京 都 大 学 防 災 研 究 所 年 報 第 49 号 C 平 成 18 年 4 月
49 C 183
Annuals of Disas. Prev. Res. Inst., Kyoto Univ., No. 49 C, 2006
6Pressure Waves derived from Eruption Movie Images of Sakurajima Volcano
Akihiko YOKOO* and Kazuhiro ISHIHARA* COE researcher, DPRI, Kyoto University
Synopsis
We analyzed some eruption movies of Sakurajima and Suwanosejima volcanoes.
Propagation of pressure waves produced by these eruptions were clearly recognized as changes inluminance using an image processing of movies, even for the case that we could not see them bybear eyes. The apparent speeds of propagation were around 340-570 m/s at Sakurajima.
Furthermore, information on wavelengths of pressure waves has been investigated quantitatively toestimate the source conditions of volcanic explosions; such as time-spatial dimension of the waves.
Keywords: Sakurajima volcano, image processing, Vulcanian explosion, visualized pressure wave

1.

Introduction

It is known that atmospheric pressure waves areproduced by volcanic eruptions. In last decades, thesewaves have been observed around volcanoes using lowfrequency microphones all over the world. Results of theobservations were attributed to identification of eruptive
or non-eruptive activities, as well as to determination of
accurate locations of eruptive vents (Okada et al., 1990;
Ripepe and Marchetti, 2002). Other successes ofunderstanding source processes of eruptive mechanisms
or dynamics were inferred from a theoretical point ofview (Buckingham and Garcés, 1996; Vergniolle andBrandeis, 1996).
Propagation of these pressure waves, in particular
shock waves, excited by explosive eruptions have beenoccasionally visualized as “flashing arc” (Perret, 1912)or as the disappearance of pre-existing clouds and/or theformation of cloud (Nairn, 1976; Ishihara, 1985; Yokooet al., 2004). Perret (1912) interpreted flashing arc as thechange of the refractive index of the air. In contrast,
others interpreted that these phenomena were induced by
the phase change of H2O due to compression phase ofshock waves and following rarefaction phase.

Velocities of their propagation were estimated to be~600 m/s based on the shape of the condensation cloudat Ngauruhoe (Nairn, 1976), and to be 300-440 m/s forthe clouds spreading out with time at Izu-Oshima(Yokoo et al., 2004). On the other hands, Ishihara (1985)
estimated them 440-550 m/s to trace the disappearance
position of the clouds at Sakurajima.
Besides, Yokoo and Taniguchi (2004) developed amethod to detect pressure waves from eruption movies
using image processing. As a result, they succeeded tovisualize pressure waves within the ash-laden plume at
Aso volcano, though the phenomenon was too weak to
be recognized directly by bear eyes.
For confirmation of usefulness of their method, weanalyzed some eruption movies of Sakurajima andSuwanosejima volcanoes. In this paper, we mainly
describe the results of Sakurajima; both of estimatedapparent velocities to propagate and spatial dimensionsof atmospheric pressure waves. And then, we presume
source information of the volcanic explosion. In addition,
for the case of weak eruption, the result of a
Suwanosejima eruption is also reported in Appendix.
2.

㧙㧙

Eruption Movies of Sakurajima

Table 1 Characters of pressure waves determined from the movie images
No.

Date & TimePa[Pa]

A05
A09
A23
A83
A98

1982/05/23 11:28
1982/06/07 11:26
1982/12/15 11:25
1987/12/08 10:40
1988/01/07 18:15

140
90
110
320
230

Vpwb
[m/s]

vcompb[m/s]

342 (±28)459 (±22)354 (±20)441 (±12)vrareb[m/s]Ȝcompc[m]

574 (±21)Ȝrarec
[m]
>280

150
175

464 (±14)514 (±14)364 (± 7)

>390
>210
250

a

Amplitude of pressure wave recorded at HAR, 2.7 km apart from the summit crater.
Propagation speed of pressure wave (Vpw) and that of compression and rarefaction phase of the wave (vcomp, vrare).
cLengths of compression and rarefaction phases of pressure waveb

Altitude [m]
A83

3000
2800
2600
2400

A05

2200

A09A23

2000
1800

A98

1600
1400
1200

1000

Fig. 1 Index map of Sakurajima. A TV camera and a
microbarograph were installed at SVO and HAR,
respectively.

