Bull. Volcanol. Soc. Japan
Vol. 58 (2013) No. 1, pp. 163-181

ArticleProcesses Prior to Outbursts of Vulcanian Eruption
at Showa Crater of Sakurajima Volcano
Akihiko YOKOO＊,＊＊, Masato IGUCHI＊＊, Takeshi TAMEGURI＊＊ and Keigo YAMAMOTO＊＊(Received September 27, 2011 ; Accepted December 26, 2012)
Showa crater of Sakurajima volcano became active in June 2006 after 58 years of quiescence. From multiparametric geophysical observations, we have identified the processes that typically occur prior to an explosive eruption
at the crater. A few hours prior to the onset of an eruption, magma starts to migrate and accumulates at a depth of about
1 km. This accumulation of magma can be clearly observed in strain change records as an inflation process. Several
tens of minutes prior to an eruption, the SO2 gas emission rate gradually decreases, indicating that a sealing process istaking place in the crater bottom as the eruption nears. During the same time period, the volcanoʼs inflation rate starts toaccelerate due to the formation of a plug above the conduit that prevents the gas from escaping, with the result that a gas
pocket forms beneath the crater. In nighttime events, a volcanic glow is also seen, which weakens and then disappears.
A few minutes prior to an eruption, a small tremor starts to occur. Its amplitude grows as the strain changes frominflation to deflation as the stored gas is released through new fractures within the plug that had been confining the gas
pocket, leading to a minor depressurization in the conduit. Then, an expansion process starts, that could explain
seismically the first motion of an explosion earthquake. This is probably when the effect of depressurization downward
from the crater bottom reaches the magma head and a sudden magma expansion with degassing starts. After a shortperiod (about half a second), this expanding magma rises and pushes the gas pocket upward, leading to a swelling of thecrater ground along with the radiation of the preceding phase of infrasound waves, and then a breakup occurs. After theplug fails due to deformation, the accumulated gasses and expanding magma are ejected together from the crater as thesurface eruption phenomena starts.
Key words : Vulcanian eruption, eruption process, Sakurajima volcano, Showa crater

1．Introduction
Recent interdisciplinary and multi-parametric observations are attempting to elicit in detail the volcanic eruption
processes at several active volcanoes including Stromboli,
Italy (Calvari et al., 2008) and Soufrière Hills, Montserrat(Voight and Sparks, 2010).
In Japan, Iguchi et al. (2008a) discussed the eruption
process of the summit crater of Sakurajima volcano, basedon integrated geophysical observations, as follows. The
process commences with a gradual volcano inflation,
which is thought to be induced by a combination of theeffects of the ascent of magma and the accumulation of
volcanic gas being emitted from the magma. Ishihara(1990) argued that such a gas accumulation leads to theformation of a “gas pocket” beneath the crater bottom.
The term “gas pocket” corresponds to a shallow transitionzone characterized by complex mingling between vesicularand dense magma a few hundred meters thick, which was＊

Aso Volcanological Laboratory, Institute for Geothermal
Science, Graduate School of Science, Kyoto University,
Minami-Aso, Kumamoto 869-1404, Japan.
＊＊
Sakurajima Volcano Research Center, Disaster, Prevention Research Institute, Kyoto University, 1722-19, Sakura

mentioned as occurring at the Soufrière Hills volcano(Burgisser et al., 2011). This zone is covered by a dense
and strongly degassed plug a few tens of meters thick.
When the accumulated pressure inside the gas pocket
exceeds the strength of the plug as gas accumulates, afailure of a tiny part of the plug, such as a fracture within
it, may occur (Lyons et al., 2012). Then, a minor amountof gas will start escaping into the atmosphere. This
leakage of gas induces a pressure decrease in the uppermost part of the conduit as well as within the gas
pocket. It has been recognized in the strain change 1.5
min before an eruption (Iguchi et al., 2008a). The depressurization in the conduit can then propagate deeper into thevolcano as an expansion wave (Turcotte et al., 1990). When
this happens, a sudden outgassing of water-saturated magma
occurs at a certain depth, which corresponds to the onset ofan explosion earthquake in the conduit around 2 km beneath Sakurajima (Ishihara, 1985; Tameguri et al., 2002).
jima Yokoyamacho, Kagoshima 891-1419, Japan.
Corresponding author: Akihiko Yokooe-mail: yokoo@aso.vgs.kyoto-u.ac.jp

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Akihiko YOKOO, Masato IGUCHI, Takeshi TAMEGURI and Keigo YAMAMOTO

An additional process of expansion occurs at the top of the
conduit (〜0.5 km deep) 1 s after the onset of the explosion
earthquake. During this second, a pressure wave is believed to propagate in the conduit from the source to the
explosion earthquake to the shallow expansion source, at a
propagation speed of 1.4-1.9 km/s (Tameguri et al., 2002).
This secondary effective expansion suggests that the gas
pocket starts to push up both the plug (the bottom of thecrater; Yokoo et al., 2009) and the rock of the conduit walls.
Upon the disruption of the plug, a strong air shock wave
is radiated by release of the built-up gas pressure. The
instantaneous (〜1 s) opening of the crater bottom necessarily leads to an abrupt but shallow pressure decrease(strain-step; Ishihara, 1990). Simultaneously, a mixture offragmented magma and volcanic gas starts to be emitted
from the conduit as an eruption cloud. This causes the
local ground deformation observed as volcano deflation.
Some geophysical and visual observations suggest theexistence of a pressurized gas pocket (a highly viscous andhighly pressurized region confined by a plug and the
conduit walls) that disrupts at the time of eruption, indicating it was formed prior to the eruption. Pressure
accumulation is thought to play an important role inprecipitating the occurrence of an eruption (Burgisser etal., 2011; Gottsmann et al., 2011). However, there havenot been sufficiently clear manifestations of evidence toallow us to understand the processes involved in theformation (and disruption) of a gas pocket with a correcttime scale compared with the accumulation of the magmapressure. Although the storage of pressurized volcanic gas
within the gas pocket prior to an explosive eruption
necessarily induces gradual and relatively local inflation
around the crater, we have not yet been able to uniquely
identify this process in observed records of the summit
crater eruptions. One possible reason for this is that geodetic records at commonly distant stations, without those atnearer dangerous stations, are more sensitive to the large
inflation induced by deeper magmatic sources (apparently,
magma migration itself) than to the tiny signals that stemfrom the gas accumulation process (Ishihara, 1990).
Basically similar processes are thought to occur during
eruptions in the newer Showa crater of the Sakurajima
volcano (the Showa crater is described in the next section).
The aim of this paper is to present clear evidence that will
help to elicit the dynamic processes of gas pocket regions,taking the Showa crater eruptions as an example. First, we
describe the activity of the Showa crater in Section 2, andour observation network targeting Showa crater eruptionsin Section 3. Next, we detail the shallow processes thattake place prior to eruptions observed by the observation
systems in Section 4, and finally, in Section 5, we discuss
the entire eruption process and its dynamics in the Showa
crater by making a comparison with eruptions of thesummit crater.

