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Broadband seismic signals associated with the 2000 volcanic unrest of Mount Bandai, northeastern Japan
Journal of Volcanology and Geothermal Research 119 (2002) 51^59 www.elsevier.com/locate/jvolgeores
Broadband seismic signals associated with the 2000 volcanic unrest of Mount Bandai, northeastern Japan Takeshi Nishimura , Sadato Ueki, Teruo Yamawaki, Satoru Tanaka, Hironori Hashino, Minemori Sato, Haruhisa Nakamichi 1 , Hiroyuki Hamaguchi Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan Received 19 November 2001; received in revised form 11 March 2002; accepted 11 March 2002
Abstract We have observed volcano-tectonic (VT) earthquakes, volcanic tremor and very long-period events (VLPEs) associated with the volcanic unrest of Mount Bandai since 2000 using a broadband seismic network closely deployed around the volcano. VT earthquakes are characterized by high-frequency ( s 10 Hz) signals with clear onsets of P and S phases, whereas volcanic tremor is characterized by a long coda comprising frequencies ranging from a few hertz to more than 10 Hz. In contrast, waveforms of VLPEs consist of very long-period (V10 s) signals preceded by shortperiod ( 6 1 s) components. Hypocenters of the VT earthquakes are concentrated in two clusters at depths of 0^2 km beneath the summit of Mount Bandai and to the northwest of the volcano. Conversely, the hypocenters of the volcanic tremor and the source of the short-period signals associated with VLPEs are located in the gap between the two clusters of VT earthquakes. Since this gap lies near a low-resistivity zone detected by magnetotelluric soundings, the source mechanisms of the tremor and short-period VLPE precursors are considered to be related to magmatic fluids consisting of hot water and/or magma. The short-period source of VLPEs is inferred to trigger inflation and deflation at a depth of about 5 km beneath the volcano’s north-northwestern flank and to thereby excite the subsequent very long period signals. Our analysis of these broadband seismic signals clarifies the recent activity of magmatic fluid beneath Mount Bandai, a volcano previously quiescent for more than 110 years. : 2002 Elsevier Science B.V. All rights reserved. Keywords: Mount Bandai; volcano-tectonic earthquakes; volcanic tremor; very long period events; broadband
1. Introduction
1
Present address: Earthquake Research Institute, Tokyo University, Yayoi 1-1-1, Bunkyo-ku, Tokyo, Japan. * Corresponding author. Tel.: +81-22-225-1950; Fax: +81-22-264-3292. E-mail address: [email protected] (T. Nishimura).
Mount Bandai, a 1816-m-high stratovolcano located in the center of northeastern Japan, is well known for its catastrophic eruption of 1888. On 15 July 1888, this eruption commenced with a phreatic explosion at the summit of Ko-Bandai, after which eruptive activity consisting of two
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large explosions, a blast and a debris avalanche continued for 2^3 hours (Sekiya and Kikuchi, 1889; Yonechi, 1995). During the eruption, a horseshoe-shaped crater with a radius of approximately 1.5 km formed on the northern £ank of the volcano. In spite of such a big eruption occurring (volcano explosive index: 4), there is no evidence of magmatic emissions from the volcano. Seismic observations close to the volcano were begun in 1963 by the Japan Meteorological Agency (JMA). In 1993, Tohoku University also began monitoring the volcano using a three-component short-period seismometer, a broadband seismometer (STS-2), and a 3-m-long water-tube tiltmeter. These instruments are installed in a horizontal vault with a length of 50 m at the permanent station BND, about 3 km southwest of the summit of Mount Bandai. Seismic activity beneath Mount Bandai was low until 2000 with the exception of a seismic swarm in 1988 (Tohoku University, 1989). On 26 April 2000, a magnitude 4.7 earthquake occurred in a region about 5 km westward of the volcano’s summit, and the number of volcano-tectonic (VT) earthquakes near the summit gradually increased. Two weeks later, on 10 May 2000, the ¢rst episode of signi¢cant volcanic tremor was observed. Between April and August 2000, VT earthquakes occurred continually, sometimes as swarms associated with volcanic tremor (Fig. 1). On the 15th and 16th of August, the daily number of VT earthquakes exceeded 100 and two large earthquakes with magnitudes of 2.0 and 2.4 occurred at shallow depths near the summit. In response, JMA made a public anouncement about the possibility of a small phreatic eruption occurring near the summit. Since September 2000, seismic activity has decreased but remains at a high level relative to that prior to April 2000. In spite of this level of seismic activity, no eruptions have been reported to date (February 2002), and no signi¢cant crustal deformation has been detected by GPS networks deployed around the volcano or by the 3-m water-tube tiltmeter at BND. To clarify the seismic and volcanic activity of Mount Bandai, we deployed three-component short-period seismometers at stations URB, BWS and AKH on 7 June 2000 and broadband
Fig. 1. (a) Hourly numbers of VT earthquakes recorded at station BND with a peak-to-peak velocity amplitude of more than 0.5 Wm/s and an S^P interval of 6 1.5 s. The ¢ne line represents the cumulative number of the earthquakes. (b) Temporal changes in tremor (open diamonds) and VLPE activity (shaded diamonds). The horizontal and vertical size of each diamond represent the duration and maximum of the tremor or VLPEs, respectively, recorded by the vertical component of the short period (1 s) seismometer at BND station. The beginning and end of the tremor and VLPEs are measured with the eye on the seismograms routinely printed on papers.
seismometers (STS-2, Streckeisen) at station SAZ on 3 August 2000, at TBT, BWS on 6 September 2000, at URB on 5 February 2001, and a longperiod seismometer (LE-3D/20s, Lenartz) at TYZ on 25 May 2001 (Fig. 3a). Signals from these instruments are digitized with a sampling frequency of either 100 Hz or 50 Hz and an A/D resolution of 12^24 bits, and are continuously stored on hard disks. Since installing these seismometers, we have recorded several types of broadband seismic signals beneath the volcano, in particular VT earthquakes, volcanic tremor and very long period events (VLPEs). In this study, we investigate the waveform characteristics of these events and determine their source locations to elucidate the recent volcanic activity of Mount Bandai.
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Fig. 2. Examples of seismograms recorded by the vertical component of the short-period seismometer at BND. (a) A VT earthquake. (b and c) Tremor containing high-frequency content (10^30 Hz). (d) Tremor with a high-frequency onset followed by a low-frequency (2^10 Hz) coda. (e) Tremor with a low-frequency onset followed by a low-frequency coda. Each seismogram is normalized by the peak-to-peak maximum amplitude that is shown in the right side of each trace.
