Quantitative Analysis of Pyroclastic Flows Using Infrasonic and Seismic Data at Unzen Volcano, Japan

J. Phys. Earth, 45, 397-416, 1997

Hitoshi Yamasato

Meteorological Research Institute, Japan Meteorological Agency, Tsukuba 305, Japan

The process of the collapse of the dacitic lava dome and the development of pyroclastic flows at Unzen volcano, Japan, were studied using infrasonic, seismic and video records. Characteristic infrasonic and seismic signals were recorded corresponding to the collapse of lava blocks from the dome, the drop of blocks on the slope and the migration of pyroclastic flow on the mountain slope. Small infrasonic and seismic waves are excited when the lava dome starts to collapse. When the lava blocks fall onto the mountain slope and are fragmented, larger waves are excited. This suggests that the seismic waves are generated by the collision of pyroclastics on the mountain slope and that the infrasonic waves are excited by small fractures of the dome and the fragmentation of pyroclastics. Some of the infrasonic signals show an obvious Doppler effect, indicating that the pyroclastic flows emit infrasonic signals during their propagation. The location of dome collapse and the path of pyroclastic flows can be identified and traced by a network of low-frequency microphones. The migrating source of infrasonic signals and probably seismic signals is inferred to be located near the front of pyroclastic flows by comparison with video images. This suggests that the fragmentation of pyroclastics occurs mainly near the front of pyroclastic flows. The speed of pyroclastic flows is estimated as 10-30 m/s from the infrasonic records. The excitation of infrasonic and seismic signals is affected by the topography of the mountain slope. The infrasonic energy is almost the same order as the seismic energy but the ratio of infrasonic to seismic energies increases for larger and more mobile pyroclastic flows. This means that the development of pyroclastic flows is controlled not only by the volume of lava and gravitational force, but also by the explosivity related to the pore gases in the lava.

. Introduction

Unzen volcano is an active dacitic volcano that is located in Beppu-Shimabara graben, Kyushu, Japan. It started eruption in 1990 after 198 years of dormancy. A dacitic lava dome emerged in May of 1991 and its collapse resulted in successive gen- erations of pyroclastic flows. A detailed description of the sequence of the present volcanic activity of Unzen

Received April 17, 1997; Accepted December 24, 1997

397

1 398 H. Yamasato was given by Ohta et al. (1992). A phreatic eruption occurred at the summit craters (Jigoku-ato crater and Tsukumo-jima crater) of Fugendake, which is the main peak of Unzen, on 17 November 1990. On 20 May 1991, a dacitic lava dome emerged at Jigoku-ato crater. It grew to collapse and resulted in pyroclastic flows from 24 May. Subsequently, pyroclastic flows frequently occurred until 1995. In particular, the one on 3 June 1991, which traveled 4.6 km, killed 43 people. Many studies have been carried out on pyroclastic flows at Unzen. Several visual observations support the view that most of the dome collapses at Unzen are triggered by gravitational instability of the dome (e.g., Fukui et al., 1991; Ui et al., 1993); that is, pyroclastic flows at Unzen volcano are mostly of the Merapi type. Although pyroclastic flow is one of the most conspicuous phenomena associated with eruptions at many volcanoes in the world, the study of pyroclastic flow by means of geophysical methods has been scarce. At Unzen volcano, a wealth of geophysical data was acquired from valuable observations. For example, Uhira et al. (1994) studied the seismic signals associated with dome collapse and discussed the force system at the origin of the pyroclastic flows. According to their results, the seismic waves are excited by the collision of lava blocks on the mountain slope, in agreement with visual observations. Yamasato et al. (1993) studied not only seismic signals but also infrasonic signals from pyroclastic flows and obtained some characteristics of these signals. The Japan Meteorological Agency (JMA) operates a low-frequency microphone network and a dense seismic network. Infrasonic observation with low-frequency microphones has been made by some geophysicists at some volcanoes, and infrasonic waves excited by explosive eruptions have been studied (e.g., Tahira, 1981; Iguchi and Ishihara, 1990). Rarely, however, have observations of infrasonic waves by a low- frequency microphone network been associated with pyroclastic flows. Here, the infrasonic and seismic data obtained by JMA are analyzed more quantitatively and the mechanism of pyroclastic flows is investigated.