2．Eruption activities at Sakurajima volcano
Sakurajima volcano, located in the southern part ofJapan, is known for its repeated small, mild vulcanianeruptions (VEI 0-1). Numerous eruptions have occurredin the summit crater since 1955, and they are generallyaccompanied by strong air shocks, the ejection of volcanic
bombs, and a large amount of volcanic material (Ishihara,1985). The recorded amplitude of an air shock that occurred in the 1980s exceeded 500 Pa at the station, situated
2.7 km from the summit crater (HAR; Fig. 1a). At the sametime, meter-sized volcanic blocks occasionally reachedresidential areas 2.5-3.5 km to the south of Sakurajima.
The total mass of ash emitted from 1978 to 2000 amounts
to approximately 2×108 tons (Ishihara, 2000). Anotherremarkable characteristic of Sakurajima vulcanian eruptions is a cyclic inflation-deflation pattern of grounddeformation associated with each eruption (Ishihara, 1990;
Iguchi et al., 2008a). This cycle has come to be used toforecast eruptions as an inflationary radial tilt is observedfor periods of 10 min to 7 h prior to eruptions, enabling
automated warnings to be issued (Kamo and Ishihara,1989).
Although the total number of explosive eruptions had
reached 7902 at the end of 2009 (Fig. 2), eruption activity
at the summit crater suddenly abated in 2003. Only a few
explosions per year have been observed since then. In
contrast, in June 2006, the Showa crater on the eastern
flank of Sakurajima reactivated after a 58-year silence(Yokoo et al., 2007; Yokoo, 2009) (Fig. 1). Eruption
activity in the Showa crater is thought likely to have
replaced activity in the summit crater in recent years (Fig.
2).
The initial reactivations at the Showa crater in 2006 and2007 were relatively minor, with only intermittent small
emissions of ash accompanied by ash clouds a few
hundred meters high. The activity has gradually becomemuch more vigorous since 2008, and eruption clouds haverisen to a height of 1-4 km above the crater. Pyroclastic
density currents (PDC) have sometimes been observed
reaching <1.5 km from the crater (Figs. 1b and 1c). The
diameter of the crater is also gradually growing, reaching
about 300 m in March 2010. The annual number of
explosive eruptions at the Showa crater reached 984 at theend of November 2010 (Fig. 2). This number of events isdouble the maximum number at the summit crater in 1974
(489 events), although the scale of the eruptions at theShowa crater is much smaller than the summit crater
eruptions in the 1970s to the 1980s (Iguchi et al., 2010).
3．Observation network
The Sakurajima Volcano Research Center (SVRC) of
Kyoto University has installed a multi-parametric observation network around Sakurajima volcano, spurred by therecurrence of eruptions from the Showa crater. Fig. 1ashows SVRCʼs permanent stations.

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Fig. 1. (a) Observation network at Sakurajima volcano. The four open and seven closed circles denote short-period andbroadband seismometer stations, respectively. The four triangles indicate stations equipped with infrasound sensors.
Video cameras are set up at the six stations shown by open squares. Four of them, NOJ, KAI, KRR, and KRL, areoperated by the Japanese Ministry of Land, Infrastructure, Transport and Tourism. KUR is also equipped with an
infrared thermal camera (closed square). Tiltmeters and extensometers are installed in underground tunnels at both HAR
and ARM (crosses). The thick gray line along the coast denotes the route of traverse measurements of SO2 emission rate.
The hatched area on the southeastern flank of the volcano is the reference region of Fig. 8. (b) Aerial photograph aroundthe southeastern flank of Sakurajima taken on April 10, 2009, with the support of Kagoshima Prefecture. The lightcolored area around the Showa crater, surrounded by a broken line, is covered with deposits from pyroclastic density
currents (PDC) from the day before the aerial photography flight. (c) An eruption of Showa crater on April 9, 2009,
showing a dense PDC covering a distance of about 1 km. This photograph was taken from a position near ARM.

At opposite sides of the volcano (ARM and HAR), twocomponent water-tube tiltmeters and three-component
extensometers constructed from super inver-bars havebeen installed in underground tunnels. The signals fromthese sensors are sampled at 1 Hz. As the ground tilt andstrain records at both stations are generally synchronizedwith variations in sea level, as already reported by Ishihara(1990), both records are corrected using BAYTAP-G(Tamura et al., 1991) and the actual tides recorded at thesouthern part of Sakurajima, as noted in Tateo and Iguchi(2009). Note that ARM is operated by the Japanese
Ministry of Land, Infrastructure, Transport and Tourism(MLIT) working in collaboration with SVRC.
Visual and infrared thermal cameras (operating at 30 fps
and 1 fps, respectively) have been deployed at two
stations̶SVO and KUR (Yokoo, 2009; Yokoo et al.,2009) ̶ and are synchronized by a GPS clock. MLIT has
also established a video monitoring network with highsensitivity TV cameras and infrared thermal cameras.
Images from some of these cameras are freely available via
the Internet1, with time delays of between 0.5 min and 2
min caused by encoding/decoding procedures and network
traffic. Movies from the MLIT cameras are saved at SVO

at 1-min intervals and the time is corrected by making acomparison with our own movie records.
Short-period three-component seismometers have been
installed at 11 seismic stations. Eight of these seismometers are sited in boreholes, and the others are on the
ground surface (Tameguri et al., 2002). Seven stations of
the short-period seismometer are also equipped withbroadband seismometers. To detect infrasound waves,
low-frequency microphones have been set up at four
stations. Seismic and infrasound data are logged at 200
Hz, apart from at KUR station where infrasound is logged
at 100 Hz (Fig. 1a).
Additionally, measurements of the SO2 gas discharge
rate are occasionally conducted using miniature UV
spectrometers (Mori et al., 2007) mounted on a vehicle
driven along a road that circles the volcano near thecoastline (Fig. 1a). These SO2 measurements were taken
roughly once a week in 2008 (62 observations), once every
2.5 weeks in 2009 (20 observations), and once a month in2010 (11 observations). Fewer observations were made in1

http: //www. qsr. mlit. go. jp/osumi/camera_sabo. htm (inJapanese)166

Akihiko YOKOO, Masato IGUCHI, Takeshi TAMEGURI and Keigo YAMAMOTO

Fig. 2. Annual numbers of explosive eruptions accompanying large infrasound waves occurring at (a) the summit crater (>10 Pa at HAR or >5 Pa at SVO) and (b) Showa crater (>10 Pa at ARM). The inset diagram in (b) shows the monthly
numbers of Showa crater eruptions. The data cover eruptions counted until the end of November, 2010.

