Bull. Volcanol. Soc. Japan
Vol. /, (,**1) No. 0, pp. ,23ῌ-*3

ArticleVolcano-tectonic Earthquakes during the Stage of Magma Accumulation
at the Aira Caldera, Southern Kyushu, JapanSri HIDAYATIῌ, Kazuhiro ISHIHARAῌῌ and Masato IGUCHIῌῌ
(Received, December ,*, ,**0 ; Accepted, September +., ,**1)
At Sakurajima volcano, activity of volcano-tectonic (VT) earthquakes has gradually increased since ,**,, as
the inﬂation of the Aira caldera progressed since +33-. In particular, VT earthquakes SW o# the volcano and in
the caldera swarmed during November ,**-ῌFebruary ,**., and then a baseline of GPS across the volcano
indicated a signiﬁcant extension. However, no signiﬁcant increase in eruptive activity was observed. Location andfocal mechanism of VT earthquakes, which occurred during +332ῌ,**/, are determined and the relationshipbetween the seismic activity of VT earthquakes and volcanic activity is discussed. The VT earthquakes originatedin an elongated zone extending in NE-SW direction, namely from the NE part of the Aira caldera throughSakurajima volcano to SW o# the volcano. This zone coincides with active tectonic zone. The focal zones of VT
earthquakes are distinguished into three regions : (+) * to . km deep beneath the summit of Sakurajima, (,) 0 to
3 km deep SW o# the volcano, and (-) . to +. km deep in the Aira caldera. At the summit area, VT earthquakes
occurred mostly south of the crater and a few located north, and very few VT earthquakes were found inside thecrater. Focal mechanism of VT earthquakes beneath the summit area has variation with depth. Reverse fault typewas dominantly obtained at depths from * to , km. Meanwhile at deeper portion of ,ῌ. km, strike-slip fault typeis predominant. In contrast, most of VT earthquakes SW o# Sakurajima indicated uniquely normal fault type withhorizontal T-axes oriented to WNW-ESE direction. Whereas the mechanism of those in the Aira caldera is
strike-slip types which one of the nodal lines oriented in NE-SW direction. These are consistent with regional
stress ﬁeld and direction of depression zone inferred from geological study. A hypothetical model adding a tensile
fault to dual Mogi’s pressure source is proposed to examine the relation of inﬂation of Aira caldera and thefollowing seismicity SW o# the volcano.
Key words : Sakurajima volcano, volcano-tectonic earthquake, ground deformation, magma supply system+. Introduction
Various types of earthquakes are observed at active
volcanoes. From the nature of waveform, Minakami(+31.) classiﬁed volcanic earthquakes into four types :A-type, B-type, explosion earthquakes and tremors.
A-type earthquakes usually originate beneath the volcanoes at the range of depth from + km to ,* km. Their
waveform is similar to shallow tectonic earthquakes,
and P- and S-phases of seismic waves are clearly recognized. On the other hand, B-type earthquakes occur
at shallow depth and their S-phases are not clear. On
the basis of source mechanism, Latter (+32+) reclassiﬁed
earthquakes observed at Ruapehu and Ngauruhoe volcanoes into two groups, “volcanic” and “tectonic” types.
“Volcanic” type usually has emergent onset with poorlydeﬁned phases and originates in partially molten material by some extended source mechanism. This category

includes B-type, explosion earthquakes and tremor in
the Minakami’s classiﬁcation. “Tectonic” type is characterized by sharp onset and well-deﬁned phases. A term
“volcano-tectonic” is proposed to distinguish between
earthquakes of “tectonic” type originate on or beneathvolcanoes and those which occur at some distance.
“Volcano-tectonic” type takes place in competent rockas a result of some instantaneous source mechanism.
This category covers Minakami’s A-type. The terminology of volcano-tectonic (VT) earthquakes has been widelyused instead of A-type earthquakes. In the present
study, the terminology of volcano-tectonic earthquakes
will be used instead of A-type earthquake, althoughA-type has been used at Sakurajima volcano.
An increase in VT earthquake activity is often an
early sign of volcanic unrest and potentially leading toeruption. At Merapi volcano, occurrence of VT earth-ῌ Center for Volcanology and Geological Hazard Miti-+.+3, Japan.

gation, Jl. Diponegoro /1 Bandung .*+,,, Indonesia.
ῌῌ Sakurajima Volcano Research Center, DPRI, Kyoto
University, Sakurajima-Yokoyamacho, Kagoshima 23+ῌCorresponding author : Sri Hidayati
e-mail : ichi@vsi.esdm.go.id

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Sri HIDAYATI, Kazuhiro ISHIHARA and Masato IGUCHI