2. VT earthquakes and volcanic tremor Fig. 2a illustrates the seismogram from a VT earthquake recorded at BND station. Distinct P and S phases with a high-frequency content of 10^ 30 Hz are recognizable. VT earthquakes like this are probably generated by faulting processes in the shallow part of the volcano (e.g. McNutt, 1996). Examples of the waveforms of volcanic tremor observed at the same station are shown
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in Fig. 2b^e. These events are characterized by a long coda wave with a duration of more than 5 s, and may be classi¢ed into three types based on their dominant frequencies and waveform characteristics. The ¢rst type of tremor (Figs. 2b and 2c) exhibits high-frequency spectra (10^30 Hz). The second type is tremor having a high-frequency onset followed by a low-frequency (2^10 Hz) coda (Fig. 2d), and the third has a relatively low-frequency onset followed by a low-frequency (2^6 Hz) coda (Fig. 2e). As is often observed at other active volcanoes, we observe tremor whose waveform characteristics are intermediate between two of the three types described above and seismic events consisting of successive occurrences of volcanic tremor. Volcanic tremor sometimes exhibits clear P and S phases at the beginnings of the seismogram, and both compressional and dilatational P wave ¢rst motions are observed at di¡erent seismic stations for individual events. Hence, the tremor is not generated by a simple volumetric (in£ationary or de£ationary) source. Using P and S onset times for the VT earthquakes and volcanic tremor, we can determine their hypocenters. We assume a one-dimensional (1-D) velocity structure based on the three-dimensional (3-D) model determined by the National University Team of Japan in 1997 (Yamawaki, 1999), which includes a low velocity surface layer of 2.5^5.5 km/s for P waves and 1.4^3.2 km/s for S waves in the uppermost 2 km. Fig. 3 shows the hypocenter distribution of the VT earthquakes for which more than four P phase onsets and two S phase onsets were accurately measurable. Relative uncertainties in the calculated hypocenters are estimated to be O 0.3 km in average and to be less than O 0.5 km in maximum. The hypocenters of most of the VT earthquakes are distributed at depths of 0^2 km below sea level in two clusters: one is around the summit area (hereafter referred to as the ‘summit cluster’) and the other is located beneath the northwestern region of the summit (‘NW cluster’). The two clusters occurred simultaneously and no obvious temporal or spatial changes of the hypocenters have been identi¢ed. The NW cluster appears to be somewhat shallower than the summit cluster but preliminary results of hypocenter determinations using the 3-D
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velocity model, which takes into account a highvelocity zone beneath the summit, do not show such a di¡erence (Yamawaki et al., 2000). Additionally, the preliminary 3-D results suggest that the epicenters determined using the 1-D structure are o¡set to the west by 500^1000 m. In Fig. 3, the hypocenters of the volcanic tremor are indicated by solid diamonds. It is clear that the volcanic tremor events are located between the two clusters of VT earthquakes at a depth of 0.1^ 2.1 km. We have not observed any systematic
variation in location of the three di¡erent types of volcanic tremor, perhaps because of the limited resolution of the hypocenter determinations.
3. VLPEs The tremor observed on 10 May 2000 included VLP signals with a period of about 10 s (0.1 Hz) as well as short-period waves of less than 1 s ( s 1 Hz). Between May 2000 and May 2001, this kind of tremor was recorded at station BND with a good signal-to-noise ratio six times. VLPEs are characterized by strong short-period signals followed by very long-period signals that oscillate several times (Fig. 4). The short-period signals of the VLPEs occurring in 2000 often show emergent onsets and indistinct S phases, but some of the VLPEs observed in 2001 have clear onsets of both P and S phases. Hence, we have been able to determine the hypocenters of VLPEs using the same algorithm as for VT earthquakes and tremor. In Fig. 3b, the hypocenters of the VLPEs are represented by shaded star marks. These events are located in the same source region as those of the volcanic tremor; that is, in the gap between the two VT clusters. This suggests that the source process of the short-period signals is somehow similar to those of the volcanic tremor. It is noted that precursory signals are also observed before the short-period signals (see Fig.
Fig. 3. (a) Locations of the seismic stations. Plus, open square and solid square symbols represent the stations equipped with short-period seismometers, broadband seismometers (STS-2) and long-period seismometers (LE-3E/20s), respectively. The hatchured line north of the summit (solid triangle) indicates the horseshoe-shaped crater that formed during the 1888 eruption. The contour interval is 100 m. The inset shows the location of Mount Bandai in northeastern Japan. (b) Hypocenter distributions of VT earthquakes (open circles), tremor (solid diamonds) and the short-period source of VLPEs on 30 January 2001 and 30 May 2001 (stars) near the summit of Mount Bandai. The dotted curve in the vertical cross-section delineates a low-resistivity region observed during a magnetotelluric survey along an east^west pro¢le 500 m north of the summit (after Inoue et al., 1995). Note that the epicenters calculated with a 3-D seismic velocity model lie 500 m to the east of those illustrated here.
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4), but that their source locations and excitation mechanisms are unclear because of their small amplitudes and ambiguous phases. The very long-period components of the VLPEs are distinguished by linear motions in the horizontal plane and by elliptical particle trajectories in vertical sections (Fig. 5a) at all of the stations. As illustrated in Fig. 5b, the major axes of the trajectories projected onto the horizontal plane intersect at one point (indicated by the large star). Hence, we have presumed that the epicenters of the very long period signals lie at this intersection point, assuming that the source radiates predominantly compressive waves. The epicenter
Fig. 4. (a) Example of a VLPE velocity seismogram recorded by the vertical component of a STS-2 seismometer at station BND. The top trace shows the raw record, and the second and the third traces show the seismogram after band-pass ¢ltering from 0.05 to 0.12 Hz and 1 to 16 Hz, respectively, with zero phase shifts. The arrow in the bottom trace indicates the onset of the short-period signals that were analyzed for hypocenter determination. The earliest precursory signals have emergent onsets and unclear phases, and have not been used in determining the location. (b) The displacement (black line) and noise spectra (gray line) of the VLPE.