2009 and 2010̶despite the increasing eruption activity(Fig. 2) ̶because we choose to make observations whenconditions are favorable for DOAS (fair skies, strong
winds, and no ash dispersion in the air) to reduce errors in
the SO2 estimation.
4．Recorded signals
The results of our multi-parametric observations are
summarized in Table 1, with details provided below.
4-1 Prior to eruption
Ground deformation
The tilt and strain changes that accompanied the cyclic
vulcanian eruptions over three days in August 2009 areshown in Fig. 3. They were recorded in the undergroundtunnel at ARM (Fig. 1a), 2.3 km from the Showa crater.
Although they were corrected as described in the previous
section, the radial and tangential tilt changes still haverelatively large fluctuations of 40-60 nano-radians within
a half-day period (Fig. 3a) caused by tidal effects.
Although tilt changes corresponding to each eruption are
identified the same as for summit eruptions (Ishihara,
1990), the changes ̶especially in the expected inflation
stage (20-90 nrad/h at the summit eruptions; Tateo andIguchi, 2009) ̶ prior to the onset of each eruption are not
identified.
As opposed to the tilt record, characteristic strain changes
prior to the occurrence of each eruption are clearly recognizable (Fig. 3b). For instance, the radial component
shows a 48 nano-strain contraction starting 6.5 h prior toan eruption at 09 : 52 on August 6, 2009, while thetangential component shows an extension of around 39
nano-strains. At the onset of the eruption, an inverse trend
is recorded in both components; that is, the radial and tangential strains show a rapid extension and contraction,
respectively. Such patterns of inverting the trend in thetilt, and also in the strain at HAR, sited 3. 1 km in ahorizontal direction on the opposite side of the volcano(Fig. 1a), are smaller than for ARM. The geodetic signals
at HAR may be obscured not only by the distance, but also
by the complicated topography between the Showa craterand the station, which includes the summit crater with adepth >500 m.
Here we categorize typical patterns of strain change
prior to an eruption into the three stages.
i) 1st StageA radial contraction and tangential extension prior to theonset of an eruption is generally observed (Fig. 3b).
However, the duration is quite different for each vulcanianeruption event. The average duration of the signals ranged
between 4 and 16 h in May 2008 (Iguchi et al., 2008b), but
in December 2009, it decreased to only a few tens ofminutes to 1.5 h. In some cases, a very complicated strainchange, consisting of a contractions-and-extensions patternin the two components, followed by a hiatus lasting 10 min
to 3 h prior to the eruptions, were recognized. During thisperiod, no surface phenomenon such as ash emission was
visible in the movie records contrary to the case at thesummit crater where ash emission was observed during

Table 1.

Summary of observation results of processes leading to a Showa crater eruption.

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Akihiko YOKOO, Masato IGUCHI, Takeshi TAMEGURI and Keigo YAMAMOTO

Fig. 3. Tilt and strain records at ARM during a 3-day period from August 6 to 9, 2009. Six thin vertical black lines witharrows at the top of the diagram indicate the onset times of major eruptions counted routinely for Fig. 2. Numerous graycolored periods denote periods of ash venting from the crater. (a) Tilt change of radial and tangential components withrespect to the crater (red and blue lines, respectively). (b) Strain changes of radial (red) and tangential (blue)
components. Preparing patterns toward the next eruption are clearly identified for both components as indicated by theset of thick arrows.

“suspension process in tilt” (Tateo and Iguchi, 2009). In
addition, no gas emissions were observed during those
periods.
ii) 2nd Stage
The rate of strain change increased slightly from a few
tens of minutes to a few minutes prior to the onset of an
eruption (Fig. 4). The amplitudes of the rate changes were
not much larger than those of regular fluctuations in thestrain records (a few nano-straions; see Appendix), thus it
was difficult to determine the period of this stage correctlyfrom only the strain data. However, the period of anincreased rate of strain change coincided with various
surface phenomena at the crater, as described later.
iii) 3rd StageA few minutes prior to the onset of an eruption, thepattern of ground deformation inverted from contractions
to extensions in radial strain, and from extensions tocontractions in tangential strain (Fig. 5a). Fig. 5b shows ahistogram of the time differences between the timing of
this strain change and the onset of an eruption estimatedfor 179 events recorded between August and September
2009. There is a clear unimodal time difference peak a
few minutes prior to an eruption (>81.6 % from 0-5 min).
Seismograms
During the 1st stage periods of strain records, small
earthquakes were sometimes observed at HIK, one of thenearest seismic stations to the Showa crater (2.2 km; Fig.
1a). Fig. 6 shows an example of such an earthquake
sequence prior to the 01:15 eruption recorded on August23, 2009. In this case, a few minutes after ash stopped
venting from the crater (21:38-22:37), a multitude of small
earthquakes started at 22:39, averaging one event every 3.5
min (circles in Fig. 6). The spectral peak of theseearthquakes was of 3-8 Hz, so they are classified as “BHtype” earthquakes (Iguchi, 1995) in Sakurajima. Becauseof their small amplitudes (<10μm/s at HIK), they werehardly recorded at the other seismic stations except for
ARM. This makes it impossible to determine their source
locations, depths, and mechanisms.
Fig. 7 shows the vertical component of the velocity
seismogram at HIK (Fig. 1a) and raw extensometer data(the radial component at ARM) for the eruption on
September 27, 2009. There was a tremor starting at 01:38:
02, about 1.5 min prior to the eruption, and its amplitude
started to increase at around 01:38:25. The change in thetrend of ground deformation (that is, the 3rd stage of thestrain record) seems to start at the same time (verticaldashed line in Fig. 7). The spectral peak of these tremors
was around 4 Hz or less.
Video records
A volcanic glow was sometimes seen at night in the
middle of 2009. As shown in Fig. 8a, a strong volcanic
glow emerged just above the crater at 02:43 on August 27,
2009. That glow weakened gradually and disappeared(02:57), and 2 minutes after, at 02:59, an eruption rich inincandescent blocks occurred.
We obtain the time series of the changing glow strengthby using the R-channel values (8 bit; 0-255) of the RGB

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Fig. 4. Strain changes recorded at ARM on (a) August 27, (b) August 28, (c) September 2, and (d) September 27, 2009.
Shaded periods denote the times of weakening and then disappearance of a volcanic glow (Fig. 8). The broken verticallines with black arrows on the right denote the onsets of the eruptions. Thick arrows show the trend in changes of theradial component. Note that the tangential record is disordered as a contractive trend, in particular in (a), but it also
shows a changing trend.