2. Data The locality of Unzen volcano and the distribution of observation stations used here are shown in Fig. 1. Short-period seismographs with a natural frequency of 1 Hz are operated at these seismic stations. The Meteorological Research Institute of JMA (MRI) installed a long-period seismograph with a natural frequency of 0.1 Hz (Den et al., 1984) at the observatory (Unzendake Weather Station, O on Fig. 1) in April 1991. Infrasonic observation with four low-frequency microphones started in the spring of 1992. Three of the low-frequency microphones used in this study have a constant pressure response in the frequency band between 0.1 and 10 Hz, the other at station K2 since October 1992, has a constant pressure response between 1 and 40 Hz. For waveform analysis, these records were transformed by filtering so that the records are comparable to each other. The filtering coefficients were determined using the relative amplitude and phase responses between the two types of microphones, which were obtained from a comparative observation. The seismic and infrasonic signals are telemetered to the observatory and are

J. Phys. Earth Quantitative Analysis of Pyroclastic Flows 399

Fig. 1. Location of Unzen volcano and observation stations that produced data analyzed in this paper. Closed circles indicate seismic stations; triangles, infrasonic stations; lozenges, video stations; and the square, the observatory of JMA (Unzendake Weather Station). The hatched area shows the pyroclastic flow deposit. recorded continuously on digital tape recorders with a sampling frequency of 50 Hz. Visual data were collected by a time-lapse video recorder at station T1 and by temporary observation with a portable 8 mm video camera at station T2. At the be- ginning of the recordings, the time codes of the video recorders were synchronized at the observatory to the time of the clock with an accuracy of 0.5 s. An example of the record of a low-frequency microphone is shown with the corresponding seismogram in Fig. 2. On the seismic record, two types of seismic signal appear; one is that associated with a pyroclastic flow and the other is a low-frequency earthquake. The same events are recorded in the infrasonic signal with amplitudes of 1 to 2 Pa. The infrasonic signal due to the pyroclastic flow has a long duration, and that from the low-frequency earthquake is impulsive. The impulsive infrasonic signal from the low frequency earthquake is investigated in the another paper (Yamasato, 1998). The infrasonic record is often disturbed by wind noise, as appears in the lower part of the record in Fig. 2.

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Fig. 2. Examples of the (a) seismic and (b) infrasonic records obtained by a vertical seismograph and a low-frequency microphone at station E1. PF and LF indicate records of a pyroclastic flow and a low-frequency earthquake, respectively.

In this paper, the data associated with pyroclastic flows during the period from June 1992 to November 1993 are analyzed. During this period, dome collapses suc- cessively occurred and pyroclastic flows ran in three directions as shown in Fig. 1.

3. Seismic and Infrasonic Signals Associated with Dome Collapse

If we examine the waveforms of the initial phases of seismic signals related to pyroclastic flows, we may get more detailed information on the process of generation of pyroclastic flows. Yamasato et al. (1993) investigated video and seismological data from some dome collapses at Unzen and clarified the time sequence of the dome collapse and the excitation of the corresponding seismic waves. Figure 3 shows an example of the time sequence of a dome collapse, which was obtained from video data obtained at station T2, and the corresponding seismic signal. In this case, an unstable tip of the dome collapsed and generated a pyroclastic flow. From photographs, the volume of the collapsed lava was estimated as 5.7•~104m3. The flow distance from the dome collapse was 2.5 km as estimated from the video record. By combining the seismic data with the visual data, the following sequence was identified.

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Fig. 3. Illustrations of the dome collapse sequence (left) and the corresponding seismic signals (right). A sketch of the dome from station T2 is shown based on a video record taken by a portable 8 mm video recorder. SPs are seismograms obtained by short-period seismographs; LP is by a long-period seismograph. The records are arranged according to the horizontal distance from the source. Seismic records for the vertical component are shown here.