Fig. 5. (a) Example of particle trajectories of the VLPE on 30 May 2001. The three-component seismograms have been band-pass-¢ltered from 0.08 to 0.12 Hz. (b) Horizontal projections of the particle trajectories observed at the ¢ve stations for the event on 30 May 2001. The axis of maximum displacement at each station is represented by a gray line. Station SAZ is o¡ the map. Note that the maximum displacement axes of all four elliptical trajectories intersect at approximately a single point (large star), considered to be the epicenter of the very long-period components of the VLPEs.
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we have calculated vertical and radial displacements using the peak-to-peak amplitude of each component after band-pass ¢ltering the displacement seismograms in the 0.08^0.12 Hz band. We further presume several source depths for the Mogi model and estimate the best-¢t volume change for each depth by using the method of the least squares. Fig. 6 plots the observed displacements of the vertical and radial components and the calculated displacements for the source depths of 2.5, 5.0 and 7.5 km. From a comparison of the observed and calculated displacements, we estimate the depth of a Mogi source to be about 5 km assuming a semi-in¢nite medium with a Poisson ratio of 0.25. A source depth of 2.5 km seems to be too shallow especially for the radial component, and 7.5 km is too deep to explain the vertical components. The volume change at the 5 km source is estimated to be 240 m3 . Discrepancies between the observations and model predictions are inferred to be due to non-spherical radiation of the seismic waves from the source, ¢nite source region e¡ects (e.g. Nishimura et al., 2000), and topographic or 3-D structural e¡ects of the volcano on the waveforms. The displacement seismograms of the ¢ve VLPEs recorded at BND are quite similar in shape (Fig. 7a) and their particle trajectories do not show any clear changes between May 2000
Fig. 6. Epicentral distance versus vertical and radial displacement of the VLPEs on 30 May 2001. The curves show the vertical and radial displacements predicted for a Mogi source at a depth of 2.5, 5.0 and 7.5 km.
is about 2 km from the epicenters of the accompanying short-period signals. To estimate the depth of the VLPEs, we apply a spherical pressure source model (so-called Mogi model) for the observed displacement ¢eld of the VLPEs. Since we analyze the very long-period signals (10 s) at nearby stations (less than 10 km), we may apply the Mogi model which is often used for static deformation analyses. Assuming the epicenter to be at the location of the large star symbol in Fig. 5b,
Fig. 7. (a) Displacement seismograms of VLPEs observed at station BND. The instrument response has been removed. (b) Short period (1^10 Hz) velocity seismograms.
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and May 2001. These results suggest that the very long-period seismic waves of the VLPEs are repeatedly generated by the same process at approximately the same source location. On the other hand, the short-period signals reveal di¡erent waveforms (Fig. 7b), suggesting some changes in their source mechanisms and locations.
4. Discussion Inoue et al. (1995) detected a low-resistivity zone extending from shallow depths to approximately 4 km beneath the summit of Mount Bandai using magnetotelluric soundings. The location of this feature is close to the hypocentral region of the tremor and the short-period source of VLPEs, especially if we take into account preliminary estimates of the e¡ects of 3-D heterogeneity (Yamawaki et al., 2000). Since the low resistivity is considered to be caused by the presence of £uid (Inoue et al., 1995), we infer that the source mechanisms of the tremor and VLPEs are strongly related to magmatic £uids, principally hot water or magma. The existence of magmatic £uids beneath the summit is also suggested by the distributions of VT earthquakes’ hypocenters and tremor elsewhere. For example, at Sakurajima volcano (Iguchi, 1994), the pattern in which the source regions of long period events or isolated tremor (so-called ‘B-type’ earthquakes) are surrounded by the hypocenters of VT earthquakes (‘A-type’ earthquakes) is interpreted to indicate the presence of a volcanic conduit ¢lled with magmatic £uid. At Hawaii, the source of long-period events has been considered to coincide with magma transport systems (Koyanagi et al., 1987). Although the source mechanisms of the tremor at Mount Bandai may not be the same as those of the events at Sakurajima or Hawaii, the consistency of the spatial separation of VT earthquakes and tremor strongly suggests the existence of a £uid-¢lled volcanic conduit or chamber beneath Mount Bandai. Fig. 8 shows composite focal mechanisms determined from P wave polarities of the VT earthquakes, tremor and the short-period precursors of VLPEs. The mechanism for the summit cluster of VTs reveals a strike slip fault with an almost east^
Fig. 8. Composite P wave focal mechanisms for the (a) summit cluster of VT, (b) NW cluster of VT (c) tremor, and (d) short-period components of the VLPEs. The open and ¢lled circles represent compressional and dilatational motions, respectively. Each stereonet is an equal-area lower hemisphere projection of the focal sphere. Reliable nodal planes can be estimated for the summit cluster only.
west P axis, which is consistent with the strike slip earthquakes occurring in the shallow crust of NE Japan. On the other hand, the source mechanisms for the NW cluster are unclear, probably because of di¡erent mechanisms within the cluster. The composite source mechanisms for the tremor and the short-period signals of VLPEs are very poorly determined, and their source mechanisms cannot be reliably determined. However, since both compressional and dilatational ¢rst motions are observed at di¡erent stations for individual events, and since S phases are clearly visible in some seismograms, their source mechanisms cannot be entirely volumetric ; that is, it must involve some component of shear. The amplitudes of the very long-period components of the VLPEs are small in the transverse directions at all stations (as indicated by the horizontal projections of the particle trajectories in Fig. 5). This implies that the source of the very
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long-period signals is excited by in£ationary and de£ationary motions of a vertically elongated pipe or a spherical chamber. Such motions are also reported for VLPEs at other volcanoes, and magmatic £uid motions in chambers or cracks have been suggested as the mechanisms generating VLPEs at Mount Aso and Mount Iwate (Yamamoto et al., 1999; Nishimura et al., 2000). Since the VLPEs at these locations also consist of short period signals followed by very long-period (V10 s) codas, they are considered to be excited by quite similar processes involving magmatic £uid activity beneath the volcanoes. It is noted that the source region of the very long-period signals of VLPEs at Mount Bandai is about 3^4 km from the hypocenters of the VLPEs’ short-period components. Such a large spatial separation of the sources of volcanic tremor has also been observed at Izu Oshima Volcano (Oikawa et al., 1991), where the short-period tremor source near the crater was interpreted to generate changes in pressure that were transmitted via the vent to a magma reservoir at a depth of 4 km, causing it to expand or contract. Although quantitative and physical relations between the two separate sources are still unknown, the short period signals of VLPEs at Mount Bandai are also considered to play a role in triggering the subsequent very long-period oscillations. It is important, however, to also mention several di¡erences that exist between the Mount Bandai VLPEs and those of other volcanoes. VLPE activity at Mount Bandai has been low compared to that at Iwate and Aso. While the total number of VLPEs at Mount Iwate exceeded 800 in 1998 (Nishimura et al., 2000) and VLPE activity at Mount Aso occurs steadily (Kaneshima et al., 1996), only six VLPEs have been observed at Mount Bandai in a period of almost 1 year ^ an average of only one event every few months. Mounts Aso and Iwate also show signi¢cant crustal deformation, frequent eruptions and geothermal activity, but Mount Bandai has not shown any evidence of shallow magma intrusion. These observations may provide some constraints on the source mechanism of VLPEs at Mount Bandai. If magmatic £uid motions are considered to be the most plausible VLPE source mechanism (e.g. Nishimura et al., 2000) then to continue generat-
ing VLPEs in the same place every few months without reinjecting fresh magma requires the magma to remain liquid for at least 1 year. If magma in a cylindrical conduit with a radius of approximately 1 m is surrounded by cold rocks, it cools and solidi¢es on time scales of a few to 10 days without a new magma supply. Hence, we infer that the magmatic £uid must consist principally of hot water or that any magma has a su⁄cient heat capacity to remain liquid for more than a few days. Finally, it should be noted that the short period signals in VLPE at Mount Bandai appear to be stronger than those of either Mount Aso or Mount Iwate. Strong radiation of short period signals may be related to di¡erences in the triggering process or the mechanical conditions of the vent connecting the short period and very long period signal source regions.