Fig. 2 Field-of-views of the movies for each eruption
listed in Table 1. A white dashed line indicates theprofile of the crater.
from archived movies taken by this system, as shown inTable 1 and Fig. 2.

volcanoSakurajima volcano located at southern Kyushu haserupted more than 8000 times since 1955 at a summit
crater (Fig. 1). The eruption type of Sakurajima is calleda Vulcanian explosion. A mechanism of this type ofexplosion is proposed as a bursting of a gas pocket justbeneath the crater (Tameguri et al., 2002). At the time ofbursting, a strong air-shock is emitted from the crater. Anobserved general waveform at Sakurajima volcano iscomposed of an impulsive compression phase and afollowing longer rarefaction phase (Iguchi and Ishihara,1990; Garcés et al., 1999).
In early 1980s, Sakurajima Volcano Observatory,
DPRI, Kyoto University, constructed a monitoring
system for eruptions by combining a TV camera withseismometers. This system succeeded to capture flashingarcs associated with explosive eruptions (Ishihara, 1985).
Five eruption movies analyzed here were selected

3.

Image Processing and Result ofAnalyzing

Firstly, movies recorded on U-matic video tapes wereconverted to digital files with VGA sized resolution (640pix×480 pix) and 30 fps. Then, an operation of ageometric transformation with linear interpolation usingnear 4 points was conducted (Jähne, 2002).
For deriving characters of pressure waves, we paid
attention to luminance data of the images (Yokoo andTaniguchi, 2004). The data of luminance, Y, is an indexof brightness on digital images and it is calculated fromthe color data of RGB (8bit×3) for each pixel byfollowing equation:㧙㧙

Y = 0.299×R + 0.587×G + 0.114×B

00

306090(a)

(b)
0200

020200Fig. 3 Four snapshots of the A23 eruption with 1.0 sec
intervals (frames #00, #30, #60, and #90). Verticalluminance profile along the rectangular zone, as
indicated in the frame #00, is illustrated at the rightside of each snapshot.
The index Y is generally corresponding to a grayscale of an image. That is, smaller and larger values
mean darker and brighter colors on digital images,
respectively (0-255; black to white).
Here, we picked the image of the eruption on Dec. 15,
1982 (A23 in Table 1) up for explaining the nature of avisualized pressure wave (Fig. 3). Before the eruption,
there were white clouds just above the crater (frame#00). One second later, some parts of them were turned
to darker (#30) and then flushed with white color at theupper (#60). It quickly returned to the state before theeruption (#90). Thus, the visualized pressure wave wasrecognized as going upward from the crater.
To make this phenomenon quantitatively by theimage processing, we focused on a rectangular zone of
20 pix width, as indicated in Fig. 3. Then, average valuesof luminance Y for all horizontal lines in this zone arecalculated for each frame.
The sequential change of averaged luminance in thezone was illustrated in Fig. 4a. It was clearly recognizedthat both of a darker and a bright ones propagatedupward in the range of 1100-1450 m. The luminancechange at the altitude of 1400 m resembled the pressurechange have been recorded by an infrasonic microphone(Iguchi and Ishihara, 1990; Fig. 4b). Namely, the dark
and bright portions corresponded to the compression andrarefaction phase of the pressure wave, respectively.
Duration of a compression phase of the wave wasfound about 0.4 sec and it did not change much value foreach altitude (Fig. 5a). On the other hand, that of afollowing rarefaction phase became longer and longer
than the preceding compression one as altitude being
higher (about 0.5-2.0 sec).