A) Seismic waves with small amplitude and predominant frequency of 2-3 Hz were excited almost simultaneously with the collapse of the lava dome. B) The amplitude became larger a few seconds later as lava blocks fell onto the slope. The seismic waves with large amplitude contained a low-frequency component (about 0.5 Hz). C) The fragmented pyroclastics started to flow generating high-frequency seismic waves (more than 2 Hz). In Fig. 4, an example of a clear infrasonic signal from a dome collapse is shown. As mentioned above, the seismic signal started with a small amplitude and became large within a few seconds. The times, A and B in the figure, are considered to correspond respectively to the times of the start of the dome collapse and of the fall of lava blocks onto the slope. A small impulsive infrasonic signal was excited at the time of the dome collapse and larger infrasonic waves were excited when the lava blocks fell onto the slope. In some cases, long-period infrasonic signals were observed in association with dome collapses as shown in Fig. 5. The apparent period of the signal, 3-6 s, varied depending on the station, indicative of a Doppler effect. In such a case, the velocity of the moving source can be estimated from the difference of the observed frequency. If apparent periods (Pi) of the signal at i-th stations (i=1, 2, 3) can be obtained, the speed

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Fig. 4. An example of seismic (left) and infrasonic (right) signals from a dome collapse. SPs are seismograms obtained by short-period seismographs; LP is by a long-period seismograph. Seismic records are for the vertical component. Infrasonic records were obtained by low-frequency microphones. The records are arranged according to the horizontal distance from the source. The phases, A and B, are assumed to correspond respectively to the start of dome collapse and the fall of lava blocks onto the slope.

(ƒÒ) and direction (D) of the source, and the true period of the signal (P0) can be calculated by solving simultaneous equations,

where Ai and c are the azimuth of the stations from the source and the sound speed, respectively. Examples are listed in Table 1. The direction of the moving sources estimated above generally agree with that from visual observations. The speed of the source was estimated as 10-40 m/s, which is considered to be the initial speed of the descending pyroclastic flows.

4. Source Location and Amplitude Variation of Infrasonic and Seismic Signals from Pyroclastic Flows To clarify the characteristics of the seismic and infrasonic signals from pyroclastic flows, their source locations and amplitude variations were investigated from the waveforms obtained by the infrasonic and seismic networks which were compared with video images. An example illustrating the migration of a medium-sized pyroclastic flow front and

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Fig. 5. An example of the infrasonic signal showing an obvious Doppler effect. Upper panel shows an example of infrasonic (MIC) and seismic records from a pyroclastic flow. Lower panel is for the low-pass filtered (the cut off frequency is 1 Hz) records for the bracketed parts of the infrasonic signals (upper). Corresponding phases are indicated by lines.

Table 1. Examples of apparent frequencies of infrasonic signals observed by low-frequency microphones at the time of dome collapse.

The initial velocities of the pyroclastic flows were estimated from the apparent periods that showed a Doppler effect. The data obtained at station K2 were not used because the frequency response of the microphone differs from those of other stations.

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Fig. 6. Illustration of an advancing pyroclastic flow and the corresponding seismic and infrasonic signals. The stars indicate the source locations of the infrasonic signals. Corresponding phases are indicated by lines. The descent of the pyroclastic flow is traced from the record of a time-lapse video tape recorder at station T1. the corresponding seismic and infrasonic signals is shown in Fig. 6. The video data obtained at station Ti were used in this analysis. In this case, the pyroclastic flow ran eastward and the tip of the flow reached a distance of 3 km from the dome. Seismic waves were excited while the pyroclastic flow front was running down the slope. They gradually decayed and finally vanished when the pyroclastic flow came to a stop. The amplitude variation of the infrasonic signal is similar to that of the seismic signal, and the infrasonic signal also became attenuated when the seismic waves decayed. As seen in the figure, the ratio of the seismic signal amplitudes observed at station K3, 5 km west of the dome, to those observed at station C, 2.5 km southeast of the dome, gradually decreased. It indicates that the seismic source migrated eastward. The source location of the seismic signals could be traced using the amplitude variation of seismic signals observed by the dense seismic network. The variations of the source location and the amplitude of the source were estimated from root-mean- square (RMS) amplitudes observed at the stations around the volcano every 10 s. In Fig. 7, the relation between the epicentral distance and the amplitude of the initial phase corresponding to the dome collapse is shown, where the seismic source is assumed

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Fig. 7. Relation between the epicentral distances and the amplitude of the seismic signals from the dome collapses of Fig. 6. The RMS amplitudes of the initial phase (10 s) are shown; they are normalized to that at station El . The solid and dashed lines show the theoretical curves assuming that the signal is composed of body waves and surface waves, respectively.

