Bull. Volcanol. Soc. Japan
Vol. /, (,**1) No. /, pp. ,1-ῌ,13Article

Source Mechanisms of Harmonic Tremors at Sakurajima Volcano
Takeshi TAMEGURIῌ, Sukir MARYANTOῌῌ and Masato IGUCHIῌ
(Received, November ,+, ,**0 ; Accepted, August ,+, ,**1)
Source mechanisms of harmonic tremors observed after B-type earthquake swarm (HTB) and those immediately after explosive eruption (HTE) at Sakurajima are estimated by inversion of root mean square (RMS) seismic
amplitudes of - components at / stations. HTB and HTE had a similar source mechanism that indicates isotropic
components of more than /*ῌ. Source depths of the HTB and HTE correspond to the location of a gas pocket
formed at uppermost part of the conduit. We infer that HTB is generated by resonance of the gas pocket formed
after swarm of B-type earthquake and that HTE is related with resonance of the gas pocket of which top ispartially collapsed by the explosion.
Key words : harmonic tremor, moment tensor, Sakurajima volcano+. Introduction
Eruptive activity with Vulcanian style has continuedat the summit crater, Minamidake of Sakurajima since+3//. Harmonic tremor (C-type tremor) have been
observed in active eruptive stages at Sakurajima volcano(Kamo et al., +311 ; Nishi, +32.), and are characterized
by regular peaks of spectra, composed of fundamental
frequency and its overtones from analyses of spectrum
and particle motion (e.g. Kakuta and Idegami, +31* ;Kamo et al., +311). As a result, the harmonic tremors
are classiﬁed into two types. One is the tremors observed several hours after B-type earthquake swarm(HTB) and the other is the tremors observed a few
minutes after explosive eruption (HTE). Peak frequencies of HTB are constant in duration of vibration.
In contrast, those of HTE gradually increase (Maryantoet al., ,**/). During +32,῍,**,, 33- HTBs occurredand only / HTEs were recorded. HTEs were rareharmonic tremor at Sakurajima volcano.
Mechanisms of harmonic tremors have been investigated by analyses of spectra at other volcanoes. Harmonic tremor at Semeru volcano was caused by repetitive triggering sources at shallow part (Schlindweinet al., +33/). Hellweg (,***) proposed that harmonic
tremor at Lascar volcano was excited by movement ofwater and gases near the surface. These studies discussed mechanisms on regular repetition of triggering
source for harmonic tremors ; however source mechanism of the trigger source has not been investigated.
Radiation pattern or moment tensor of the harmonic,. Data
Seismograms at / stations ranging +.1῍... km fromthe active crater (Fig. +) are used for analyses. At .
stations, three-component short-period seismometers(natural period T*ῌ+ s, damping constant hῌ+.*) areinstalled in boreholes at depths of 2/ to ,3* m. Station
HIK is equipped with the seismometers installed on thesurface in vaults. One horizontal component seismometer is set parallel to the direction from the station to thesummit crater (longitudinal component) and the otheris set perpendicular to the direction (tangential component). Velocity seismograms recorded before ,**+
were digitized with an A/D resolution of +, bit and asampling rate of ,** Hz from analog magnetic tapes.
Seismic signals recorded after ,**+ were recoded with
an A/D resolution of ,, bit and a sampling rate of ,**Hz.

῍ Sakurajima Volcano Research Center, Disaster Preven-University, Jln. Veteran ++, Malang, Indonesia.

tion Research Institute, Kyoto University, SakurajimaYokoyama, Kagoshima 23+῍+.+3, Japan.
῍῍ Department of Physics, Faculty of Science, Brawijaya

Corresponding author : Takeshi Tameguri
e-mail : tamekuri@svo.dpri.kyoto-u.ac.jp

tremors were also practically not reported. And also,
their source locations were inferred at shallow parts
near the surface, but the depths were not quantitatively
determined in the previous studies. Maryanto et al.
(,**/) indicated that Rayleigh waves were dominant inharmonic tremors at Sakurajima volcano from analysesof particle motion.
In this study, we estimate moment tensor and source
depths of the two kinds of harmonic tremors, HTB andHTE, at Sakurajima volcano from spatial distributionsof RMS seismic amplitudes. We further discuss common and di#erent factors in their source mechanisms.

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