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,**3 5 - 6 +0 789 ,**3 5 1 6 ,1 78:Ground Deformation Associated with BL-type Earthquake Swarms at Sakurajima Volcano
Yuki TATEO῍ and Masato IGUCHI῍
Sakurajima volcano experiences, in addition to repeated vulcanian eruptions, intermittent small eruptions
similar in style to strombolian eruptions. These strombolian-like eruptions are associated with swarms of BL-type
earthquakes dominated by low frequency components (+ῌ- Hz). Ground deformation associated with BL-type
earthquake swarms was detected by water-tube tiltmeters and extensometers in an underground tunnel. Tilt andstrain records were corrected by BAYTAP-G to account for the tidal e#ect. Gradual tilt change of crater-side-up(,*ῌ-,* nrad) and extension of the ground (2ῌ+1* nstrain) continued for -ῌ-* hours before the BL swarms. Theinﬂation was temporarily suspended for *./ῌ+3 hours, and was then followed by deﬂation associated with BLswarms. The degree of tilt and strain change was in the same order as that for vulcanian eruptions ; however theduration of inﬂation processes is longer than that of a vulcanian explosion (by several minutes to several hours).
The inﬂation rates (,ῌ,2 nrad/h, ,ῌ+0 nstrain/h) prior to BL swarms are smaller than those prior to vulcanianeruptions (,*ῌ3* nrad/h, +*ῌ/* nstrain/h). In the deﬂation process, tilt change of crater-side-down (.*ῌ-**nrad) and contraction of the ground (,*ῌ+0* nstrain) continued for +ῌ0./ hours and was accompanied by BLswarms. The deﬂation rates (+1ῌ++- nrad/h, +,ῌ/1 nstrain/h) accompanying BL swarms are small. The
deﬂation rates of +/- of the vulcanian eruptions exceeded the upper limit of the deﬂation rate that accompanied
the BL swarms. The depth of the source of pressure that is thought to induce the ground deformation associatedwith BL swarms is estimated to be -ῌ. km for both inﬂation and deﬂation processes (assuming the Mogi source tobe horizontally located at the center of crater). No di#erence in depth is detected for vulcanian eruptions.
Volcanic gases were emitted in the inﬂation process prior to BL swarms, however volcanic gas emission stopped
in the inﬂation process prior to vulcanian eruptions. It is inferred that prior to vulcanian eruptions, the top of theconduit is plugged by a lava dome derived from cooled and degassed magma, and the internal pressure rapidlyincreases. In contrast, prior to BL swarms, the upper conduit is loosely choked and the internal pressure
gradually increases due to the intrusion of new magma from a deeper source. The di#erence in inﬂation rates may
be caused by the degree of choking of the upper conduit. BH-type earthquakes dominated by high frequency
components (/ῌ2 Hz) occurred alongside inﬂation prior to a BL swarm. The inﬂation rate almost reached themaximum rate prior to BL swarms that occurred without pre-BH-type earthquakes. It is inferred that a highinﬂation rate due to choking of the conduit is the cause of BH-type earthquakes.
Key words : ground deformation, BL-type earthquake swarms, Sakurajima volcano+ῌ ῐ ῎ ῑ ῏;<=!+> + ?6@AB +./!=!,HIJ/!KL,MNM(OPQ4RS

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Sakurajima Volcano Research Center of DPRI, Kyoto
Univ., Sakurajima-Yokoyama, Kagoshima, 23+ῌ+.+3,3 y@ABFJapan.
Corresponding author : Masato Iguchi
e-mail : iguchi@svo.dpri.kyoto-u.ac.jp