0123

456time [sec]78910

To estimate the propagation speed of this wave, weFig. 4 (a) Sequential luminance data during 10 sec
from approximately 1 sec before the onset of the A23eruption. (b) Luminance change at the altitude of 1400
m. Decrease in luminance is plotted upwards.
(a)
0

1

2time [sec]345

671450
20

1400
1350
1300
1250
12001150
1100

300(b)

1.060

90
120
time [frames]
time [sec]
2.0

1.5

180

150

210

3.0

2.5

1600
sky

20

1400

dilutecloud

1200

rim

1000

flank ofSakurajima

800
30

40

50

60
70
time [frames]

8090Fig. 5 (a) Luminance changes with time at the altitudes
ranging from 1100 to 1475 m. (b) Profiles of changes
in luminance. Difference of luminance data between
each frame and the first frame (#00). Increase inluminance is plotted rightwards.
checked the three times in luminance changes every 5-8m altitudes; (a) the time starting to be darker, (b) the timeof the primary negative peak, and (c) the secondary
positive peak. As a result, the velocities of (a), (b), and㧙㧙

(a)(c) were estimated as 459 m/s (±22 m/s), 441 m/s (±12m/s), and 464 m/s (±14m/s), respectively (Table 1). Allof them were significantly higher than the speed of soundin the air (~335 m/s).
Moreover, we subtracted the luminance data of thefirst frame #00 from that at each frame for the purpose ofgetting information on spatial-dimension (a kind of wave
length). The compression phase of the wave was about
175 m at frame #45, and the rarefaction phase was >390
m at #60 (Fig. 5b).
Results of the analyses for other eruptions were also
listed in Table 1. Apparent propagation speeds of thewaves were faster than the sound velocity of the air. The
spatial scales of the compression phases were about 150
m and those of the rarefaction phases were about 150 mand two times longer than the compression’s.
4.

Discussion

One of our interests is source conditions of gas
pocket causing the volcanic explosion. In order to getsome information on that, we calculated averagevelocities of pressure waves at the distance of 250-600 m
from the source using 1D spherical explosion model(Saito and Glass, 1979; the code was provided by Prof. T.
Saito). This range of distances was equivalent to that ofaltitudes of 1100-1450 m from the crater bottom, within
which the waves of A23 eruption were visualized on themovie. We assumed that the explosion source wascomposed of 500°C H2O vapor degassed from magma
and the outside was the air for the calculation.
In Fig. 6a, the relationship between internal pressureand velocity was illustrated for each radius of the source;
5 m, 10 m, 15 m, 20 m, and 25 m. If we adopt a pressureof the explosion source to be 140-280 bars which were
estimated from ejection speeds of ballistics (Iguchi et al.,
1983; Ishihara, 1985; 1990), a radius of that would bededuced for 10-15 m. This evaluated value was roughly
consistent with the size of red-growing part in the lava
dome which had disappeared by the explosion, and thepressure source of the explosion estimated from the dataof ground deformation (Ishihara, 1990).
The waveform of pressure waves at 550 m distancewas calculated with parameters mentioned above, andillustrated with the luminance change from the movie inFig. 6b. The duration of the calculated compression
phase almost coincided with that of the luminancechange. However, the following phase did not have so525
r=25 mr=20 mr=15 m

500
459
22 m/s475

r=10 m

450
425

r=5 m

400
375
Ishihara (1985)

350
325(b)

050

100

150
pressure [bar]

200

250

300

2
primary shock
140 bar/15 m

1.5

secondary tertially
20280 bar/10 m

1
luminance data0.500.5

1

1.5
2
time after explosion [sec]

2.5

3

3.5

Fig. 6 (a) Relationship between internal pressure andaverage velocities of pressure waves at the range ofdistance from 250 to 600 m calculated from 1D
spherical explosion model. r denotes the radius ofpressure source. (b) Comparison between calculated
waveforms of pressure waves at 550 m from thesource and observed luminance change 550 m above
the crater bottom, as shown in Fig. 4b.
good agreement. We think that this may be mainly
caused by simplicity of the explosion model we used
here; e.g., an instantaneous release of a high-pressurizedgas.
5.

