic waves. The code used to calculate the spectrogram was provided by Dr. Yorihiko Osaki. We are thankful to two anonymous reviewers and Editor Diana Roman who helped us to greatly improve this manuscript.
The authors declare that they have no competing interests.
Availability of data and materials
The data that support the findings of this study are available on request from the corresponding author.
This study was supported by a Grant-in-Aid for Young Scientists (B) (JSPS KAKENHI No. 15K17755).
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