quakes preceded the increase in Multiphase (MP)events which are related to formation of lava dome
(Hidayati et al., +332 ; Ratdomopurbo, +33/). Increase
in VT earthquakes and their hypocenter migration
toward the summit were also observed prior to +33*ῌ
+33. eruption at Unzen volcano (Umakoshi et al., ,**+).
However, increase in VT earthquakes is not necessarily
accompanied with eruption. For examples, Guntur
volcano, Indonesia (Suantika et al., +332) and Mt. Vesuvius, Italy (De Natale et al., ,***) have experienced
several signiﬁcant increases in VT events during theirlong-time dormant period, but eruptions have not takenplace yet since +2.1 and +3.., respectively. Thus,
relationship between VT earthquakes and the dynamics
of volcanoes is still unclear. Therefore, VT seismicity
could be a long-term sign of potential unrest as well asa good short-term indicator (Chouet, +330).
Seismic events originating at Sakurajima volcano havebeen conventionally classiﬁed into A-type, B-type, explosion earthquakes and C-type tremor. A-type earthquakes, namely VT earthquakes have often preceded
swarms of B-type earthquakes and the following repeatof explosive eruptions (Kamo, +312). The swarm ofB-type earthquakes is inferred to be related to extrusionof lava to the summit crater (Nishi, +32.). VT earthquakes have originated mostly beneath the summit andoccasionally in a deeper zone SW o# the volcano (Nishi,+312). The foci of VT earthquakes occasionally migrated from deep SW o# to shallow part beneath thevolcano before eruptive activity increased (Kamo,+312). This suggested a magma pathway from SW of
Sakurajima. In addition, ground deformation studies
have indicated magma pathway from the Aira caldera,
north of the Sakurajima (Eto, +301 ; Mogi, +3/2 ;Yokoyama, +320 ; Yoshikawa, +30+). Based on seismological and ground deformation studies, Kamo (+323)
proposed a model composed of , magma paths to explain the volcanic process at Sakurajima volcano. Oneis from the northern part beneath the Aira caldera andthe other from the southern part which is inferred byhypocenter migration of VT earthquakes from deep SW
o# to shallow part beneath the volcano.
Locations of VT earthquakes beneath the summit of
the volcano are separated from those of the other typesof volcanic earthquake ; BH-type, BL-type and explosion earthquakes (Iguchi, +33.). VT earthquakes enclose the hypocentral zone of the other types of volcanic
earthquakes and the zone suggests a vertically elongatedmagma conduit (Ishihara, +33*). Iguchi (+33.) determined source mechanism of these earthquakes. VT
earthquakes have a faulting source and originate incompetent rock, while BH-type, BL-type and explosion
earthquakes are caused by the release of volumetricstrain related to the expansion process of a gas pocket
along the conduit. Nishi (+312) found temporal changeof the mechanism of VT earthquakes beneath the summit
with eruptive activity. Normal fault type predominantly
occurred when eruptive activity began to increase, andreverse fault type occurred while the activity declined.
During the period of +310-+312, VT earthquakes frequently occurred SW o# the Sakurajima (Kamo, +312),
however, fault plane solutions of the VT earthquakes
could not be determined due to insu$cient coverage ofseismic stations. Focal mechanisms of VT earthquakes
were not clear except for the summit area.
The previous studies on VT earthquakes were mostly
carried out during the deﬂation stage of the Aira caldera. The deﬂation of Aira caldera turned inﬂation in+33- (Eto et al., +332) and seismicity of VT earthquakes
has sometimes increased since then. In the present study,
we will determine hypocenter and focal mechanism of
VT earthquakes originating at Sakurajima volcano andits surroundings during the inﬂation stage of the caldera, and examine relationship of VT earthquakes withvolcanic activity. In particular, volcanological implications of VT earthquakes out of the summit area of
Sakurajima will be discussed in relation to tectonic
setting and volcanic activity.
,. Activity of Sakurajima volcano
The Aira caldera occupies the northern part of Kagoshima Bay. Hayasaka (+321) revealed that the origin of
the bay dates back to certain age far older than theformation of the calderas and was attributed to theformation of an elongated depression from north tosouth accompanied with large scale tectonic activities as
illustrated in Fig. +. A submarine volcano, Wakamiko
is located at NE part of the caldera. At the southern
rim of the caldera, Sakurajima volcano is located as a
post-caldera cone which has grown to its present elevation of about ++** m. An E-W cross section of the southern part of the Aira caldera showed that Sakurajima
volcano lies on the graben formed by normal faults(Aramaki, +32.).
Sakurajima volcano has experienced ﬂank and summit
eruptions during historic times. Large eruptions withe#usion of lava took place in +.1+-+.10, +113 and +3+..
The +113 eruption was accompanied with submarine
eruption at NE o# the volcano, and several new islets
were formed. The current eruptive activity started in+3//, and it is characterized by explosive eruption of
Vulcanian type at the summit crater, Minamidake.
From +31. to +33,, the ground around the Airacaldera gradually subsided as detected by precise leveling measurement showing *.+- m subsidence (BM S,0referring to BM S+1) at northern part of Sakurajima,
while hundreds number of explosive eruptions anddozen million tons of volcanic ash were ejected a year,
as shown in Fig. ,. During the period, number of VT
earthquakes has gradually decreased. In particular,Volcano-tectonic Earthquakes during the Stage of Magma Accumulation at the Aira Caldera, Southern Kyushu, Japan 291

seismicity of VT earthquakes out of Sakurajima hassigniﬁcantly decayed as deﬂation of the caldera has
progressed. In +33-, eruptive activity and ash ejection
declined signiﬁcantly, and the deﬂation of the groundturned inﬂation, as illustrated in Fig. ,. It is interpreted
that magma accumulation resumed at the magma reservoir beneath the caldera (Eto et al., +332). Meanwhile,number of VT earthquakes originating beneath thesummit has gradually increased since ,**,. In November ,**-, ,0 VT earthquakes occurred SW o# thevolcano and the seismicity continued until February,**.. Furthermore, in December ,**-, while the seismic activity still persisted, several earthquakes originatedin the caldera, where few events have been detected
since +32,. However, no signiﬁcant change in eruptive
activity was observed, in spite of increase in seismicity
of VT earthquakes.

Fig. +. Map of subsurface geology of Kagoshima
Bay, Southern Kyushu, Japan. Aira caldera and
Sakurajima volcano is shown (Hayasaka, +321,
with slightly modiﬁed expression). + : basement ;, : nearly horizontal strata continue to +/*ῌ,**m below sea level, overlain by recent deposits ;- : thin parallel strata continue to depth of ,**ῌ-** m through recent deposit. This deposit buries
a depression feature like a valley ; . : probably
fore-set deposits of deltas overlain by recent
deposits ; / : non stratiﬁcation material overlain
by recent deposits /*ῌ2* m thick ; 0 : dike.

-. Seismic observation
Sakurajima Volcano Research Center (SVRC) of Kyoto
University has monitored seismic activity of the volcano
by +2 stations (Fig. -). Ten stations are distributed in arange of 1 km from the active crater of Sakurajima
volcano (Fig. -a). Most of them are borehole stations,
except for HIK and KOI, where seismometers are installed on the ground surface ones. All the stations areequipped with --component short-period (+ Hz) seismometer and seismic signals are transmitted by telephone line and radio waves to SVRC. Wider network(Fig. -b) is composed of 2 stations located outside
Sakurajima volcano in Southern Kyushu, and also equipped with --component short-period seismometer. All
the seismic signals are transmitted by telephone line to
SVRC.
The seismic signals were recorded on analogue magnetic tapes in trigger method before May ,**+, and they
were digitized with a sampling rate of ,** Hz for
analysis in this study. Since ,**+, the data have beentransformed in digital form at a sampling rate of ,** Hz
continuously at the stations.
.. Hypocenter.ῌ+ Method
To calculate hypocenter location, a velocity modelcomposed of three layers over a half space was assumed
as listed in Table +, based on the model proposed by
Yamasato (+321). A value of +.1- was assumed as Vp/
Vs ratio for all the layers. Location of VT earthquakes
was calculated by HYPOELLIPSE program (Lahr, +333).
In order to minimize time-picking errors, calculation
was done for events whose P-wave ﬁrst motions wereclearly recorded at more than 2 stations for VT events atthe volcano and more than +. stations for those out of
Sakurajima. Calculation stability was examined byvarying initial depths of +, -, / and 1 km to conﬁrm no292