Fig. 5. (a) Four examples of radial strain observed at ARM. The arrows represents the time when the strain changed to theinverse direction, while the eruption occurred at the time shown by the thin vertical line. (b) Histogram of timedifference between the strain change and the onset of an eruption between August and September 2009 (179 events).

color scale for two selected fields of view (“glow 1” and“glow 2” FOVs) in the movie imagery (Fig. 8a). “Reference” FOV on the southeastern flank of the volcano istaken as the reference region for the color measurement.
The corresponding area of the reference FOV is shown in
Fig. 1. Increases and decreases in the R-values indicate astrengthening or weakening in the reddish color on thedigital images, respectively.
At 02:47 (12 min prior to the onset of the eruption), theaverage R-values in the “glow 1” and “glow 2” FOV
regions started to decrease from around 250 to 140,
reaching almost the same values as the “reference” FOV
region at 02:53 (6 minutes elapsed time) (Fig. 8b). The Rvalue in the “reference” FOV was stable at 135 prior to the170

Akihiko YOKOO, Masato IGUCHI, Takeshi TAMEGURI and Keigo YAMAMOTO

Fig. 6. Seismogram of the vertical component at the station HIK lasting 3.5 h from 21 : 30 on August 22, 2009. After the
first BH-type earthquake (Iguchi, 1995) started around 22 : 40, many small earthquakes, marked by circles, occurred as
the 01 : 15 eruption approached. Note that ash was ejected from the Showa crater during the period of 21 : 48-22 : 37.

Fig. 7. Comparisons of the vertical component of seismic velocity at HIK and raw extensometer data at ARM (radial
component) for the eruption on September 27, 2009. About 1.5 min prior to the onset of the eruption, a small tremor
occurred, then growing in amplitude (vertical dashed line). The time of the start of the increase in amplitude in thetremor signal seems to correspond to the turning time of the strain data trend (thick arrows along with the data).

eruption, representing the nighttime background value on
the surface of the volcano flank. It suddenly increased to
240 at the onset of the eruption (02:59) because of thedispersion of incandescent ejecta around the crater. It thendropped back to the previous background value of 〜140in about 10 min as the ejected materials cooled down. The
period of the glow disappearance apparently corresponded
to the 2nd stage of the strain record (Fig. 4a).
Infrared movies
We compared the infrared thermal movies taken at KUR
with the raw extensometer data measured at ARM. Fig. 9ashows the temporal variations in the apparent temperature
just above the crater prior to and after the 04 : 32 eruption

on December 29, 2009. Although we can see the change
in average temperature is almost stable at 〜10℃ for the
10 min prior to the eruption in the upper diagram of Fig.
9a, at around 04:31:30, the temperature increased slightly
by 〜2℃ approaching the time of eruption (lower diagram).
Interestingly, the onset time of the temperature increase
coincides with the onset of the 3rd stage of the strain
record (lower diagram). However, there is no large difference in the thermal images I and II (Fig. 9b) nor in thevisible movies. Because pure hot gas has a very low
emissivity (<0.2) and it is not visible theoretically on thethermal image, this indicates that hot gas with vapor andmuch smaller amount of ashes started being exhausted atProcesses Prior to Outbursts of Vulcanian Eruption at Showa Crater of Sakurajima Volcano

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Fig. 8. (a) Sequential nighttime images on August 27, 2009, captured from movies taken by two high-sensitivity cameras
deployed around Sakurajima (upper: KRL and lower: KAI; Fig. 1a). Small square regions of 20×20 pixels in the first
picture are selected for evaluating the temporal change of the glow strengths. (b) Temporal change of averaged Rchannel values in three small selected regions in (a).

Fig. 9. (a) Temporal variations in apparent temperature above the crater (max./min.: light blue, average: blue) and raw dataof the ARM extensometer (radial component: red dots) for the eruption on December 29, 2009. The region analyzed forthe extraction of the temperature (10×5 pixels) is depicted by a white box in (b). The times when the trends change inboth data sets are synchronized at about 1 min prior to the onset of an eruption (lower diagram). (b) Thermal snapshotsof (I) 95 s before, (II) 35 s before, and (III) 25 s after the onset of eruption, respectively, taken from KUR.

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Akihiko YOKOO, Masato IGUCHI, Takeshi TAMEGURI and Keigo YAMAMOTO

Fig. 10. Traverse DOAS data prior to the 11 : 25 eruption
on February 6, 2008 (see also Fig. 12). Measured data
are shown by solid symbols and time-corrected dataconsidering advection by the wind (13 m/s) are shownby open symbols. Horizontal and vertical bars on themeasured data show times for each traverse trace and±25 % of measured SO2 values, respectively.