to be the point on the ground surface that is the same as the infrasonic source estimated by a method that is mentioned later. From this figure, it is seen that the amplitude distribution is independent on the azimuth and the seismic source is almost isotropic. In the figure, the theoretical amplitude distribution curves are shown, where the seismic source is isotropic and the signal is composed of body waves (the solid line) or surface waves (the dashed line), respectively. However, from the figure, it cannot be determined whether the signal is composed of body waves or surface waves. Therefore, the source location is determined by two methods in that the signal is assumed to be composed of body waves or surface waves. Assuming that the signal is composed of body waves, the amplitude (ai, i=1, •c, n: n is number of the stations) is approximated as

(1)

where ri, k, ƒÂi, and a0 are the epicentral distance (km), the attenuation coefficient, the station effect and a constant, respectively. The attenuation coefficient (k) and the station effect (ƒÂi) for each station can be calculated from the seismic amplitudes of the phases that correspond to the dome collapse. The attenuation coefficient (k) was obtained as 0.13•}0.04. Assuming that coefficients k and ƒÂi are constant, the seismic source location

(x, y) and the amplitude (a0) during the pyroclastic flow can be estimated from

(2) where the source is fixed on the path of the pyroclastic flow that is identified from the infrasonic data, as shown later. In this case, seismic data were used from all stations

Vol. 45, No. 6, 1997 406 H. Yamasato except E3, which is located nearest to the dome and whose records were clipped. In the case that the signal is assumed to be composed of surface waves, the following Eqs. (1') and (2') were used instead of Eqs. (1) and (2).

(1')

(2')

In this case, the attenuation coefficient (k') was obtained as 0.25•}0.04. The variation of the amplitude distribution and the source location for the pyroclastic flow in Fig. 6 is shown in Fig. 8. The difference of the source locations between the two methods is within 300 m, and it can be seen that the seismic source migrated eastward. The source location and amplitude variation are shown in Fig. 9 (c) and (e). The amplitude is normalized to that at the distance of 1 km from the source. Figure 9 (a) shows the migration of the pyroclastic flow front identified from the video image. The seismic source is considered to be near the pyroclastic flow front, and it migrates eastward with variation in amplitude.

Fig. 8. Amplitudes of the seismic signal from the pyroclastic flow shown in Fig. 6 at every 60 s after dome collapse. The amplitudes are normalized to that at station E1. Open circles and stars indicate the source locations obtained from the amplitude distributions assuming that the signal is composed of body waves and surface waves, respectively.

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Fig. 9. Temporal variations of seismic and infrasonic sources from the pyroclastic flow shown in Fig. 6. (a): The position of the pyroclastic flow front as traced by a video camera located at T1 . (b) and (c): The advancing infrasonic and seismic sources estimated from the waveform correlation observed by the low-frequency microphone network and the distribution of seismic amplitudes observed around the volcano. (d) and (e): The variation of the amplitudes of the seismic and infrasonic signals estimated from observed amplitudes; they are normalized to a distance of 1 km from the source. The distances in (a), (b), and (c) are the horizontal distances from the point where the dome collapse occurred.

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On the other hand, the source location of the infrasonic signal can be estimated from the travel time differences of conspicuous phases in the signal because a very good correlation is observed among the waveforms of different stations as shown in Fig. 4. The method of source determination is mentioned in the Appendix. Figure 9 (b) shows the estimated migration of infrasonic sources of the pyroclastic flow in Fig, 6. Also in Fig. 6, the source location is indicated by stars. It is obvious that the infrasonic sources migrated along the slope and were always confined to being near the front of the pyroclastic flow. Figure 9 (d) shows variations in the amplitude of the infrasonic wave with the amplitude (s0) estimated assuming

Fig. 10. Estimated source locations of the infrasonic signals from pyroclastic flows. Closed circles indicate the sources due to dome collapses and open circles indicate the sources of later phases that arise from the flow of pyroclastics. The data from June 1992 to November 1993 are analyzed. The hatched area indicates the locus of the lava dome. Stars indicate the locations where the amplitudes of the later phases of the seismic and infrasonic signals reached a peak.