Sri HIDAYATI, Kazuhiro ISHIHARA and Masato IGUCHI

Fig. ,. Volcanic activity at Sakurajima volcano and its surroundings during the period from +31, to ,**/. a)
Annual amount of ash-falls ejected from the summit crater. b) Annual number of explosion earthquakes isshown by histogram (left axis) and vertical ground deformation is represented by relative change of BM S,0 to
BM S+1 deduced from leveling survey (right axis). c) Annual number of VT earthquakes at SW o# thevolcano and in the caldera. d) Annual number of VT earthquakes beneath the summit crater.

signiﬁcant changes of foci.
During the period from +332 to ,**/, 112 VT earthquakes whose magnitude ranged from *./ to ,.. wererecorded, and +33 well-located events were selectedbased on the criteria shown in Table ,. The standard
deviation of arrival time di#erence between observationand calculation is smaller than *.- s for all the selected
events.
.ῌ, Results
The overall distribution of VT earthquakes during
those periods is shown in Fig. .a-c, by plotting the +33

well-located events. VT earthquakes are located in an
elongated zone from the NE part of the Aira caldera
through the Sakurajima till the SW o# the volcano. The
main activities are found in three distinct regions : (+)
beneath the summit crater, Minamidake of the Sakurajima, (,) SW o# the volcano and (-) in the Airacaldera. As shown in Fig. .b, the hypocenters in region(+) are mostly concentrated in south of the crater rim of
Minamidake, and a few events occurred north of thecrater rim. The epicenters in the region (,) are locatedin the migration zone of hypocenters of VT earthquakes

Volcano-tectonic Earthquakes during the Stage of Magma Accumulation at the Aira Caldera, Southern Kyushu, Japan 293

Fig. -. Location of seismic stations used in the present study. (a) Solid squares and crosses denote borehole andground surface stations at Sakurajima, respectively. (b) Underground tunnel stations in Southern Kyushu areshown by crosses.

Table +. P-wave velocity model used for hypocenter
calculation.

Depths are referred to sea level.

as detected by Kamo (+312). Most of the hypocentersin the region (-) are located around the submarine
volcano, Wakamiko. The focal depth of these earthquakes in region (+) ranges from * to . km below sea
level. Foci of the region (,) are deeper at 0 to 3 km.
The foci ranged from . to +. km in the region (-).
Hypocenters of VT earthquakes beneath the summit
are plotted on E-W vertical cross section (Fig. /a) andhistogram of depth is added as shown in Fig. /b. Seismicity at a depth of , km below sea level is signiﬁcantly
low. Two groups are distinguished beneath the summit
region : shallower than , km and deeper one. The deepzone extends to . km. Such a seismicity gap has been
already pointed by Ishihara (+322) based on the hypo-

Table ,. Selection criteria used for events at each region.

“Erlat”, “Erlon” and “Erz” indicate errors with respect
to latitude, longitude and depth, respectively. “rms”
is root mean square of the residual between the observed arrival times and calculated ones.

center calculation assuming homogeneous half space ofVpῌ,./ km/s, and it is interpreted that the gap corresponds to a small magma pocket along the verticallyelongated magma conduit. In the observation period in
the present study, the seismicity gap is also found. The
same velocity model as Ishihara (+322) led the seismic
gap at the same depth of , km, also in the present study.
A similar seismicity gap is clearly detected at depths of+ῌ, km beneath the summit of Merapi volcano, Indonesia, and the seismicity gap is interpreted as a small
magma pocket. VT earthquakes have been classiﬁed
into VTA (deep) and VTB (shallow) and the VTB
events a precursory phenomenon immediately before
occurrence of pyroclastic ﬂow (Ratdomopurbo, +33/).

294

Sri HIDAYATI, Kazuhiro ISHIHARA and Masato IGUCHI

Fig. .. Hypocenter distribution of VT earthquakes during the period from +332 to ,**/. (a) Epicenter distribution. Solid lines with bars indicate the outline of faults (Hayasaka, +321). Hypocenters are plotted on thevertical cross-sections of E-W (b) and N-S (c) directions, respectively. (d) Enlargement of the distribution of
hypocenters at the summit area of Minamidake. Open and solid circles indicate hypocenters shallower anddeeper than ,km, respectively.

Volcano-tectonic Earthquakes during the Stage of Magma Accumulation at the Aira Caldera, Southern Kyushu, Japan 295

Fig. /. Depth dependence of VT earthquakes beneath the summit. a) Hypocenter distribution in the E-W verticalcross-section with error bar. b) Number of events versus depth.

/. Focal Mechanism/ῌ+ Method
Focal mechanism of VT earthquakes was determinedby both polarity of P-wave ﬁrst motion and amplitude
ratio of SV to P-wave. Firstly, focal mechanism was
obtained from distribution of polarities of P-wave ﬁrst
motion. As the number of seismic stations is insu$cient
to ﬁx the focal mechanism, ranges of strike, dip and rake
were determined so as to be consistent to the polaritiesof P-wave ﬁrst motions. Secondly, the best solutions,which can explain ratios of amplitudes of SV to P-waves
of vertical component (Kisslinger, +32*), was obtainedby a grid search method in the ranges of strike, dip andrake as determined in the ﬁrst step. In the present study,
the peak-to-peak maximum amplitudes were used for P
and SV-waves, because SV-wave ﬁrst motions were notclearly picked up at most of the stations. Most of cases
for P-wave, the ﬁrst motions gave the maximum values.
Horizontal components were not used. Phase and amplitude in horizontal components are not reliable at a
ground surface station at Sakurajima due to unconsolidated ground (Tameguri et al., ,**+).
Displacements of P- and S-waves at far-ﬁeld from apoint double couple source are written as follow (Layand Wallace, +33/) ;
+
r῏ ῌ
P ῌ῍
- R M῎t῏ ῐ ῏a.pr ra
ῑ
῍,
+
r ῏῏
SV ῌ῍
USVῌr, t῍ῐ
R
M
t῏
῎ bῐῑ
.pr rb῎UPῌr, t῍ῐ