the same time as the ground deformation changed.
SO2 release
The timing of the increase in the strain rate (start of the
2nd stage) may correspond with the decrease in the releaseof volcanic gas. Changes in the SO2 discharge rate wereoccasionally observed during DOAS measurements on thesame time scale as other phenomena preceding eruptions
in a few tens of minutes to a few minutes prior to theeruption. Fig. 10 shows an example in which there is aclear decrease trend in the gas discharge rate, more or less,30 min prior to the eruption. The SO2 discharge dropped
from 2500 t/day to 1000 t/day, even although we cannot
strictly identify the onset using DOAS data as the firsttraverse took place from 10:40-10:46. A drop in the SO2
release about 20 min prior to an eruption at Sakurajima
was also measured directly by a UV camera system in
December 2009 (Kazahaya and Mori, 2010).
4-2 At time of eruption
Seismograms
Yokoo and Tameguri (2009) analyzed 81 explosion
earthquakes of the 2008 Showa crater eruptions recordedat the HAR station (Fig. 1a) and found three characteristicphases in the first three seconds of the earthquakes (thefirst two columns in Fig 11). The first part consists of the
P-wave first motion produced in a deeper location of the
volcano. The particle motion of this phase plotted on thevertical-radial motions plane (Yokoo and Tameguri, 2009)is quite similar to that of the P-phase of explosion
earthquakes associated with eruptions from the summit
crater; thus, relatively the same source depth (about 2 km
below the crater) of the summit explosion could beassumed for the Showa crater eruptions. The second
phase, showing a retrograde rotation on the vertical-radialplane, though, had not been observed in the summit crater
eruptions. The apparent travel speed of this phase isalmost the same as the first P-wave, but the duration of thisphase seems to become longer as the distance increases.
These facts suggest that this phase mainly consists ofRayleigh wave. The third phase is also composed ofRayleigh waves, in the same manner as in the LP phase of
explosion earthquakes of the summit eruptions (Tameguri
et al., 2002). These results suggest that similar mechanisms are at work in Showa crater eruptions as thesummit crater eruptions (Temeguri et al., 2002) although
the cause producing the observed second phase are notsurely unknown yet; different source mechanism or theeffect of the structures through which the waves propagate.
InfrasoundWe also observed a somewhat complicated set of
infrasound waveforms as well as the seismograms (Yokoo
and Tameguri, 2009). The last two columns of Fig. 11
show four examples of infrasound waves recorded in 2009that show a wide variation during the phase preceding thelarge amplitude onset of the shock-like main phase. Fig.
11a and 11b, respectively, are without and with the“preceding phase” (Yokoo et al., 2009), that is a weak andslow pressure increase. Fig. 11c and 11d are, respectively,
an example in which an increase in pressure started slowly
̶in a similar way to the normal preceding phase ̶butthen stopped due to the stagnation: and an example, with apulse-like pressure change. The duration of the preceding
phase (the time difference from the onset of a precedingpressure change to the onset of the main phase) has anegative relation with the amplitude of the main phase(Yokoo and Tameguri, 2009). The arrival time difference
between the preceding infrasound phase at ARM and thethird seismic wave (LP) phase at HAR is almost constantat about 4.5 s, regardless of how the type of preceding
phase varies. During the eruptions without an infrasoundpreceding phase (Fig. 11a), the 4.5 s differences were also
observed for the arrival times of the infrasound main phaseat ARM and the seismic third LP phase at HAR. This time
approximately equals the travel time difference between
infrasound wave from the crater to ARM and seismic wave
from the crater to HAR, of 6.8 s and 2.3 s, respectively.
Video records
The sequence of surface phenomena for the explosive
eruption at the Showa crater at 11:25 on February 6, 2008is shown in Fig. 12, as a typical example. Prior to theonset of the eruption, there was only a diluted volcanic gas
steam was emitted from the crater. At 11:25:55, a main
shock-like infrasound wave (Yokoo et al., 2009) suddenly
occurred, followed by a dense volcanic cloud. The formation of an instantaneous condensation cloud (phase changeof H2 O in the air from vapor to steam) occurred byadiabatic expansion and cooling due to the passage of therarefaction phase of such a strong infrasound wave, which
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Fig. 11. Seismic and infrasound waveforms for the Showa crater eruptions observed at HAR (first two columns on the left)
and ARM (on the right), respectively. Events at (a) 17 : 21 on February 3, (b) 16 : 58 on April 2, (c) 01 : 06 on March 10,
and (d) 05 : 22 on March 10, 2009. The right-hand images are close-up waveforms of each initial part with a fixed time
window of 3 sec. Labels marked 1st, 2nd, and 3rd indicate the phases as determined by Yokoo and Tameguri (2009).

period. A volcanic cloud contains numerous volcanic
bombs, but it is not easy to see them during daytime
eruptions, in contrast to the nighttime eruptions in which
many incandescent bombs are clearly recognizable preceding the volcanic cloud (Fig. 12f).
5．Discussion
5-1 Magma migration and storage
The strain change in the 1st stage is considered to bepressurization induced by magma migration within the
volcano, which is a view proposed for the summit crater
eruptions (Ishihara, 1990; Iguchi et al., 2010).
In the summit crater eruptions (Ishihara, 1990), bothradial and tangential strains were extensions, and the tilt
was crater-side-up prior to the eruptions, and both wereinverted during the eruption. The deformation pattern in
the Showa crater eruptions is similar to that in the summit
eruptions in terms of the strain change, which is inverted
during eruptions, although only the tangential strain isextensional while the radial component is in contraction(Figs. 3 and 5). This difference can be explained by different source depths. The radial-to-tangential strain ratio(β=εr /ετ) and the distance from the source to the stationr are related to the source depth d in the most simple way

by an inflating sphere in a homogeneous elastic half space(Mogi, 1958):
×r.
 2+β
1−β

d=

In the summit eruptions, β was positive and the pressuresource was estimated to be located 2-6 km beneath thevolcano, with a suggested volume change of 103−106 m3(Ishihara, 1990). On the other hand, in Showa crater
eruptions, the negative β gives a smaller d. For example,
the source of the 09:52 eruption on August 6, 2009 (Fig. 3)
was estimated to be 1.2 km beneath the crater. The depthsof almost all the explosions in the last half of the year 2009
range from 0 km to 2 km with a mean distribution around 1
km (Iguchi et al., 2010). This result suggests that thepressure for a Showa crater eruption is concentrated in amuch shallower portion of the conduit than for the
Sakurajimaʼs main magma chamber, which is 2-6 km deep
(Ishihara, 1990). This is a reason that the tilt prior to theeruption was small as same as the background variation(Fig. 3a). We, however, should note that neglecting
topography in this simplest approximation could cause thedepth of the pressure source to be overestimated (Williams
and Wadge, 1998). The actual depth of the pressure174

Akihiko YOKOO, Masato IGUCHI, Takeshi TAMEGURI and Keigo YAMAMOTO

Fig. 12. (a-e) Screenshots of the 11 : 25 eruption on February 6, 2008, obtained by the movie system at KUR. The
condensation cloud caused by the passage of a strong shock-like infrasound wave (rarefaction phase) shown by a circlein (b) has returned to its original appearance in (c), which resembles (a). (d-e) Outburst growing volcanic cloud withnumerous ballistic bombs at the head of the cloud. (f) Typical image of a nighttime eruption (23 : 18 on October 14,2009). The three letters on each image are the names of the MLIT camera sites (Fig. 1a).