5. Summary We have observed several broadband seismic signals associated with the volcanic unrest of Mount Bandai in 2000: VT earthquakes, volcanic tremor and VLPEs. The main results are as follows: (1) VT earthquakes occurred between May 2000 and May 2001 in two clusters at depths of 0^ 2 km beneath the summit and the northwestern £ank of the volcano; (2) the hypocenters of volcanic tremor were located in a gap between the two VT earthquake clusters, perhaps indicating the presence of a volcanic conduit or a chamber containing hot water or magma; (3) very longperiod codas (V0.1 Hz) of the VLPEs are concluded to have been excited by in£ation and de£ation of a volcanic pipe or spherical chamber at a depth of about 5 km beneath the north-northwestern £ank of the volcano. The short-period precursory signals ( s 1 Hz) of the VLP events, which are generated in the same region as the volcanic tremor, play a role in triggering such in£ation and de£ation. Mount Bandai has not exhibited any detectable crustal deformation or eruptive activity during the period of observation, and the observed seismic activity is therefore concluded to represent magmatic £uid motion beneath the volcano.
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Acknowledgements We are indebted to the Alts Bandai and Urabandai ski ¢elds, Biwasawa Campground, the town o⁄ces of Inawashiro and Bandai, Department of Geography of Tohoku University and all members of the Research Center for Prediction of Earthquakes and Volcanic Eruptions for their kind cooperation during the deployment of our temporary seismic network. This research was partially supported by the National Project for Prediction of Volcanic Eruptions and a Grant for Scienti¢c Research from the Japanese Ministry of Education, Culture, Sports, Science and Technology (11640404). References Iguchi, M., 1994. A vertical expansion source model for the mechanisms of earthquakes originated in the magma conduit of an andesitic volcano: Sakurajima, Japan. Bull. Volcanol. Soc. Jpn. 39, 49^67. Inoue, J., Kawakami, N., Takasugi, S., Tanaka, K., Takeuchi, M., 1995. Resistivity structure beneath the Bandai volcano by the magnetotelluric soundings. In: Bandai Volcano ^ Recent Progress on Hazard Prevention. National Research Institute for Earth Science and Disaster Prevention, pp. 31^ 41. Kaneshima, S., Kawakatsu, H., Matsubayashi, H., Sudo, Y., Tsutsui, T., Ohminato, T., Ito, H., Uhira, K., Yamasato, H., Oikawa, J., Takeo, M., Iidaka, T., 1996. Mechanism of phreatic eruptions at Aso Volcano inferred from near-¢eld broadband seismic observations. Science 273, 642^645. Koyanagi, R.Y., Chouet, B., Aki, K., 1987. Origin of volcanic
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tremor in Hawaii, Part I: Data from the Hawaiian Volcano Observatory 1969^1985. Volcanism in Hawaii. US Geological Survey Professional Paper 1350, 1221^1257. McNutt, S.R., 1996. Seismic monitoring and eruption forecasting of volcanoes: A review of the state-of-art and case histories. In: Monitoring and Mitigation of Volcano Hazards. Springer, Berlin, pp. 99^146. Nishimura, T., Nakamichi, H., Tanaka, S., Sato, M., Kobayashi, T., Ueki, S., Hamaguchi, H., Ohtake, M., Sato, H., 2000. Source process of very long period seismic events associated with the 1998 activity of Iwate Volcano, northeastern Japan. J. Geophys. Res. 105, 19135^19147. Oikawa, J., Ida, Y., Yamaoka, K., Watanabe, H., Fukuyama, E., Sato, K., 1991. Ground deformation associated with volcanic tremor at Izu-Oshima volcano. Geophys. Res. Lett. 18, 443^446. Sekiya, K., Kikuchi, Y., 1889. The eruption of Bandai-san. J. Coll. Sci. Imp. Univ. Jpn. 3, 91^172. Tohoku University, 1989. Recent seismic activity around Mount Bandai from October 1897 to January 1989. Rep. Coord. Comm. Volcan. Predict. 43, 52^55. Yamamoto, M., Kawakatsu, H., Kaneshima, S., Mori, T., Tutsui, T., Sudo, Y., Morita, Y., 1999. Detection of a crack-like conduit beneath the active crater at Aso volcano, Japan. Geophys. Res. Lett. 26, 3677^3680. Yamawaki, T., 1999. Three-dimensional Shallow Velocity Structure of Bandai Volcano Inferred from Explosion Experiment. M.Sc. Thesis, Tohoku University, Tohoku, 85 pp. Yamawaki, T., Nishimura, T., Ueki, S., Tanaka, S., Hamaguchi, H., 2000. Hypocenter determination at Bandai volcano by using three-dimensional velocity structure from active seismic experiment. Abstract, 2000 Fall Meeting of the Seismological Society of Japan, p. 71. Yonechi, F., 1995. Reconstruction of the scenario of the eruption and the old landform of Mt. Bandai interpreted from the records by local people. Bandai Volcano ^ Recent Progress on Hazard Prevention. National Research Institute for Earth Science and Disaster Prevention, Japan, pp. 181^188.