(+)

where
UPῌr, t῍, USVῌr, t῍ : far-ﬁeld displacement for P- and SVwaves,

RP, RSV: radiation pattern for P- and SVwaves,r: distance,ῌMῌt῍: moment rate function,a, b: velocities of P and S-wave,r: density (assumed to be ,/** kg/m-).
In the present study, only velocity amplitude data
were used. Velocity amplitudes are obtained from thederivative of the equations (+) and time terms of UPῌr,t῍ and USVῌr, t῍ are omitted. The ratio of the velocity
amplitude of SV to P-wave can be written as ;ῌ
USVῌr῍ RSV a(,)ῐ P - .
ῌR b
UPῌr῍
Meanwhile, radiation patterns of P-wave (RP) andS-wave (RSV) is described as follows (Aki and Richards,+32*) ;ῌ῏῏
῏cos l cos d sin ,ix cos ῌf῏fs῍῏῎sin l sin ,d ῌcos,ix῏sin,ix sin, ῌf῏fs῍῍ ῏῏῎sin l cos ,d sin ,ix sin ῌf῏fs῍
ῑ
ῒ (-)
῍,
῏
RSVῐsin l cos ,d cos ,ix sin ῌf῏fs῍῏῏cos l cos d cos ,ix cos ῌf῏fs῍῏+῏
῎ cos l sin d sin ,ix sin ῌf῏fs῍
,
῏
ῑ
ῒ
+
῏ sin l sin ,d sin ,ix ῌ+῎sin, ῌf῏fs῍῍῎RPῐcos l sin d sin,ix sin , ῌf῏fs῍,

where fs, d and l represent strike, dip and rake of thefault plane, respectively. f and ix are azimuth and take-o#angle from the source, respectively, which are deduced
from hypocenter calculation. Therefore, velocity ampli-

296

Sri HIDAYATI, Kazuhiro ISHIHARA and Masato IGUCHI

Fig. 0. An example of focal mechanism. The event
occurred at SW of Sakurajima volcano at *- :+.
on December -+, ,**-. Residual between observed SV/P amplitude ratio and calculated ones
are plotted against strike, dip and rake. The
parameters with strike of ,,/ῑ, dip of ,/ῑ andrake of ῎3/ῑ give the minimum residual as
shown by downward arrows. At the top, nodal
plane solution and polarities of P-wave ﬁrst motion are plotted on the upper-hemisphere of the
focal sphere by stereographic projection. Openand solid circles indicate dilatation and compression, respectively.

tude ratio of SV to P-wave at a station x from the sourcex* could be written as follows ;ῌRSVῌfs, d, l, x*῍aUSV.
(.)ῌ
P ῌx῍῏
U
RPῌfs, d, l, x*῍bGrid search method was applied for strike, dip andrake, so as to minimize residual of the calculated SV/P
amplitude ratios from observed ones with / degrees
increment. An example of focal mechanism obtained by
the calculation method is shown at the top of Fig. 0 by
plotting the polarities of P-wave ﬁrst motions on upperhemisphere of the focal sphere by stereographic projection. This event occurred SW o# Sakurajima volcano at
a depth 0.3 km, at *- :+. on December -+, ,**-. Polarities of P-wave ﬁrst motions suggest a normal fault type.
In this case, minimum residual was obtained at ,/, ,,/and ῎3/ degrees, for dip, strike and rake angles, respec-tively. To avoid ambiguity of determination of focal
mechanism, amplitude data ofῐ, mm/s for VT earthquakes with Mῐ+ were used for analysis. We could
determine focal mechanism of -2 VT events out of +33
well-located ones.
/ῌ, Results
Fault plane solutions of VT earthquakes beneath thesummit region are shown in Fig. 1. Polarities of P-wave
ﬁrst motion are plotted on upper-hemisphere of the
focal sphere by stereographic projection. VT earthquakes shallower than , km have dominantly reverse
fault type mechanism with / out of 1 events and the restof them are normal fault type. The orientations ofT-axes of VT earthquakes at shallow part beneath thesummit region are mostly nearly vertical, while P-axes
are nearly horizontal in NE-SW direction for reverse
fault type (Fig. +*a). No strike-slip type is found. On
the other hand, at deeper portion, focal mechanisms of
VT earthquakes show strike-slip or reverse fault type.
Strike-slip fault types are predominant as represented by. out of / events. T-axes are oriented horizontally E-W
direction for strike-slip and nearly vertical for reverse
fault type. P-axes are oriented N-S direction in the two
types (Fig. +*b). Normal fault type events were notdetected in the analysis period.
In the SW region, fault plane solutions of ,* events
are well determined as illustrated in Fig. 2. All thesolutions indicated normal fault type. P- and T-axes
were concentrated in the narrow portion. T-axes arenearly horizontal in WNW-ESE direction and P-axes
are mostly vertical with dip angles ofῐ 0*ῑ (Fig. +*c).
Meanwhile the focal mechanism of those VT earthquakes in the caldera is strike-slip fault type (Fig. 3). Pand T-axes are oriented nearly horizontal in E-W and
N-S direction, respectively (Fig. +*d). Strike of one of
the nodal planes coincides with the direction of faults inthe caldera (Fig. 3).
0. Discussions
0ῌ+ Shallow magma conduit beneath the summit crater
Epicenter distribution of VT earthquakes beneath thesummit region is shown in Fig. ++a. There are few
events observed inside the crater rim, but most of them
are distributed south of the crater and a few north of it.
Ishihara (+33*) and Iguchi (+33.) inferred that thezone enclosed by VT earthquakes was near-surface
magma conduit connecting the summit crater and shallow reservoir located at a depth of approximately / km,
because focal zone of explosion and B-type earthquakes
are surrounded by VT earthquakes. If VT earthquakes
originated in competent rock outside presumed magma
conduit where the other types of volcanic earthquake
are generated, it might be expected that seismic waves
passing through the conduit is distorted.
To discuss the e#ect of distortion on seismic waves

Volcano-tectonic Earthquakes during the Stage of Magma Accumulation at the Aira Caldera, Southern Kyushu, Japan 297

Fig. 1. Fault plane solutions of VT earthquakes beneath the summit area. Polarities of P-wave ﬁrst motions andthe fault plane solutions are plotted on upper-hemisphere of the focal sphere by stereographic projection. Solidand open circles indicate compression and dilatation of P-wave ﬁrst motions, respectively. Directions ofpressure and tension axes are shown by squares with “P” and “T”, respectively.