Summary

We re-examined some characters of atmosphericpressure waves associated with Vulcanian eruptions atSakurajima more quantitatively by an image processing.
The apparent propagating speeds of the waves were
faster than the sound velocity of the air (342-574 m/s).
We also found that a spatial scale of a compression phase
was about 150 m and that of a rarefaction phase was twotimes more than the former. The method to combine animage processing of eruption movies with a numericalsimulation of propagation of pressure wave would makea contribution to modeling of volcanic explosions andphysical parameters of explosive sources
Acknowledgements
The authors are grateful to all staff members of SVO
for their helpful supports, especially, to Dr. M. Iguchi
providing us a lot of constructive comments. We also
acknowledge Prof. Y. Tanaka for his invitation to㧙㧙

observe Suwanosejima eruption. This research wasmainly supported by the Japanese Ministry of Education,Culture, Sports, Science and Technology (MEXT) 21stCOE program for DPRI, Kyoto University (No.
14219301, Program Leader: Prof. Y. Kawata) and partly
supported by Grant-in-Aid for Scientific Researchoffered by MEXT (No. 14080203 for Prof. K. Tanaka).
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AppendixHere, we report a brief result of analysis for the weakpressure waves excited by a small eruption at
Suwanosejima.
Suwanosejima volcano is a volcanic island locatedabout 200 km southwards from Kyushu. On Aug. 10,
2005, a few tens of eruptions were occurred in thedaytime. An eruption occurred at 16:11:02 was thelargest one, but the peak pressure of this eruption wasobserved 8.4 Pa at a station 620 m apart from the vent.
Before the eruption, there were quite dilute steam
above the crater (Fig. A1a). The first volcanic cloud (jet)
(a)

(b)

(c)

steam

rim2

16:11:00

C

16:11:03

16:11:06

(d)

(e)

(f)

16:11:09

16:11:12

16:11:15

rim1

Fig. A1 Sequential images of 16:11 eruption on Aug.
10, 2005, with 3 sec time intervals.

㧙㧙

(a)

?

750

volcanic cloud700(b)

650

10600

volcanic cloud

16:11:00

16:11:02

16:11:04
time

16:11:06

16:11:08

Fig. A3 Luminance data at each altitude. Note that
decrease in luminance is plotted upside.

Fig. A2 (a) Luminance change in a 20 pix width zone
at the center of the image (Fig. A1f), started from
16:11:00. (b) Differences between the luminance of
two consecutive frames. Bluish and reddish colors
denote darker and blither before each one frame,respectively.

upper part of the image (Fig. A2a), although they weremuch weak compared with Sakurajima eruptions (Fig.
4a). Therefore, we calculated difference of the luminancebetween two consecutive frame’s data (Yokoo andTaniguchi, 2005). Then, propagation was recognized in
the altitude ranging from 550 m to 750 m (Fig. A2b).
We estimated an apparent speed of propagation usingluminance change with time at each altitude in thevertical rectangular zone shown in Fig A1f. The
estimated speed of velocity was about 228 m/s based on
the upward migration of 42 peaks of luminance (Fig.
A3a), which was slower than the sound speed of the air.

could be seen as a vertical emission within the crater(Fig. A1b) and about 7 sec later the secondary cloudwith ballistics began to rise up toward an inclined
direction (Fig. A1d).
It was very difficult to recognize propagation of apressure wave associated with this eruption from thenormal image.
Using the method of image processing described intext we found the slight changes in luminance data at theἫጊྃἫᤋ௝߆ࠄ᛽಴ߒߚ࿶ജᵄߩ․ᓽߦߟ޿ߡ
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