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Broadband seismic signals associated with the 2000 volcanic unrest of Mount Bandai, northeastern Japan Takeshi Nishimura , Sadato Ueki, Teruo Yamawaki, Satoru Tanaka, Hironori Hashino, Minemori Sato, Haruhisa Nakamichi 1 , Hiroyuki Hamaguchi Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan Received 19 November 2001; received in revised form 11 March 2002; accepted 11 March 2002
Abstract We have observed volcano-tectonic (VT) earthquakes, volcanic tremor and very long-period events (VLPEs) associated with the volcanic unrest of Mount Bandai since 2000 using a broadband seismic network closely deployed around the volcano. VT earthquakes are characterized by high-frequency ( s 10 Hz) signals with clear onsets of P and S phases, whereas volcanic tremor is characterized by a long coda comprising frequencies ranging from a few hertz to more than 10 Hz. In contrast, waveforms of VLPEs consist of very long-period (V10 s) signals preceded by shortperiod ( 6 1 s) components. Hypocenters of the VT earthquakes are concentrated in two clusters at depths of 0^2 km beneath the summit of Mount Bandai and to the northwest of the volcano. Conversely, the hypocenters of the volcanic tremor and the source of the short-period signals associated with VLPEs are located in the gap between the two clusters of VT earthquakes. Since this gap lies near a low-resistivity zone detected by magnetotelluric soundings, the source mechanisms of the tremor and short-period VLPE precursors are considered to be related to magmatic fluids consisting of hot water and/or magma. The short-period source of VLPEs is inferred to trigger inflation and deflation at a depth of about 5 km beneath the volcano’s north-northwestern flank and to thereby excite the subsequent very long period signals. Our analysis of these broadband seismic signals clarifies the recent activity of magmatic fluid beneath Mount Bandai, a volcano previously quiescent for more than 110 years. : 2002 Elsevier Science B.V. All rights reserved. Keywords: Mount Bandai; volcano-tectonic earthquakes; volcanic tremor; very long period events; broadband
1. Introduction
1
Present address: Earthquake Research Institute, Tokyo University, Yayoi 1-1-1, Bunkyo-ku, Tokyo, Japan. * Corresponding author. Tel.: +81-22-225-1950; Fax: +81-22-264-3292. E-mail address: [email protected] (T. Nishimura).
Mount Bandai, a 1816-m-high stratovolcano located in the center of northeastern Japan, is well known for its catastrophic eruption of 1888. On 15 July 1888, this eruption commenced with a phreatic explosion at the summit of Ko-Bandai, after which eruptive activity consisting of two
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large explosions, a blast and a debris avalanche continued for 2^3 hours (Sekiya and Kikuchi, 1889; Yonechi, 1995). During the eruption, a horseshoe-shaped crater with a radius of approximately 1.5 km formed on the northern £ank of the volcano. In spite of such a big eruption occurring (volcano explosive index: 4), there is no evidence of magmatic emissions from the volcano. Seismic observations close to the volcano were begun in 1963 by the Japan Meteorological Agency (JMA). In 1993, Tohoku University also began monitoring the volcano using a three-component short-period seismometer, a broadband seismometer (STS-2), and a 3-m-long water-tube tiltmeter. These instruments are installed in a horizontal vault with a length of 50 m at the permanent station BND, about 3 km southwest of the summit of Mount Bandai. Seismic activity beneath Mount Bandai was low until 2000 with the exception of a seismic swarm in 1988 (Tohoku University, 1989). On 26 April 2000, a magnitude 4.7 earthquake occurred in a region about 5 km westward of the volcano’s summit, and the number of volcano-tectonic (VT) earthquakes near the summit gradually increased. Two weeks later, on 10 May 2000, the ¢rst episode of signi¢cant volcanic tremor was observed. Between April and August 2000, VT earthquakes occurred continually, sometimes as swarms associated with volcanic tremor (Fig. 1). On the 15th and 16th of August, the daily number of VT earthquakes exceeded 100 and two large earthquakes with magnitudes of 2.0 and 2.4 occurred at shallow depths near the summit. In response, JMA made a public anouncement about the possibility of a small phreatic eruption occurring near the summit. Since September 2000, seismic activity has decreased but remains at a high level relative to that prior to April 2000. In spite of this level of seismic activity, no eruptions have been reported to date (February 2002), and no signi¢cant crustal deformation has been detected by GPS networks deployed around the volcano or by the 3-m water-tube tiltmeter at BND. To clarify the seismic and volcanic activity of Mount Bandai, we deployed three-component short-period seismometers at stations URB, BWS and AKH on 7 June 2000 and broadband
Fig. 1. (a) Hourly numbers of VT earthquakes recorded at station BND with a peak-to-peak velocity amplitude of more than 0.5 Wm/s and an S^P interval of 6 1.5 s. The ¢ne line represents the cumulative number of the earthquakes. (b) Temporal changes in tremor (open diamonds) and VLPE activity (shaded diamonds). The horizontal and vertical size of each diamond represent the duration and maximum of the tremor or VLPEs, respectively, recorded by the vertical component of the short period (1 s) seismometer at BND station. The beginning and end of the tremor and VLPEs are measured with the eye on the seismograms routinely printed on papers.
seismometers (STS-2, Streckeisen) at station SAZ on 3 August 2000, at TBT, BWS on 6 September 2000, at URB on 5 February 2001, and a longperiod seismometer (LE-3D/20s, Lenartz) at TYZ on 25 May 2001 (Fig. 3a). Signals from these instruments are digitized with a sampling frequency of either 100 Hz or 50 Hz and an A/D resolution of 12^24 bits, and are continuously stored on hard disks. Since installing these seismometers, we have recorded several types of broadband seismic signals beneath the volcano, in particular VT earthquakes, volcanic tremor and very long period events (VLPEs). In this study, we investigate the waveform characteristics of these events and determine their source locations to elucidate the recent volcanic activity of Mount Bandai.
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Fig. 2. Examples of seismograms recorded by the vertical component of the short-period seismometer at BND. (a) A VT earthquake. (b and c) Tremor containing high-frequency content (10^30 Hz). (d) Tremor with a high-frequency onset followed by a low-frequency (2^10 Hz) coda. (e) Tremor with a low-frequency onset followed by a low-frequency coda. Each seismogram is normalized by the peak-to-peak maximum amplitude that is shown in the right side of each trace.