passing through the presumed magma conduit, thewaveforms of VT earthquakes are compared at two
stations : KAB located -.. km NW of the summit craterand ARI ,.1 km to the south (Fig. ++b). The seismic
waves generated by VT earthquake at northern portionof the crater were attenuated stronger at ARI than at
KAB (Event + in Fig. ++b). In this case, the seismic
waves passed through the conduit under the summit
crater and arrived at station ARI. In contrast, seismic
waves generated by the event located at the southern
rim were strongly attenuated at KAB (Event , in Fig.
++b). For the event located west of the crater (Event in Fig. ++b), seismic waves to the two stations did notpass through beneath the crater, and the amplitudes ofseismic waves are almost same at the , stations. The
region occupied by volcanic conduit seems to inﬂuenceof attenuation of seismic waves. In order to conﬁrm theattenuation e#ect, focal mechanism is investigated.
Focal mechanism of the event -, which did not show
anomalous attenuation, is shown in Fig. ++c. The

mechanism is normal fault type and take-o# to stations
ARI and KAB are near the T-axis. It is estimated thatradiation pattern does not inﬂuence on anomalous
attenuation as shown on seismograms of the , stations.
The mechanism of event , is illustrated in Fig. 1a(Event .), showing reverse fault type with P-axis in
ENE-WSW direction. In this case, radiation patternfrom the source does not contribute the anomalous
attenuation to stations ARI and KAB. It is possiblethat seismic waves at distant stations are attenuated due
to anelastic attenuation of propagation path. As shownin the spectra (Fig. ++b), spectra amplitude at station
KAB is smaller by + order at frequencies of .ῌ0 Hz thanthose at ARI. It is di$cult for geometric spreading andanelastic attenuation (mean attenuation of SakurajimaQῌ,* ; Iguchi, +33.) to explain such an anomalous
attenuation at station KAB. For the event + north ofthe crater, the mechanism seems to be reverse fault,
however it was not well determined because of small
magnitude. It is possible that small amplitude at station

298

Sri HIDAYATI, Kazuhiro ISHIHARA and Masato IGUCHI

Fig. 2. Fault plane solutions of VT earthquakes SW o# Sakurajima volcano. Symbols follow the convention
in Fig. 1.

ARI is caused by radiation pattern of which take-o# isnear nodal plane. If the take-o# is near the nodal plane,larger amplitude of S-wave would be expected at station
ARI. In the case of event +, no such a large S-wave was
detected. It is more reasonable that anomalous attenuation of the seismic waves passing beneath the crater iscaused by anelastic medium beneath the crater. These
results may suggest that the presumed conduit was
occupied or ﬁlled with materials probably magma which
attenuated seismic waves.
Activity of VT earthquakes beneath the summit area
is signiﬁcantly low at the depth of , km below sea level,compared with shallower and deeper portions, as illustrated in Fig. /. Gravity anomaly around Sakurajima
indicated that a density discontinuity lies at a depth ofabout ,./ km beneath the volcano. Pyroclastic deposite
overlies the Mesozoic basement rock at a depth of ,./km (Yokoyama and Ohkawa, +320). Considering thismodel, Ishihara (+322) interpreted that the seismicity
gap corresponds to a small magma chamber formed at
the geological boundary. Ishihara (+33*) estimatedthat the depth of pressure source associated with explosive eruptions ranged from , to 0 km and thought thatthe magma conduit at deeper portion than , km became
larger, compared with shallower part. In the present
study, it is shown that predominant fault plane solutionsof VT earthquakes at depths of shallower than , km arereverse fault type with vertical T-axes and horizontal
P-axes in NE-SW direction, and those at depths of morethan , km are strike-slip type (Fig 1). P-axes orient
NE-SW direction for the earthquakes occurring shallower than , km, and N-S direction for those deeper
than , km. The di#erence in focal mechanism seems to
be related to structure of the conduit and magma chamber. A conceptual model of magma conduit is shown in
Fig. +, to explain the di#erence in focal mechanisms. A

Volcano-tectonic Earthquakes during the Stage of Magma Accumulation at the Aira Caldera, Southern Kyushu, Japan 299

Fig. 3. (a) Fault plane solutions of VT earthquakes in the Aira caldera. Symbols follow the convention in Fig. 1.
(b) Locations of hypocenters are shown by numerals.

Fig. +*. Projections of P- and T-axes for VT earthquakes at shallow (ῌ, km) part beneath thesummit (a), deep part (ῌ, km) beneath thesummit (b), SW o# the volcano (c) and in thecaldera (d). P- and T-axes are plotted on
upper-hemisphere of the focal sphere by stereographic projection.

small magma chamber at the depth of , km is inferredfrom the seismicity gap as shown by the present study
and Ishihara (+322). Repeat of precise leveling meas-

urement shows the deep magma chamber at a depth of/ km (Eto and Nakamura, +320). Pressure decrease in
the magma chamber provide contraction stress ﬁeld
above the magma chamber. In the contraction stressﬁeld, reverse fault or strike-slip fault type is expected
and the orientation of P-axis is related with the stressﬁeld caused by the magma chamber. If the magma
chamber at a depth of / km was an isotropic contraction
source, the P-axes of the events above the source would
be oriented toward the source. As the VT earthquake
deeper than , km occurred at south of the crater, theP-axes would be oriented N-S direction. On the other
hand, it is di$cult to explain the E-W orientation of
P-axes of the VT earthquakes shallower than , km by anisotropic contraction source beneath them, because the
VT earthquakes shallower than , km are also locatedsouth of the crater. If the conduit was partially extended at a depth of , km crack-likely striking in NW-SE
direction, the NE-SW orientation of P-axes of VT earthquakes shallower than , km could be explained. Closing
crack in the direction of NW-SE would cause contraction ﬁeld dominantly NE-SW direction above it. Crack
striking NW-SE direction may be reasonable because
the crater of Minamidake have extended toward SE
since +30*’s, forming crater B.
Temporal change of focal mechanism is shown in Fig.
+-. During the period of the increase in seismicity since,**,, the focal mechanism is dominated by reverse fault
at shallower than , km and strike-slip fault at deeper
than , km. In contrast to the recent seismicity of
dominance of reverse fault type, Nishi (+312) showedthat VT earthquakes of normal fault type were dominant at the beginning of active period of eruptive activ-

300

Sri HIDAYATI, Kazuhiro ISHIHARA and Masato IGUCHI

Fig. ++. Attenuation of seismic wave beneath the summit crater. (a) Epicentrer distribution of VT earthquakes
beneath the summit area. On the map, broken lines indicate paths of seismic waves which experienced stronger
attenuation. Note that broken lines pass across the crater zone. Solid lines indicate paths of seismic waves
with no signiﬁcant attenuation. For the VT events +--, seismograms observed at stations ARI and KAB andtheir spectra are shown in (b). Focal mechanism of event - is shown in (c), and the mechanism of event , isreferred to Fig.1a (#.).

ity. Although the reason why normal type events
dominantly occurred only at the beginning of the active
period of eruptivity was not mentioned by Nishi (+312),the normal fault events may be generated by tension
stress ﬁeld caused by opening of the crack-like conduitat the depth of , km due to intrusion of magma. In theanalysis period of +31/ῌ+311 by Nishi (+312), about,** explosive eruptions occurred a year, and magma
was frequently supplied to the shallow part of the conduit. In contrast, recent eruptive activity, especiallyafter ,**-, declined comparatively to +31*’s and +32*’s(Figs. , and +-). Very little magma has been supplied
to the shallow part of the conduit, and the conduit
formed by the previous eruptive activities began to becollapsed causing contraction ﬁeld. Nishi (+312) also
pointed out that reverse faulting occurred during thecalm periods.