source preceding the eruption may be far shallower thanthe estimated value.
During summit eruptions in 1990s, small earthquakes
with peak frequencies in the 3-8 Hz band, classified as
BH-type earthquakes, occurred 2. 5-3. 5 km beneath thecrater in association with ground inflation (Iguchi, 1995).
Multitudes of small BH-type earthquakes have beenargued to be produced by the migration process of magma
moving into the lower part of a conduit. Because thismagma migration leads to an accumulation of stress in thewall rock around the conduit, some brittle failures such as
VT earthquakes were assumed to occur. This type ofearthquake is also sometimes observed in the 1st stage of
the Showa crater eruptions (Fig. 6).
On the other hand, we rarely observed LP (long-period)
earthquakes with low peak frequencies of around 1-3 Hz(“BL-type earthquakes” in Sakurajima; Iguchi, 1995)
during this inflation period associated with Showa crater
eruptions. The BL-type earthquakes are tied to volumetric
expansion caused by the formation of bubbles within themagma (Iguchi, 1995). One speculative explanation for
the absence of BL-type earthquakes in Showa crater eruptions would be that no effective bubble formation occurs
during this stage because intruding magma maintains
relatively high pressures within the conduit due to thehigher ascension speed of the magma than for the summit
eruptions.
As described above, it is thought that the grounddeformation associated with BH-type earthquakes in the
1st stage of the strain change is produced by magma
migration in a volcano to a shallow location of about 1 km
deep. However, in some cases, ground inflation ceases
and stagnates without any kind of surface phenomenon as
observed during the summit eruptions (Tateo and Iguchi,2009), e.g., ejection of volcanic materials. This means thatthe stagnation period of the strain change prior to theeruption is not due to a balance between the input and theoutput of the mass of magma through the crater. The
intrusion of magma stopped and it stayed in situ more orless in a stable condition. In other words, there would

Processes Prior to Outbursts of Vulcanian Eruption at Showa Crater of Sakurajima Volcano

already be a cap covering the crater that would bear downon the magma in the conduit to prevent an eruption. For
that matter, we posit that the pressure accumulation
process caused by magma intrusion occurs in preparation
for the next eruption, but it is not the process that triggers
the onset of eruption itself, as an increase in pressure alone
is not sufficient to lead to an immediate eruption. The
mechanism that directly triggers the eruption, releasing the
intruded magma, is considered to be another phenomenon.
5-2 Crater bottom sealing process and formationof gas pocket
The 2nd stage is defined by the increase in the strainchange (Fig. 4). Because the change overlapping thestrain trend has the same characteristics as the 1st stage
strain change (radial contraction and tangential extension),
it is inferred that the ground deformation in this stage
includes an additional inflation source at a shallow depth.
An important observation is that a volcanic glow weakens
and then disappears at the beginning of the strain 2nd stage(Fig. 8). It is considered that the crater glow observed atnight is simple evidence of the presence of incandescenthigh-temperature materials in the shallow portion of thecrater (Yokoo, 2009), like a lava dome/pond. The weakening and disappearance of the volcanic glow indicates either
the sealing of a shallow conduit and the cooling of incandescent rocks (or molten lava) inside the crater, nor a
deepening of the magma level in the conduit. However,the latter possibility can not explain the additional inflation
source, thus rejected here. Besides, it has been reported
that the SO2 discharge rate as measured with a UV camera
gradually decreases for 20 min prior to an eruption(Kazahaya and Mori, 2010), that is same time range as forthe 2nd strain stage. The decrease in gas release can also
be explained by the process of the sealing of the gas
pathway in the shallower portion of the conduit. The heat
supplied by the volcanic gas into the atmosphere decreases
and eventually ceases, which explains the gradual decreaseof the apparent temperature above the crater observed in
the infrared images (Fig. 9a). A decrease in the thermalradiance of the crater walls has also been observed at
Stromboli 1 hour before the violent explosive paroxysm ofApril 5, 2003 (Ripepe and Harris, 2008).
Details of the sealing process that occurs at the Showa
crater are still poorly understood. However, recent experiments (Otsuki and Nakamura, 2012) may be used to
explain the processes in the 2nd stage. They have demonstrated that a permeable melt foam with deformed
bubbles may change into a melt with isolated spherical
bubbles due to the surface tension of a melt in a relatively
short time (<30 minutes), which suggests that this canresult in the formation of impermeable lava caps in thecrater in the same time scale. As this kind of sealing
process proceeds, gas consequently accumulates in theuppermost part of the conduit, which is plugged by animpermeable magma that is cooling. This leads to the175

formation of a gas pocket beneath the plug, which generates an overpressure, detected as an inflating deformation
signal.
We can also estimate the source pressure of the gas
pocket using these overlapping strain data, assuming thesource is a sphere of 10 m radius and 109 Pa for Lameʼscoefficient in the Mogi model (Mogi, 1958), to be 0.5×
107 to 7×107 Pa. These pressures are almost the sameorder but slightly higher than relatively larger examples ofthe summit explosions estimated by a shock-tube model, 1.
4×107 to 2.8×107 Pa (Ishihara, 1985). If the source size
is 3×104 m3 equivalent to a cylinder with a 100 m lengthand a 10 m radius, the source pressure becomes one-order
lower value, 1×106 to 9×106 Pa, which sounds reasonable in terms of the small scale of Showa crater eruptions(Iguchi et al, 2010).
Processes similar to those in the 2nd stage of the strainchange at Sakurajima have been observed in other volcanoes (e.g., Ripepe and Harris, 2008; Genco and Ripepe,
2010; Bonaccorso et al., 2012), but in different time scales.
Hirabayashi et al. (2005, 2007) detected a decrease in the
discharge rate of SO2, 3 min prior to explosions at the
Suwanosejima and Semeru volcanoes. They interpreted
this as gas entrapment below the cap beneath the crater, but
without accounting for why the shallow part of the conduit
seals to prevent gas escaping. Upward ground displacement and tilt indicating shallow inflation beneath thecrater, have been detected 1-2 min and 3-30 min prior to
the explosions at the Suwanosejima and Semeru volcanoes,
respectively, and these inflation-related deformations arethought to represent the accumulation of volcanic gas in
the upper part of the conduit (Iguchi et al., 2008a).
At Asama volcano, which is an andesitic volcano whose
activity closely resembles that of Sakurajima, the internal
structure of the volcano was estimated by using theleading-edge Muon tomography method (Tanaka et al.,2010). Tomography shows a region with low bulk density(0.8±0.2 g/cm3, implying a porosity of 25-45 %) beneaththe crater floor covered by an extruded lava dome. Thisevidence is not directly comparable to the suggested gas
pocket we infer as being formed below the crater floor of
Sakurajima, but the presence of this lower-density region
just beneath the dense lava plug at the vent is very
important.
5-3 Gas and pressure leakage
Strain with radial contraction and tangential extension isinverted during the 3rd stage, a few minutes prior to
eruptions (Fig. 5). The inverted trend of the strain record
is interpreted as the pressure decreasing in the shallow
portion of the volcano. Infrared movie observation showsa slight temperature increase above the crater during thisstage (Fig. 9). The shallow decompression and the temperature increase could be produced by volcanic gases leaking
from the gas pocket into the atmosphere.
A small-amplitude tremor with a 4 Hz spectral peak