2. VT earthquakes and volcanic tremor Fig. 2a illustrates the seismogram from a VT earthquake recorded at BND station. Distinct P and S phases with a high-frequency content of 10^ 30 Hz are recognizable. VT earthquakes like this are probably generated by faulting processes in the shallow part of the volcano (e.g. McNutt, 1996). Examples of the waveforms of volcanic tremor observed at the same station are shown
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in Fig. 2b^e. These events are characterized by a long coda wave with a duration of more than 5 s, and may be classi¢ed into three types based on their dominant frequencies and waveform characteristics. The ¢rst type of tremor (Figs. 2b and 2c) exhibits high-frequency spectra (10^30 Hz). The second type is tremor having a high-frequency onset followed by a low-frequency (2^10 Hz) coda (Fig. 2d), and the third has a relatively low-frequency onset followed by a low-frequency (2^6 Hz) coda (Fig. 2e). As is often observed at other active volcanoes, we observe tremor whose waveform characteristics are intermediate between two of the three types described above and seismic events consisting of successive occurrences of volcanic tremor. Volcanic tremor sometimes exhibits clear P and S phases at the beginnings of the seismogram, and both compressional and dilatational P wave ¢rst motions are observed at di¡erent seismic stations for individual events. Hence, the tremor is not generated by a simple volumetric (in£ationary or de£ationary) source. Using P and S onset times for the VT earthquakes and volcanic tremor, we can determine their hypocenters. We assume a one-dimensional (1-D) velocity structure based on the three-dimensional (3-D) model determined by the National University Team of Japan in 1997 (Yamawaki, 1999), which includes a low velocity surface layer of 2.5^5.5 km/s for P waves and 1.4^3.2 km/s for S waves in the uppermost 2 km. Fig. 3 shows the hypocenter distribution of the VT earthquakes for which more than four P phase onsets and two S phase onsets were accurately measurable. Relative uncertainties in the calculated hypocenters are estimated to be O 0.3 km in average and to be less than O 0.5 km in maximum. The hypocenters of most of the VT earthquakes are distributed at depths of 0^2 km below sea level in two clusters: one is around the summit area (hereafter referred to as the ‘summit cluster’) and the other is located beneath the northwestern region of the summit (‘NW cluster’). The two clusters occurred simultaneously and no obvious temporal or spatial changes of the hypocenters have been identi¢ed. The NW cluster appears to be somewhat shallower than the summit cluster but preliminary results of hypocenter determinations using the 3-D
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velocity model, which takes into account a highvelocity zone beneath the summit, do not show such a di¡erence (Yamawaki et al., 2000). Additionally, the preliminary 3-D results suggest that the epicenters determined using the 1-D structure are o¡set to the west by 500^1000 m. In Fig. 3, the hypocenters of the volcanic tremor are indicated by solid diamonds. It is clear that the volcanic tremor events are located between the two clusters of VT earthquakes at a depth of 0.1^ 2.1 km. We have not observed any systematic
variation in location of the three di¡erent types of volcanic tremor, perhaps because of the limited resolution of the hypocenter determinations.
3. VLPEs The tremor observed on 10 May 2000 included VLP signals with a period of about 10 s (0.1 Hz) as well as short-period waves of less than 1 s ( s 1 Hz). Between May 2000 and May 2001, this kind of tremor was recorded at station BND with a good signal-to-noise ratio six times. VLPEs are characterized by strong short-period signals followed by very long-period signals that oscillate several times (Fig. 4). The short-period signals of the VLPEs occurring in 2000 often show emergent onsets and indistinct S phases, but some of the VLPEs observed in 2001 have clear onsets of both P and S phases. Hence, we have been able to determine the hypocenters of VLPEs using the same algorithm as for VT earthquakes and tremor. In Fig. 3b, the hypocenters of the VLPEs are represented by shaded star marks. These events are located in the same source region as those of the volcanic tremor; that is, in the gap between the two VT clusters. This suggests that the source process of the short-period signals is somehow similar to those of the volcanic tremor. It is noted that precursory signals are also observed before the short-period signals (see Fig.
Fig. 3. (a) Locations of the seismic stations. Plus, open square and solid square symbols represent the stations equipped with short-period seismometers, broadband seismometers (STS-2) and long-period seismometers (LE-3E/20s), respectively. The hatchured line north of the summit (solid triangle) indicates the horseshoe-shaped crater that formed during the 1888 eruption. The contour interval is 100 m. The inset shows the location of Mount Bandai in northeastern Japan. (b) Hypocenter distributions of VT earthquakes (open circles), tremor (solid diamonds) and the short-period source of VLPEs on 30 January 2001 and 30 May 2001 (stars) near the summit of Mount Bandai. The dotted curve in the vertical cross-section delineates a low-resistivity region observed during a magnetotelluric survey along an east^west pro¢le 500 m north of the summit (after Inoue et al., 1995). Note that the epicenters calculated with a 3-D seismic velocity model lie 500 m to the east of those illustrated here.
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4), but that their source locations and excitation mechanisms are unclear because of their small amplitudes and ambiguous phases. The very long-period components of the VLPEs are distinguished by linear motions in the horizontal plane and by elliptical particle trajectories in vertical sections (Fig. 5a) at all of the stations. As illustrated in Fig. 5b, the major axes of the trajectories projected onto the horizontal plane intersect at one point (indicated by the large star). Hence, we have presumed that the epicenters of the very long period signals lie at this intersection point, assuming that the source radiates predominantly compressive waves. The epicenter
Fig. 4. (a) Example of a VLPE velocity seismogram recorded by the vertical component of a STS-2 seismometer at station BND. The top trace shows the raw record, and the second and the third traces show the seismogram after band-pass ¢ltering from 0.05 to 0.12 Hz and 1 to 16 Hz, respectively, with zero phase shifts. The arrow in the bottom trace indicates the onset of the short-period signals that were analyzed for hypocenter determination. The earliest precursory signals have emergent onsets and unclear phases, and have not been used in determining the location. (b) The displacement (black line) and noise spectra (gray line) of the VLPE.
Fig. 5. (a) Example of particle trajectories of the VLPE on 30 May 2001. The three-component seismograms have been band-pass-¢ltered from 0.08 to 0.12 Hz. (b) Horizontal projections of the particle trajectories observed at the ¢ve stations for the event on 30 May 2001. The axis of maximum displacement at each station is represented by a gray line. Station SAZ is o¡ the map. Note that the maximum displacement axes of all four elliptical trajectories intersect at approximately a single point (large star), considered to be the epicenter of the very long-period components of the VLPEs.