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Fig. 11. Migration of infrasonic source and amplitude variation of infrasonic signal from pyroclastic flows. Events that furnished very clear infrasonic waves were selected for each direction. The distances are horizontal from the center of the lava dome. The amplitudes are normalized to those of the phases corresponding to dome collapse. An event in B-course shows very low speed, which seems to be a multi-flow. where s and r are the observed infrasonic amplitude and distance from the source, respectively, and RMS infrasonic amplitudes, observed every 10 s, were used. The infrasonic amplitude is also normalized to a distance of 1 km from the source. The pattern of amplitude variation is similar to that of the seismic amplitude. The above results indicate that both the seismic and infrasonic sources are near the pyroclastic flow front and migrate along the mountain slope, and that their amplitudes vary with time in a similar pattern. The distribution of infrasonic source locations is shown in Fig. 10 for dome collapses and for following pyroclastic flows for the interval June 1992 to November 1993. Closed circles indicate locations determined from phases corresponding to dome collapse, and open circles are for infrasonic sources for pyroclastic flows as determined from later phases. The infrasonic sources corresponding to pyroclastic flows show migrations ei- ther northeastward (A-course), eastward (B-course) or southeastward (C-course). The pyroclastic flows during the above period were in these three directions (see Fig. 1). Thus, the direction of a pyroclastic flow can be determined from analysis of infrasonic data even if visual observation is impossible. In Fig. 9, the infrasonic and seismic amplitudes decreased just after dome collapse but increased again about 50 s later at a site about 1 km from the lava dome. Such variation was also found in other seismic signals excited by pyroclastic flows at Unzen. Figure 11 shows the migration and amplitude variation of the infrasonic sources of pyroclastic flows between June 1992 and November 1993. The events that furnished

Vol. 45, No. 6, 1997 410 H. Yamasato very clear infrasonic waves were selected for the three directions. The amplitude variation differs among the flows but the locations where the amplitude increased again are almost the same for each direction. These locations are indicated by stars in Fig. 10 and are almost identical for all pyroclastic flows in the same direction. The locations coincide with topographic irregularities, such as waterfalls (Nakada and Fujii, 1993), where the slope angle changes abruptly. From Fig. 11, the migrating speeds of the infrasonic sources could be determined to be 10-30 m/s. In this analysis, horizontal distance is used, therefore, the speed along the mountain slope may be as much as 10% larger. These values are almost the same as that for pyroclastic flows measured by visual observations (e.g., Fukui et al., 1991; Yamamoto et al., 1993).

5. Infrasonic and Seismic Energies The amplitudes of the infrasonic and seismic signals from pyroclastic flows vary due to topographic effects, as mentioned in the previous section. In this section the energies of the infrasonic and seismic waves excited by pyroclastic flows are estimated based on the results shown in Fig. 9. If an isotropic infrasonic source exists on the surface of an infinite half space, the infrasonic energy (Ea) can be represented as

where pe is the effective sound pressure at horizontal distance r from the source and p, c are the density and the sound speed of the atmosphere. For the pyroclastic flow in Fig. 9, this gives a total infrasonic energy of 1.3•~106 J. However, an absolutely precise estimation of the infrasonic energy is difficult because it is not easy to measure the directionality of the source and the effects of diffraction, reflection, scattering and absorption of the infrasonic waves related to topographic and meteorological conditions. The absolute estimation error may be large but the ratio of energies estimated from the records at different stations is within 0.6-1.6. Therefore, the factor of the relative estimation error is considered to be within 2. Seismic energy is estimated by making comparisons to the seismic energy of low-frequency earthquakes. Low-frequency earthquakes are believed to occur in a very shallow area beneath the summit (Shimizu, 1993), and their spectra are similar to those of the seismic signals from pyroclastic flows (Uhira et al., 1994). Here, we assume that the seismic energy (Es) is proportional to the square sum (P) of the waveform recorded, that is

The seismic energy (Es, in J) of low-frequency earthquakes can be estimated from the seismic magnitude (M) using Gutenberg and Richter's formula,

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Table 2. Parameters for pyroclastic flows that flowed eastward (B-course).

The pyroclastic flows are those for which clear infrasonic signals were observed from August 1992 to July 1993, and which flowed more than 3 km. The seismic duration is the period of time for which the amplitude of seismic signals was more than 1.25•~10-5 m/s at E3. * Multi-flow.