0ῌ, Regional tectonics
The relationship among regional tectonics, volcanic
activity and VT earthquakes out of the summit region is
discussed. From geological and geophysical data compiled by Kimura (+32/), regional tectonic stress acting
on Aira caldera and its surroundings is dominated by
WNW-ESE extension. Hayasaka (+321) revealed anelongate depression from north to south, as illustrated
by the geologic structure map along the Kagoshima Bay
in Fig. +. The mechanism of VT earthquakes SW o#
Sakurajima volcano seems to be consistent with regional
tectonics inferred from these geological studies.
Meanwhile, VT events at the NE part of the caldera
have strike-slip type and one of the nodal lines seems tocoincide with the direction of depression line oriented in
NE-SW direction (Fig. 3). However, the relationshipbetween these earthquakes and volcanic activity stillVolcano-tectonic Earthquakes during the Stage of Magma Accumulation at the Aira Caldera, Southern Kyushu, Japan 301

Fig. +,. Conceptual model of the magma conduit
beneath the summit crater. Magma conduit andthe shallow magma reservoir are indicated bydotted area Open circles indicate hypocenters of
VT event. Solid and gray arrows show depression of magma reservoirs at depths of , km and/km, and contraction stress ﬁeld above them,
respectively.

cannot be explained. Previous seismological studies revealed that volcano dynamic is controlled by both regional tectonics and volcanic activity (Frazzeta andVillari, +32+ ; Umakoshi et al., ,**+). To understandthe nature of VT earthquakes around Sakurajima volcano, further studies on focal mechanism of earthquakes
and tectonics around the Aira caldera is needed.
0ῌ- An alternative model on magma supply system
The period in the present study is characterized byinﬂation of the Aira caldera and signiﬁcant seismic
activity around Sakurajima volcano and in the caldera.

In particular, VT earthquakes swarmed SW o# thevolcano in November ,**- and were followed by several
earthquakes at the NE portion of the caldera, where noearthquakes were observed from +32,. Following theincrease in seismicity, GPS baseline across the volcano
indicated remarkable extension in October ,**.. However, no signiﬁcant change in eruptive activity wasrecognized, as illustrated in Fig. +.. As Kriswati andIguchi (,**-) and Iguchi (,**0) discussed, the supply
and accumulation of magma under the caldera havecontinued at an average rate of +ῌ+*1 m- /year. The
increase in seismic activity out of the Sakurajima volcano is surely related to the accumulation of magma.
Models on magma supply system of Sakurajima volcano have been proposed mainly from analysis of grounddeformation data (Mogi, +3/2 ; Yoshikawa, +30+ ; Eto,
+301 ; Yokoyama, +320). These proposed model consistof deep magma reservoir under the Aira caldera andshallow one beneath Sakurajima volcano. This model
could explain fairly well the pattern of deformation
obtained by leveling survey (e.g., Eto and Nakamura,+320). Kamo (+312) found migration of VT earthquakes from deep SW o# the volcano to shallow portion
beneath the summit and subsequent increase in explosive activity, and Kamo (+323) proposed a revised modelof magma supply system with two magma reservoirs byadding a temporal magma pathway from SW of the
volcano toward the shallow magma reservoir beneaththe Sakurajima. In this model, two magma paths from
north and south are independent of each other.
Some characteristics of seismic activity and grounddeformation in the period of the present study from+332 to ,**/ are di#erent from them in the analysis
period in +31/-+311 by Kamo (+312), and it is di$cult
to explain the following feature of recent seismicity andground deformation by the revised model by Kamo(+323). (+) The hypocenters of VT earthquakes SW o#the Sakurajima in ,**- are concentrated in the narrow
cubic zone of , km and migration of hypocenters was
not detected, unlikely to the migration of hypocenters at
the same region in +310. (,) If magma was intruded
from SW to Sakurajima, ground at southern part of
Sakurajima would be uplifted. However, the center ofground deformation was still in the northern part of
Sakurajima or center of the Aira caldera, and no remarkable ground deformation was detected at thesouthern part. (-) In the period of +31/ῌ+311, theground of Aira caldera and northern part of Sakurajima
began to be deﬂated in spite of the increase in seismicity
SW o# the volcano. The two phenomena seem to occur
independently. On the other hand, the inﬂation of the
Aira caldera has repeated several times since +33- (Fig.
+.) and the VT earthquakes frequently occurred SW o#
the volcano in the inﬂation stage. It may be morereasonable to infer a relation between the inﬂation of

302

Sri HIDAYATI, Kazuhiro ISHIHARA and Masato IGUCHI

Fig. +-. Relationship of focal mechanisms of VT earthquakes with eruptive activity. (a) Monthly numbers of
explosive eruptions. (b) Hypocenter depths of VT earthquakes are plotted against time. Types of focal
mechanism are shown by solid, open, half-shaded circles for reverse, normal and strike-slip fault types,
respectively. Small open circle denote VT earthquakes whose mechanisms could not be determined.

the caldera and increase in the seismicity. Therefore, analternative model on magma supply system is proposed
to explain the relation between accumulation of magma
and the seismicity SW o#. In the model, it is assumed
that a tensile fault from the magma reservoir beneath
the Aira caldera extending toward SW o# the
Sakurajima was opened by lateral intrusion of magmafrom the magma reservoir and VT earthquakes occurredat the tip of SW edge of the tensile fault.
Next, some evidence for the tensile fault striking fromthe magma reservoir beneath the Aira caldera to SW o#the Sakurajima in NNE-SSW direction will be shown.
Location of VT earthquakes seems to align in a zone of
NNE-SSW direction from the caldera through Sakurajima, as shown in Fig. .. Along the zone, signiﬁcantvolcanic activities occurred in historical time. These are
active fumaroles at the submarine volcano, Wakamiko,and submarine eruption NE o# the Sakurajima during
the +113 eruption, and Sakurajima volcano itself. In
addition, a large earthquake (M1) originated SW o# thevolcano (Abe, +32+), related to the +3+. eruption. This
zone also corresponds to geologically active tectonic
area at the northern Kagoshima Bay (Fig. +). These
geological facts might suggest the existence of the tensile fault.
Ground deformation also suggests the tensile fault.

