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 inthe 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 isstrike-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 activevolcanoes. 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. Theirwaveform 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 presentstudy, 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