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we have calculated vertical and radial displacements using the peak-to-peak amplitude of each component after band-pass ¢ltering the displacement seismograms in the 0.08^0.12 Hz band. We further presume several source depths for the Mogi model and estimate the best-¢t volume change for each depth by using the method of the least squares. Fig. 6 plots the observed displacements of the vertical and radial components and the calculated displacements for the source depths of 2.5, 5.0 and 7.5 km. From a comparison of the observed and calculated displacements, we estimate the depth of a Mogi source to be about 5 km assuming a semi-in¢nite medium with a Poisson ratio of 0.25. A source depth of 2.5 km seems to be too shallow especially for the radial component, and 7.5 km is too deep to explain the vertical components. The volume change at the 5 km source is estimated to be 240 m3 . Discrepancies between the observations and model predictions are inferred to be due to non-spherical radiation of the seismic waves from the source, ¢nite source region e¡ects (e.g. Nishimura et al., 2000), and topographic or 3-D structural e¡ects of the volcano on the waveforms. The displacement seismograms of the ¢ve VLPEs recorded at BND are quite similar in shape (Fig. 7a) and their particle trajectories do not show any clear changes between May 2000
Fig. 6. Epicentral distance versus vertical and radial displacement of the VLPEs on 30 May 2001. The curves show the vertical and radial displacements predicted for a Mogi source at a depth of 2.5, 5.0 and 7.5 km.
is about 2 km from the epicenters of the accompanying short-period signals. To estimate the depth of the VLPEs, we apply a spherical pressure source model (so-called Mogi model) for the observed displacement ¢eld of the VLPEs. Since we analyze the very long-period signals (10 s) at nearby stations (less than 10 km), we may apply the Mogi model which is often used for static deformation analyses. Assuming the epicenter to be at the location of the large star symbol in Fig. 5b,
Fig. 7. (a) Displacement seismograms of VLPEs observed at station BND. The instrument response has been removed. (b) Short period (1^10 Hz) velocity seismograms.
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and May 2001. These results suggest that the very long-period seismic waves of the VLPEs are repeatedly generated by the same process at approximately the same source location. On the other hand, the short-period signals reveal di¡erent waveforms (Fig. 7b), suggesting some changes in their source mechanisms and locations.
4. Discussion Inoue et al. (1995) detected a low-resistivity zone extending from shallow depths to approximately 4 km beneath the summit of Mount Bandai using magnetotelluric soundings. The location of this feature is close to the hypocentral region of the tremor and the short-period source of VLPEs, especially if we take into account preliminary estimates of the e¡ects of 3-D heterogeneity (Yamawaki et al., 2000). Since the low resistivity is considered to be caused by the presence of £uid (Inoue et al., 1995), we infer that the source mechanisms of the tremor and VLPEs are strongly related to magmatic £uids, principally hot water or magma. The existence of magmatic £uids beneath the summit is also suggested by the distributions of VT earthquakes’ hypocenters and tremor elsewhere. For example, at Sakurajima volcano (Iguchi, 1994), the pattern in which the source regions of long period events or isolated tremor (so-called ‘B-type’ earthquakes) are surrounded by the hypocenters of VT earthquakes (‘A-type’ earthquakes) is interpreted to indicate the presence of a volcanic conduit ¢lled with magmatic £uid. At Hawaii, the source of long-period events has been considered to coincide with magma transport systems (Koyanagi et al., 1987). Although the source mechanisms of the tremor at Mount Bandai may not be the same as those of the events at Sakurajima or Hawaii, the consistency of the spatial separation of VT earthquakes and tremor strongly suggests the existence of a £uid-¢lled volcanic conduit or chamber beneath Mount Bandai. Fig. 8 shows composite focal mechanisms determined from P wave polarities of the VT earthquakes, tremor and the short-period precursors of VLPEs. The mechanism for the summit cluster of VTs reveals a strike slip fault with an almost east^
Fig. 8. Composite P wave focal mechanisms for the (a) summit cluster of VT, (b) NW cluster of VT (c) tremor, and (d) short-period components of the VLPEs. The open and ¢lled circles represent compressional and dilatational motions, respectively. Each stereonet is an equal-area lower hemisphere projection of the focal sphere. Reliable nodal planes can be estimated for the summit cluster only.
west P axis, which is consistent with the strike slip earthquakes occurring in the shallow crust of NE Japan. On the other hand, the source mechanisms for the NW cluster are unclear, probably because of di¡erent mechanisms within the cluster. The composite source mechanisms for the tremor and the short-period signals of VLPEs are very poorly determined, and their source mechanisms cannot be reliably determined. However, since both compressional and dilatational ¢rst motions are observed at di¡erent stations for individual events, and since S phases are clearly visible in some seismograms, their source mechanisms cannot be entirely volumetric ; that is, it must involve some component of shear. The amplitudes of the very long-period components of the VLPEs are small in the transverse directions at all stations (as indicated by the horizontal projections of the particle trajectories in Fig. 5). This implies that the source of the very
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long-period signals is excited by in£ationary and de£ationary motions of a vertically elongated pipe or a spherical chamber. Such motions are also reported for VLPEs at other volcanoes, and magmatic £uid motions in chambers or cracks have been suggested as the mechanisms generating VLPEs at Mount Aso and Mount Iwate (Yamamoto et al., 1999; Nishimura et al., 2000). Since the VLPEs at these locations also consist of short period signals followed by very long-period (V10 s) codas, they are considered to be excited by quite similar processes involving magmatic £uid activity beneath the volcanoes. It is noted that the source region of the very long-period signals of VLPEs at Mount Bandai is about 3^4 km from the hypocenters of the VLPEs’ short-period components. Such a large spatial separation of the sources of volcanic tremor has also been observed at Izu Oshima Volcano (Oikawa et al., 1991), where the short-period tremor source near the crater was interpreted to generate changes in pressure that were transmitted via the vent to a magma reservoir at a depth of 4 km, causing it to expand or contract. Although quantitative and physical relations between the two separate sources are still unknown, the short period signals of VLPEs at Mount Bandai are also considered to play a role in triggering the subsequent very long-period oscillations. It is important, however, to also mention several di¡erences that exist between the Mount Bandai VLPEs and those of other volcanoes. VLPE activity at Mount Bandai has been low compared to that at Iwate and Aso. While the total number of VLPEs at Mount Iwate exceeded 800 in 1998 (Nishimura et al., 2000) and VLPE activity at Mount Aso occurs steadily (Kaneshima et al., 1996), only six VLPEs have been observed at Mount Bandai in a period of almost 1 year ^ an average of only one event every few months. Mounts Aso and Iwate also show signi¢cant crustal deformation, frequent eruptions and geothermal activity, but Mount Bandai has not shown any evidence of shallow magma intrusion. These observations may provide some constraints on the source mechanism of VLPEs at Mount Bandai. If magmatic £uid motions are considered to be the most plausible VLPE source mechanism (e.g. Nishimura et al., 2000) then to continue generat-
ing VLPEs in the same place every few months without reinjecting fresh magma requires the magma to remain liquid for at least 1 year. If magma in a cylindrical conduit with a radius of approximately 1 m is surrounded by cold rocks, it cools and solidi¢es on time scales of a few to 10 days without a new magma supply. Hence, we infer that the magmatic £uid must consist principally of hot water or that any magma has a su⁄cient heat capacity to remain liquid for more than a few days. Finally, it should be noted that the short period signals in VLPE at Mount Bandai appear to be stronger than those of either Mount Aso or Mount Iwate. Strong radiation of short period signals may be related to di¡erences in the triggering process or the mechanical conditions of the vent connecting the short period and very long period signal source regions.