176

Akihiko YOKOO, Masato IGUCHI, Takeshi TAMEGURI and Keigo YAMAMOTO

emerged a few tens of seconds prior to the strain 3rd stage(Fig. 7). We assume that this tremor is produced by thefracturing of the magma lid because the accumulated gas
pressure beneath the crater exceeded the threshold level ofthe strength of the material. The amplitude of tremor starts
to grow at the time of the onset of the 3rd stage (verticaldashed line in Fig. 7). An amplitude increase might
indicate an increase in the flow volume of the gas into thefractures, or the progression of the fracturing process.
The effect of the decompression of the gas pocket due tothe gas leakage propagates deeper into the conduit. This
thus induces the start of degassing from both dense andvesiculated magma in the mingling zone beneath the gas
pocket. When the effective decrease in pressure reaches
the head of the water-saturated dense magma body, it
culminates in a sudden increase in the total volume of the
mixture of (fragmented) magma and the exsolved gases.
This expanding process may correspond to the onset of
explosion earthquakes (Tameguri et al., 2002). We canassume the relatively resemblance of the source depth of
the Showa crater eruptions to the same as the case of thesummit eruptions, about 2 km below the volcano (Tameguri
et al., 2002), because the source direction inferred from theparticle motion of the first phase of explosion earthquakes
is similar (Yokoo and Tameguri, 2009). Considering thisseems slightly deeper than the depth of pressure accumulation, around 1 km on average, estimated from the strain
record, it may be necessary to determine details of thesource processes and depths by a more precise analysis of
explosion earthquakes.
5-4 Failure of the crater ground
Upon the sudden out-gassing and fragmentation of themagma, a pressure wave is radiated toward the surface inthe conduit (Tameguri et al., 2002; Iguchi et al., 2008a).
At the same time, the expanding mixture of magma andgases moves upward. This induces a lower confiningpressure of the mixture responsible for the exsolution of
the water from the magma as they migrate upward. The
total volume of mixtures, then, increases.
When the mixture of fragmented magma and exsolved
gases reaches the shallower gas pocket, the sealing plug
starts to swell to failure, as proposed by Yokoo et al.
(2009) based on the results that the shallow expansion
phase (LP phase) of the explosion earthquake and thepreceding phase of infrasound wave were produced at
almost the same time (Yokoo and Tameguri, 2009). The
upward motion of the plug surface is considered to be
represented by the infrasound preceding phase. The
upheaval volume of the plug just before bursting, butduring the time of radiating preceding infrasound, isestimated from its waveform to be around 250 m3 for the
11:25 eruption on February 6, 2008, based on Yokoo et al.
(2009). This is the largest value in the past three years.
Almost all cases are less than 100 m3. These values areabout the same size as for the eruption at Suwanosejima

volcano (Yokoo and Iguchi, 2010), which suggests that thevent size of the Showa crater is much smaller than that of
the summit crater (〜20 m; Ishihara, 1990). This was
confirmed by aerial photographs taken in May 2011 by theJapan Meteorological Agency, as being 〜5 m. However,this 5 m is half the size of the pressure source we assumed
in the previous section (10 m radius of a cylinder with a
100 m thick). It suggests that size of the conduit would
taper near the surface.
When the plug fails, the strong main phase of an
infrasound wave is radiated together with the release of
mixtures of high-pressurized expanding gas and magma
fragments (Figs. 11 and 12). At the summit eruptions,
Tameguri et al. (2002) found that the amplitude of the LPphase of an explosion earthquake and that of the main
phase of infrasound waves are positively related: however,no such good correlation is seen in the Showa crater
eruptions (Iguchi et al., 2010). The amplitude of the main
phase of infrasound was compared with the duration of thestrain 3rd stage, during which the gas pocket wasconsidered to be formed, but no clear correlation was seen(Fig. 13a). The amplitude of the main phase of infrasoundalso has almost no positive correlation to the amplitude of
the strain changes during the 2nd stage (Fig. 13b). These
two results mean that accumulated gas in the gas pocket,
which induces acceleration in the strain change inapparently the 2nd stage, is not directly related to theintensity of the radiation of the main phase of infrasoundwave. Which parameters control the intensity of theinfrasound radiation? An additional observed fact, thenegative relation of the amplitude to the duration of thepreceding phase (Yokoo and Tameguri, 2009), may haveto be considered. The deformation process of the lid will
become a good indicator in accounting for the observed
amplitude of the main phase, which is suggested by anexperimental approach of a bubble bursting (Kobayashi etal., 2010).
5-5 Time sequence of Showa crater eruptions
The time series of recorded signals and their characteristics as obtained from our multi-parametric geophysical observations are very instructive in understanding theeruption processes at the Showa crater of Sakurajima volcano. Here, we summarize the processes preceding theonset of an eruption as shown in Fig. 14, focusing inparticular on the processes forming and destroying a gas
pocket beneath the crater.
A few hours prior to the onset of an eruption, magma
starts to migrate and be stored at depths of around 0-2 km(Fig. 14a). This is clearly recognizable in the record ofstrain change as an inflation process. Starting a few tensof minutes prior to the eruption, the SO2 gas emission rate
gradually decreases, indicating that a sealing process isprogressing at the crater bottom as the eruption approaches(Fig. 14b). A few tens of minutes to a few minutes prior to
the eruption, the rate at which the volcano is inflating starts

Processes Prior to Outbursts of Vulcanian Eruption at Showa Crater of Sakurajima Volcano

177

Fig. 13. (a) Relationship between periods of the 2nd stage of strain change when the volcanic glow disappears just prior tothe onset of an eruption, as identified on the MLIT video network, and the excess pressure of infrasound waves at ARM(40 nighttime events from August 2009 to January 2010). (b) Excess infrasound pressures corresponding to inflating
amplitudes at ARMʼs radial strain (32 events from August to December 2009).

to accelerate due to the construction of a plug at themagma surface, which causes the formation of a gas
pocket beneath the crater (Fig. 14c). This process is also
observed as the weakening and then disappearance of a
volcanic glow during nighttime events. A few minutes
prior to the eruption, a small tremor starts to emerge (Fig.
14d). Its amplitude grows, and the inflating strain changes
to a deflating process. This is thought to occur as gas isreleased through newly formed fractures within the plug.
Seismic signals show that dynamic processes resembling
the eruptions at the summit crater also occur during Showa
crater eruptions (Fig. 14e). Expanding magma then rises
and pushes the gas pocket up, which leads to a swelling of
the crater ground, and a phase of radiating weak infrasoundwaves. After the failure of the plug due to its deformation,
the accumulated gasses and expanding magma itself areejected together from the crater, following the ejection of
volcanic bombs at the start of the eruptionʼ s surface
phenomena (Fig. 14f).
The model described in this paper still contains much
speculation, especially in the process relating to theshallow decompression due to leakage, explosion earthquakes at depth, and the swelling and failure of the plug.
We think these processes will become important clues fora better understanding of the Showa crater eruptions.
However, these subjects are left for a future work because
the main aim of this paper is to present observed evidences
related to the formation of a gas pocket within a time scale.
6．Conclusions
From our multi-parametric observations at Sakurajima
volcano, Japan, typical processes that occur prior to the
outburst of explosive eruptions at Showa crater have been
revealed and the following conclusions were reached:(1) Magma starts to migrate to a depth of 0-2 km