Therefore, the coefficient (C) can be estimated from the relation between the seismic magnitudes and the square sum of the waveforms of the low-frequency earthquakes. Here, seismic magnitudes are estimated from seismic amplitudes observed around the volcano using a formula obtained by Hashimoto et al. (1997). As the source moves, the waveform recorded at station K6 is used, where the epicentral distance stays nearly constant within 4.3-4.5 km during the pyroclastic flow in Fig. 7. In the case of this pyroclastic flow, the seismic energy is estimated as 1.5•~106 J; that is equal to the seismic energy of an earthquake with M=0.9 . Also in the case of seismic energy, the absolute estimation error may be large, however, the ratio of energies estimated from the record at different stations is within 0.7-1.3. Therefore, the factor of the relative estimation error is considered to be within 2, similar to that of infrasonic energy. The infrasonic and seismic energies for other pyroclastic flows in the same direction

(B-course) as the pyroclastic flow in Fig. 9 can be similarly estimated. Results are indicated in Table 2 for pyroclastic flows of more than 3 km (estimated by the Unzendake Weather Station from video or infrared camera data). For medium to large-sized pyroclastic flows at Unzen, the total energy of infrasonic and seismic waves is about 106-108 J, and the infrasonic energy is almost the same order as the seismic energy. The relation between flow distances and the ratios of infrasonic energy to seismic energy are shown in Fig. 12. The ratio increases as pyroclastic flows cover longer distances.

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Fig. 12. Relation between the flow distance and the energy ratio of the infrasonic waves to the seismic waves from pyroclastic flows. Only the pyroclastic flows that flowed eastward (B-course) and have clear infrasonic signals were used. The flow distances are horizontal as observed by the Unzendake Weather Station. Closed circles indicate the pyroclastic flows for which the flow fronts were out of the range of the video camera; therefore, the actual distances might be larger than those shown here.

6. Discussion From the analysis mentioned before, the infrasonic waves corresponding to dome collapses are believed to be excited in the following sequence. 1) When the dome starts to collapse, some small fracture occurs in/on the dome. A small impulsive infrasonic wave is excited by small volcanic gas emission. 2) Lava blocks fall onto the mountain slope and collide with the mountain terrain. The lava blocks are fragmented and excite large infrasonic waves. 3) Fragmented blocks and/or pyroclastics start to flow along the slope. Infrasonic waves are excited by the fragmentation of advancing pyroclastics. The infrasonic and seismic signals show amplitude variations as the pyroclastic flows descend the mountain slope. When the pyroclastic flows pass areas where the slope angle changes, the emission of infrasonic and seismic waves is enhanced. This indicates that pyroclastics excite infrasonic and seismic waves when they are fragmented by collision. Both sources are near the front of pyroclastic flows, and therefore, the fragmentation of pyroclastics is considered to occur mainly at the front of the pyroclastic flow. The ratio of infrasonic to seismic energies (Ea/Es) increases for larger and more mobile pyroclastic flows. As the seismic waves result from the collision of lava and pyroclastics on the mountain slope, the seismic energy is considered to be related to

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the volume of the pyroclastic flow. Therefore, it can be said that large pyroclastic flows excite large infrasonic energy per unit volume. Some visual observations found that descending pyroclastic flows were accelerated by self explosions of pyroclastics at topographic singularities (Nakada and Fujii, 1993), and that pore gas pressure plays a major role in self explosion (Sato et al., 1992). As the pore gases must be excited efficiently by fragmentation, the infrasonic wave is considered to be related to the pore gas content of a pyroclastic flow. Therefore, the development of pyroclastic flows is controlled not only by volume of lava and gravitational force, but also by the explosivity related to the pore gases in the lava. The ratio Ea/ES is believed to be a parameter of the explosivity of pyroclastic flows.

7. Conclusion The infrasonic and seismic signals from dome collapse and following pyroclastic flows at Unzen volcano were analyzed and the following results were obtained. 1) When a dome collapse occurs and a following pyroclastic flow ensues, infrasonic and seismic signals are observed. Small impulsive infrasonic waves and small seismic waves are excited when unstable parts of the lava dome start to collapse. When the lava blocks fall onto the mountain slope and are fragmented, larger waves are excited. These infrasonic signals show the Doppler effect, demonstrating that they are due to advancing pyroclastics. Thus, the infrasonic waves are excited during the ejection of volcanic gases and the fragmentation of descending pyroclastics. 2) Infrasonic and seismic waves are excited as a pyroclastic flow descends a mountain slope and decay when it stops. The sources of the infrasonic and seismic waves can be estimated from waveform correlation and amplitude distribution. Both sources are believed to be near the front of pyroclastic flows, indicating that the fragmentation of pyroclastics occurs mainly at the front of pyroclastic flows. The speed of pyroclastic flows is estimated as 10-30 m/s from infrasonic records. The excitation of infrasonic and seismic energies is affected by the topography of the mountain slope. The infrasonic energy of a pyroclastic flow is almost the same order as the seismic energy but the ratio of infrasonic to seismic energies (Ea/Es) increases for larger and more mobile pyroclastic flows. This means that the development of pyroclastic flows is controlled not only by the volume of lava and gravitational force, but also by the explosivity related to the pore gases in the lava. The parameter Ea/Es is believed to be related to the explosivity of pyroclastic flows.