Distributions of vertical displacements around Sakurajima volcano deduced by the precise leveling during .
periods from +31. to +32, are shown in Fig. +/ (Etoand Ishihara, +32* ; Eto et al., +32, ; Eto and Nakamura,+320). Although depression patterns are recognized
during the . periods, there were slightly di#erent pattern among them. The depression zones during +310ῌ
+312 and +312ῌ+32* (Figs. +/b and c) seem to haveextended southwestwards compared with the other periods. Signiﬁcant numbers of VT earthquakes SW o# thevolcano were observed in +31/ῌ+32*, as shown in Fig ,.
It is expected that a tensile fault cause depression zoneof ground surface along it (Okada, +33,), as observedin the +320 eruption at Izu-Oshima (Hashimoto andTada, +322). Recent VT earthquakes originating SW
o# Sakurajima volcano have uniquely normal fault type
and T-axes oriented WNW-ESE direction, and the direction of T-axes and the direction of ground depression
zone are approximately perpendicular to each other.
Similar relationship was observed in the +320 ﬁssure
eruption of Izu-Oshima (Yamaoka et al., +322). Considering the studies in Izu-Oshima volcano, VT earthquakes SW of Sakurajima might be an indication thatmagma intruded in laterally extended dike across
Sakurajima volcano from NE to SW, probably comingfrom magma reservoir beneath the Aira caldera.

Volcano-tectonic Earthquakes during the Stage of Magma Accumulation at the Aira Caldera, Southern Kyushu, Japan 303

Fig. +.. Relation of seismicity of VT earthquakes with eruptive activity and ground deformation during the period
from +33/ to ,**/. Top : monthly number of explosions. Second : monthly number of VT earthquakes
beneath the summit area. Third : monthly number of VT earthquakes SW o# the Sakurajima and in the Airacaldera. Bottom : temporal changes of horizontal distance measured by GPS. The daily horizontal distances ofbaselines SVOG-KURG (E-W) and FUTG-ARIG (N-S) are plotted (Iguchi, ,**0). Locations of the GPS
stations are shown at lower right corner.

304

Sri HIDAYATI, Kazuhiro ISHIHARA and Masato IGUCHI

Fig. +/. Distributions of vertical ground displacements around Sakurajima volcano measured by precise leveling
during the periods : a) +31.ῌ+310 (Eto and Ishihara, +32*), b) +310ῌ+312 (Eto and Ishihara, +32*), c) +312ῌ
+32* (Eto et al., +32,) and d) +32*ῌ+32, (Eto and Nakamura, +320). The displacement is referred to BM,120.

Eto and Nakamura (+320) explained ground deformation around Sakurajima volcano by using the modelof , pressure sources based on Mogi’s model. However,some of the observed data is not well ﬁtted by the model,
in particular, in the periods +310ῌ+312 and +312ῌ+32*(Fig. 3 of Eto and Nakamura, +320). Here, we attempt
to explain the vertical displacement data during +312ῌ
+32* by a composite model of , Mogi sources and atensile fault, as illustrated in Fig. +0a. Two Mogi
sources “A” and “B” are assumed to be located at
depths of +* km and - km, respectively, based on calculation result by Eto and Nakamura (+320). The solidline indicates the vertical tensile fault extending fromthe main pressure source “A” beneath the caldera across
Sakurajima volcano “B” to the focal zone of VT earthquakes SW o#. The length of the tensile fault is +, km
and the depths of top and bottom of it are assumed to be
0 km and 3 km, respectively, considering the depth
range of VT earthquakes SW o#. Assuming this conﬁguration, other source parameters are estimated. Calculated vertical displacements based on the composite
model are compared with observed one along levelingroute (Fig. +0b). Calculation displacements are fairly
well ﬁtted to observed data under the following condition ; volume decreases of the sources A and B : +.,ῌ+*1 m- and /.1ῌ+*/ m-, respectively, opening of thetensile fault : *.*0 m. Then, volume increase of thetensile fault is estimated to be ,.+ῌ+*0 m-. On most of
the benchmarks, calculated values based on the composite model are better ﬁtted to observed vertical displacement than those from the previous model of , Mogi
sources, except for around BM ,/** at SE edge of
Sakurajima. The model proposed in this study including
the tensile fault can explain the distribution of verticaldisplacement better. The volume decrease of the sources
A and B and volume increase of the tensile fault isbalanced by volcanic ash and gas ejected from the summit
crater. During the period between the , measurements
of precise leveling, volcanic ash of +..ῌ+*1 ton (/.0ῌ+*0 m- DRE (dense rock equivalent) ; density is assumed
to be ,/** kg/m-) was ejected. It is di$cult to estimate
contribution of volcanic gas, however, it was reported
that volcanic gas was discharged at rates of +***ῌ,***ton/day (Kamada et al., +32,)
Recent GPS data show change in the pattern ofground deformation associated with VT earthquakes
SW o# the volcano, as illustrated in Fig. +.. Overall
features of change in horizontal distance along two GPS
baselines were quite similar, and basically reﬂect theaccumulation process of magma under the Aira caldera.

Volcano-tectonic Earthquakes during the Stage of Magma Accumulation at the Aira Caldera, Southern Kyushu, Japan 305

Fig. +0. Composite model of a tensile fault and , Mogi sources. (a) Assumed location of Mogi’s pressure sources(A and B) and a tensile fault for calculation of ﬁtting calculated vertical displacements to observed ones
during the period of +312ῌ+32*. Assumed depths of A and B are +* km and - km, respectively. The solid line
indicates the location of the tensile fault, and the length, top and bottom depths are assumed to be +, km, 0 km
and 3 km, respectively. (b) The ﬁtting of the composite model of a tensile fault and , Mogi sources (solid
curve) and the model of , Mogi sources (broken curve) to vertical displacements along the leveling route isillustrated. Benchmarks are aligned clockwise from S+1 (west edge of Sakurajima). Numbers of the benchmarks are referred to the upper ﬁgure.