5. Summary We have observed several broadband seismic signals associated with the volcanic unrest of Mount Bandai in 2000: VT earthquakes, volcanic tremor and VLPEs. The main results are as follows: (1) VT earthquakes occurred between May 2000 and May 2001 in two clusters at depths of 0^ 2 km beneath the summit and the northwestern £ank of the volcano; (2) the hypocenters of volcanic tremor were located in a gap between the two VT earthquake clusters, perhaps indicating the presence of a volcanic conduit or a chamber containing hot water or magma; (3) very longperiod codas (V0.1 Hz) of the VLPEs are concluded to have been excited by in£ation and de£ation of a volcanic pipe or spherical chamber at a depth of about 5 km beneath the north-northwestern £ank of the volcano. The short-period precursory signals ( s 1 Hz) of the VLP events, which are generated in the same region as the volcanic tremor, play a role in triggering such in£ation and de£ation. Mount Bandai has not exhibited any detectable crustal deformation or eruptive activity during the period of observation, and the observed seismic activity is therefore concluded to represent magmatic £uid motion beneath the volcano.
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Acknowledgements We are indebted to the Alts Bandai and Urabandai ski ¢elds, Biwasawa Campground, the town o⁄ces of Inawashiro and Bandai, Department of Geography of Tohoku University and all members of the Research Center for Prediction of Earthquakes and Volcanic Eruptions for their kind cooperation during the deployment of our temporary seismic network. This research was partially supported by the National Project for Prediction of Volcanic Eruptions and a Grant for Scienti¢c Research from the Japanese Ministry of Education, Culture, Sports, Science and Technology (11640404). References Iguchi, M., 1994. A vertical expansion source model for the mechanisms of earthquakes originated in the magma conduit of an andesitic volcano: Sakurajima, Japan. Bull. Volcanol. Soc. Jpn. 39, 49^67. Inoue, J., Kawakami, N., Takasugi, S., Tanaka, K., Takeuchi, M., 1995. Resistivity structure beneath the Bandai volcano by the magnetotelluric soundings. In: Bandai Volcano ^ Recent Progress on Hazard Prevention. National Research Institute for Earth Science and Disaster Prevention, pp. 31^ 41. Kaneshima, S., Kawakatsu, H., Matsubayashi, H., Sudo, Y., Tsutsui, T., Ohminato, T., Ito, H., Uhira, K., Yamasato, H., Oikawa, J., Takeo, M., Iidaka, T., 1996. Mechanism of phreatic eruptions at Aso Volcano inferred from near-¢eld broadband seismic observations. Science 273, 642^645. Koyanagi, R.Y., Chouet, B., Aki, K., 1987. Origin of volcanic
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tremor in Hawaii, Part I: Data from the Hawaiian Volcano Observatory 1969^1985. Volcanism in Hawaii. US Geological Survey Professional Paper 1350, 1221^1257. McNutt, S.R., 1996. Seismic monitoring and eruption forecasting of volcanoes: A review of the state-of-art and case histories. In: Monitoring and Mitigation of Volcano Hazards. Springer, Berlin, pp. 99^146. Nishimura, T., Nakamichi, H., Tanaka, S., Sato, M., Kobayashi, T., Ueki, S., Hamaguchi, H., Ohtake, M., Sato, H., 2000. Source process of very long period seismic events associated with the 1998 activity of Iwate Volcano, northeastern Japan. J. Geophys. Res. 105, 19135^19147. Oikawa, J., Ida, Y., Yamaoka, K., Watanabe, H., Fukuyama, E., Sato, K., 1991. Ground deformation associated with volcanic tremor at Izu-Oshima volcano. Geophys. Res. Lett. 18, 443^446. Sekiya, K., Kikuchi, Y., 1889. The eruption of Bandai-san. J. Coll. Sci. Imp. Univ. Jpn. 3, 91^172. Tohoku University, 1989. Recent seismic activity around Mount Bandai from October 1897 to January 1989. Rep. Coord. Comm. Volcan. Predict. 43, 52^55. Yamamoto, M., Kawakatsu, H., Kaneshima, S., Mori, T., Tutsui, T., Sudo, Y., Morita, Y., 1999. Detection of a crack-like conduit beneath the active crater at Aso volcano, Japan. Geophys. Res. Lett. 26, 3677^3680. Yamawaki, T., 1999. Three-dimensional Shallow Velocity Structure of Bandai Volcano Inferred from Explosion Experiment. M.Sc. Thesis, Tohoku University, Tohoku, 85 pp. Yamawaki, T., Nishimura, T., Ueki, S., Tanaka, S., Hamaguchi, H., 2000. Hypocenter determination at Bandai volcano by using three-dimensional velocity structure from active seismic experiment. Abstract, 2000 Fall Meeting of the Seismological Society of Japan, p. 71. Yonechi, F., 1995. Reconstruction of the scenario of the eruption and the old landform of Mt. Bandai interpreted from the records by local people. Bandai Volcano ^ Recent Progress on Hazard Prevention. National Research Institute for Earth Science and Disaster Prevention, Japan, pp. 181^188.
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