beneath the crater and is stored there for a few hours prior
to eruption. This is clearly identifiable in strain records
with both radial contraction and tangential extension
changes. This magma intrusion induces the occurrence ofsmall earthquakes around the conduit.
(2) A few tens of minutes to a few minutes prior to
eruption, a decrease in the SO2 discharge rate and an
acceleration of volcano inflation are observed. During
nighttime eruptions, a weakening and then disappearanceof the volcanic glow is seen. This is due to the formationof a gas pocket sealed by a consolidating lava plug beneaththe crater.
(3) Once the gas amassed in the gas pocked attains
sufficient pressure to overcome the threshold value for
deforming the lid, fracturization occurs within the plug, so
minor amounts of gas and pressure leak from the gas
pocket into the atmosphere. This induces volcano contraction and produces small tremors a few minutes prior tothe onset of an eruption.
(4) An explosive eruption, accompanied by the radiationof infrasound waves, follows the explosion earthquake at
the Showa crater as well as in the case of summit crater
eruptions at Sakurajima volcano. However, not all of thesource mechanisms are described in this paper due to lackof data, so further work should be done in the future.
Acknowledgments
We are grateful to all the members of the Sakurajima
Volcanological Observatory for their support of our research. Geophysical observations carried out at ARM were
performed as collaborative work by Osumi Kasen Kokudo
Jimusho, MLIT, and SVRC, DPRI of Kyoto University.
M. Ichihara, two anonymous reviewers, and the editor T.
Nishimura are greatly acknowledged for their commentsthat improved the quality of this paper. This research is

178

Akihiko YOKOO, Masato IGUCHI, Takeshi TAMEGURI and Keigo YAMAMOTO

Fig. 14. Sequential schematic images of processes occurring prior to the onset of an eruption. (a) A few hours prior to
eruption. (b-c) A few tens of minutes to a few minutes prior to eruption. (d) A few minutes prior to eruption. (e) Half asecond prior to eruption. (e) Onset of an eruption. (g) After the onset of an eruption. Vertical and horizontal scales inthese images are not shown. See text for details.

Processes Prior to Outbursts of Vulcanian Eruption at Showa Crater of Sakurajima Volcano

partially supported by JSPS Research Fellowship AY (No.
19·126) and Tohoku Universityʼs GCOE program “Global
Education and Research Center for Earth and Planetary
Dynamics” (PI: E. Ohtani).
Appendix: Fluctuations in ground deformation record
Differing from the obvious ground deformations caused
by magma migration, strain fluctuations of only a few
nano-strains are clearly recognizable in all strain change
records. We considered the meaning of these small perturbations by comparing them with eruptive surface
phenomena. Average luminance data for two regions at
the crater and an area nearby are extracted to retrieve thetime sequence of the surface phenomenon.
By this method, eruptions with ash venting are clearly
identified by the peaky negative changes in the differencesof average luminance in these two regions (Figs. A1a andA1b). A set of oppositely directed strain change patterns
in the radial and tangential components at ARM are almost
always observed during these periods of ash venting (Fig.

179

A1c). This indicates that ejection of ashes and gases
doubtlessly causes a release of pressure from the volcano,inducing small changes in the strain record.
However, many fluctuations with the same order ofamplitude as ash emissions are also observed without
significant surface phenomenon occurring, such as in the
period prior to the eruption (08 : 00-09 : 40 on September
18, 2009). This means that these small changes areoccasionally affected by certain processes occurring within
the volcano. Small fluctuations might be caused by aprocess of pressure recovery due to the depressurization of
the magma conduit (e.g., ascending magma; Nishimura,2004). It is, therefore, difficult to distinguish why suchsmall strain changes (of the order of few nano-strains) may
be eruptive or non-eruptive without surface observations(the process works only underground). It should be notedthat peaky changes in luminance around the period from08:40 to 09:25 (Fig. A1a) were caused by a dappled shadefrom clouds.

Fig. A1. (a) Difference in luminance between two selected regions in the movie image (1 and 2 in (b)) on September 18,
2009. Higher value of the difference means brightening in the crater area. The shaded periods denote times of ash
venting, thus lower luminance values are calculated. Times denoted by dots at (I) 08 : 10, (II) 08 : 56, and (III) 09 : 49
correspond to the images in (b). (b) Images taken by camera at KRL (Fig. 1a). Two regions, the crater (1) and thereference region (2), are for evaluating changes in luminance. The size of each region is 50×20 pixels. (c) Strainchanges in each component at ARM during these times.

180

Akihiko YOKOO, Masato IGUCHI, Takeshi TAMEGURI and Keigo YAMAMOTO
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(Editorial handling Takeshi Nishimura)

桜島昭和火口におけるブルカノ式噴火の前駆プロセス
横尾亮彦・井口正人・為栗健・山本圭吾桜島において多項目地球物理観測を行い，昭和火口のブルカノ式噴火発生に至る一連のプロセスについて明らかにした．火口下 1 km の深さにマグマが貫入することによって，噴火の数時間前から山体が膨張する．
噴火開始の数 10 分前には，火口底にプラグが形成されて脱ガス通路の閉塞が進行し，火口直下に火山ガスの溜まりが形成される．二酸化イオウ放出率の減少や山体膨張率の増加として観測されるほか，夜間であれ
ば，火映が微弱化した後に消滅する．噴火数分前になると極小微動が起こり始める．プラグに亀裂が形成され，そこを通って火口底下に溜まっていたガスの放出が始まることに対応する．その後，放出されるガスが多くなることで，微動の振幅が大きくなる．山体膨張から収縮に転じ，火道内では小規模な減圧が誘引される．減圧の影響が水に飽和したマグマ深度まで到達すると，急激なマグマ発泡，破砕が始まり，爆発地震が発生する．破砕したマグマは火道内を上昇し，火口直下にあるガス溜まりを押し上げる．そのため，爆発地
震の発生開始からおよそ 0.5 秒後には，ガス溜まりの上方にある火口底プラグが隆起して，空振先行相を励起する．プラグの変形が進行して破壊に至った後は，火道内を上昇してきた破砕マグマが火山噴煙として放出される．