The author would like to express his gratitude to Prof. Kazuhiro Ishihara of Sakurajima Volcanological Observatory of Kyoto University and Drs. Masaaki Seino, Ki-iti Horai and Kohichi Uhira for their encouragementand helpful suggestions.Thanks are also due to Dr. Alan T. Linde of the Carnegie Institution of Washington, who read the manuscript and gave helpful suggestions, and to the staff of Unzendake Weather Station of JMA, who have operated the infrasonic and seismicnetworks. Discussions with Prof. Makoto Tahira of Aichi University of Education, and Messrs. Keiichi Fukui and Tetsuo Hashimoto were also useful in this study. The comments of two anonymous reviewersof JPE were useful for revising this paper.

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REFERENCES

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APPENDIX. SOURCEDETERMINATION OF INFRASONICSIGNAL

Because a good correlation is observed among the waveforms of different stations as shown in this study, the source location of the infrasonic signal can be estimated from the travel time differences of conspicuous phases in the signal. Assuming that the sound velocity is 340 m/s and the source is on the ground surface, the source location (x, y) and the origin time (t) of the phase can be estimated using the least-squares

J. Phys. Earth Quantitative Analysis of Pyroclastic Flows 415

Fig. A1. Examples of the infrasonic signals and their correlation analysis. The upper graph shows examples of infrasonic signals from pyroclastic flows that flowed along A-, B-, and C-courses. The dashed lines indicate some phases that show good correlation. The lower graph shows the travel time difference between stations K8 and C, and the variation of correlation coefficients. Crosses, open circles and closed circles indicate examples of pyroclastic flows that flowed along A-, B-, and C-courses, respectively. method. For analysis of the later phases of the seismic signals excited by pyroclastic flows, the time delays were obtained from the data taken at three or four infrasonic stations by seeking the optimum cross correlation among record segments 10 s long. Examples are shown in Fig. A1. As seen in the figure, the travel time delay varies in

Vol. 45, No. 6, 1997 416 H. Yamasato

time and the variation pattern is different for the pyroclastic flow of each course. In

general, the correlation is good in the initial phase but it becomes worse in the later phase. As the slowness of infrasonic wave is larger and the velocity structure is more homogeneous than those of seismic waves, the source location can be estimated very

precisely. The location error is considered to be due to the reading error of the phases and the variation in meteorological conditions. Sound speed in the atmosphere varies with the temperature. If the temperature varies in the range of 0-30•Ž, sound speed varies 330-350 m/s. However, the effect of such variation in sound speed on the location of the source is less than 50 m. If homogeneous wind with a speed of 10 m/s blows, the error becomes 100 m; however, in such a case, the phases of the infrasonic signals cannot be identified due to the wind noise. Therefore, the location error is considered to be less than 100 m when clear phases can be identified in the records at the four stations. However, in the case that data from only three stations is available, the source location can be estimated but the error becomes larger. For analyzing impulsive infrasonic signals as phases corresponding to dome collapse (see Fig. 4), the estimation of the travel time differences is straightforward. However, it is more difficult to analyze continuous signals as later phases from pyroclastic flows, so the location error may be rather large. The

bad correlation in the later phase is considered to be due not only to the small excitation of the signals but also to the size of the source. Infrasonic signals were often disturbed by wind noise; therefore, it was impossible to determine the source location when wind noise was large or the signal was small. For example, from October 1992 to January 1993, 663 pyroclastic flows occurred. Among them, the infrasonic sources corresponding to dome collapses could be determined for 95 events (about 14%) and those of the later phases could be determined for 12 events (about 2%).

J. Phys. Earth