Note that the trend of the two baselines slightly di#ers
from each other during the period from ,*** to middle
of ,**.. The baseline of E-W direction, SVOG-KURG
indicated extension, and that of N-S direction, ARIGFUTG did slightly contraction. The feature of horizontal deformation could be explained by assuming ashorter and counterclockwisely rotated tensile fault
starting from south of station FUTG and reaching SW
o# the volcano (Fig. +1). As the station SVOG and
KURG are located at opposite sides of the fault, thebaseline of SVOG-KURG extended due to opening of
the tensile fault. FUTG in the contraction area of the306

Sri HIDAYATI, Kazuhiro ISHIHARA and Masato IGUCHI

Fig. +1. Conceptual horizontal displacements at the. GPS stations at Sakurajima for ground deformation of the period from ,*** to middle of,**.. Dashed line is fault and “A” is deﬂationpressure source in deﬂation stage in +312-+32*
shown in Fig. +0. Circles “AL” and “AG” indicate locations of pressure source during theperiod of +33+ῌ+330 by precise leveling (Eto etal., +332) and during the period of +331ῌ+333
by continuous GPS observation (Kriswati andIguchi, ,**-) in inﬂation stage of the Airacaldera. Gray arrows indicate horizontal displacements caused by opening of the e#ective
tensile fault indicated by gray line. Solid arrows
show horizontal displacement due to inﬂation of
the pressure source AL” and “AG” beneath theAira caldera. Open circles are hypocenters of
VT earthquakes.

tensile fault moved toward the edge of the tensile fault(southward movement) and the station ARIG also
moved southward by opening of the tensile fault.
Therefore the baseline FUTG-ARIG showed no remarkable change or slight contraction. The deﬂation
Mogi source in +312ῌ+32* is located near Shinjima as
shown in Fig. +1 by “A”. Locations of the pressuresources in inﬂation stage are estimated to be located alittle north or west of the deﬂation source. Circles “AL”
and “AG” in Fig. +1 indicate locations of the sources
during the period of +33+ῌ+330 by precise leveling (Etoet al., +332) and during the period of +331ῌ+333 bycontinuous GPS observation (Kriswati and Iguchi,,**-). If the inﬂation source were located at di#erent
position from that of deﬂation source, it would bereasonable that tensile crack occupies the northern partof the Sakurajima rather than NE part. The model for
temporal changes of baselines measured by GPS corresponds to less activity at the northern part of the tensile
fault as shown in Fig. +1. During the period from ,***
to middle of ,**., remarkable inﬂation of the Airacaldera was not detected.
Finally, the model proposed in the present study isschematically illustrated in Fig. +2 comparing with themodel by Kamo (+323), and interpretation of recent
volcanic process is described according to the proposedmodel.
+) Magma has been supplied to the main reservoir
beneath the Aira caldera, and the accumulation ofmagma induces inﬂation at the caldera since +33-, as
indicated by leveling and GPS data (Figs. , and +.).
The location of pressure source is shown by “A”.
,) Magma started to intrude laterally toward the
Sakurajima volcano along the tensile fault, maybe, inearly +333, because the inﬂation of Aira caldera became
active in November +331 and the eruptive activity of
Sakurajima gradually increased in June +333 reaching at
a peak in December (Fig. +.). It is reasonable thatmagma began to move from the magma reservoir beneath the Aira caldera to the Sakurajima before theeruptive activity in late +333. As shown by changes ofpattern of baselines indicated by GPS from ,*** to the
middle of ,**., lateral movement to SW direction still
continued along the tensile fault in spite of less magma
accumulation beneath the Aira caldera.
-) As a result, VT earthquakes of normal fault typewere generated due to pressure increase at the southwestern edge of the tensile fault during the periods from
November ,**-ῌFebruary ,**.. This lateral migration
might cause change in stress ﬁeld at deeper parts aroundthe caldera. VT earthquakes at the caldera region might
be partly a#ected by adjustment of stress ﬁeld due to thisintrusion process.
.) Due to the change of stress ﬁeld of the Airacaldera, some earthquakes occurred at NE of the Airacaldera.
/) Accumulation of magma resumed beneath theAira caldera in October ,**..
During the periods from ,*** above mentioned, very
little magma moved to the magma chamber beneath thesummit. The pressure source “B” may be deﬂated as
discussed in 0ῌ+.
1. Conclusions
VT earthquakes observed at Sakurajima volcano andits surroundings during the period of +332-,**/ wereanalyzed and discussed. The results are summarized asfollows ;+) The seismicity is mainly found at three distinct
regions : beneath the summit, SW o# Sakurajima volca-Volcano-tectonic Earthquakes during the Stage of Magma Accumulation at the Aira Caldera, Southern Kyushu, Japan 307

Fig. +2. A hypothetical model on magma supply system at Sakurajima volcano. (a) Horizontal plane. (b)Vertical cross-section in SW-NE direction. Numbers correspond to volcanic processes during the period from+333 to ,**/ (see text). Open circles show VT earthquakes. Open circles with “A” and “B” are pressuresources beneath the Aira caldera and Sakurajima, respectively. Tensile fault is indicated by dashed line andrectangle, and the active zone from ,*** to middle of ,**. is indicated by gray. Magma migration inferred in
the present and previous studies is shown by arrows. A solid circle with M1 indicates the location of thelargest earthquakes in the +3+. eruption determined by Abe (+32+). Lines with branches are geological faults.
(c) Previous model modiﬁed from Kamo (+323). VT earthquakes during the period of +310ῌ+32* are plotted.

no and NE part of the Aira caldera, in particulararound Wakamiko volcano. These regions are aligned
in NE-SW direction, which corresponds to active tectonic zone.
,) Determined focal mechanism of VT earthquakes
beneath the summit region is reverse fault types in theshallow portionῌ, km, meanwhile at deeper one, strikeslip fault type is predominant. The di#erence in focal
mechanism above and below , km boundary seems to berelated to shape of the conduit at depths of , km and /km.

-) Most of VT earthquakes SW o# Sakurajima volcano indicated a unique focal mechanism of normal fault
type with horizontal T-axes oriented nearly to WNWESE direction. At the NE part of the caldera, themechanism showed strike-slip fault types and one of thenodal lines oriented in NE-SW. This orientation isconsistent with regional stress ﬁeld and direction ofdepression zone inferred from geological study.
.) The occurrence of VT earthquakes of normal fault
types at deep SW region could be explained by, forexample, a tensile fault, namely lateral dike, running308

Sri HIDAYATI, Kazuhiro ISHIHARA and Masato IGUCHI

across the volcano from the main magma reservoir
beneath Aira caldera.
Acknowledgement
The authors are grateful to Drs T. Tameguri and K.
Yamamoto of SVRC, Disaster Prevention ResearchInstitute, Kyoto University and Profs Y. Tanaka, T.
Kagiyama, Y. Sudo and T. Ohkura of Aso Volcanological Laboratory, Graduate School of Sciences for theirworthy comments and discussions. Thanks are extended to sta#s of SVRC for maintaining the instruments.
Finally, constructive reviews by Drs. H. Aoyama, J.
Oikawa and T. Tsutsui are very helpful.
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