BULLETIN OF THE VOLCANOLOGICAL SOCIETY OF JAPAN
Online ISSN : 2189-7182
Print ISSN : 0453-4360
ISSN-L : 0453-4360
Volume 52, Issue 6
Displaying 1-3 of 3 articles from this issue
  • Sri HIDAYATI, Kazuhiro ISHIHARA, Masato IGUCHI
    Article type: Article
    2007 Volume 52 Issue 6 Pages 289-309
    Published: December 28, 2007
    Released on J-STAGE: March 20, 2017
    JOURNAL FREE ACCESS
    At Sakurajima volcano, activity of volcano-tectonic (VT) earthquakes has gradually increased since 2002, as the inflation of the Aira caldera progressed since 1993. In particular, VT earthquakes SW off the volcano and in the caldera swarmed during November 2003-February 2004, and then a baseline of GPS across the volcano indicated a significant extension. However, no significant increase in eruptive activity was observed. Location and focal mechanism of VT earthquakes, which occurred during 1998-2005, are determined and the relationship between the seismic activity of VT earthquakes and volcanic activity is discussed. The VT earthquakes originated in an elongated zone extending in NE-SW direction, namely from the NE part of the Aira caldera through Sakurajima volcano to SW off the volcano. This zone coincides with active tectonic zone. The focal zones of VT earthquakes are distinguished into three regions: (1) 0 to 4km deep beneath the summit of Sakurajima, (2) 6 to 9km deep SW off the volcano, and (3) 4 to 14km 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 the crater. Focal mechanism of VT earthquakes beneath the summit area has variation with depth. Reverse fault type was dominantly obtained at depths from 0 to 2km. Meanwhile at deeper portion of 2-4km, strike-slip fault type is predominant. In contrast, most of VT earthquakes SW off Sakurajima indicated uniquely normal fault type with horizontal 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 the following seismicity SW off the volcano.
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  • Isoji MIYAGI
    Article type: Article
    2007 Volume 52 Issue 6 Pages 311-333
    Published: December 28, 2007
    Released on J-STAGE: March 20, 2017
    JOURNAL FREE ACCESS
    The volcanic activity of Hijiori volcano (N38°36’35”, E140°9’20”; WGS84) is reported in detail as a case study to understand how a new felsic volcano commences the activity. Hijiori volcano, a small caldera with approximately 2km in diameter, is an active volcano of Japan, which started a series of eruptions 12,000 years ago (in Calendar age), where no volcanic body had existed before. The eruptive history of Hijiori volcano was divided into four major stages (Stage 1, 2, 3, 4) and the subdivisions (Stage 1, Stage 2-1, 2-2, Stage 3-1, 3-2, 3-3 [a,b], Stage 4-1 [a,b,c,d,e], 4-2, 4-3). Time span of each hiatus among the four major stages is shorter enough to produce no black soils, and no significant difference in the ^<14>C age have been recognized between the stage 1 and stage 3. The major eruptive product of Hijiori volcano turns out to be valley filling non-welded (Stage1, 3-3 [a,b]) and partly welded (Stage 2-2) pumice flow deposits with total maximum thickness of about 150m and with a range 5km to the southward and 9km to the northward. The accompanying pumice fall (Stage 1, 2-2, 3-3 [a,b]) and ash fall (Stage 4-2) are dispersed 60km to the eastward. All the essential pumice from the eruptions were hypersthene-hornblende-quartz dacite. There are phreatic (or phreatomagmatic) eruptions at the beginning of Stage 2, 3, and 4 which deposited lapilli falls and flows (Stage 2-1, 3-1, 4-1 [a-e]) in the proximity. The volume of the valley filling pyroclastic flow deposit and the air fall deposit are 1.4 and 0.6km3, respectively, and the caldera filling deposit is estimated to be 0.3km3. Therefore, total eruptive volume of Hijiori volcano is estimated to be about 2.3km3.
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  • Akihiko TERADA, Yasuaki SUDO, Shin YOSHIKAWA, Hiroyuki INOUE
    Article type: Letter
    2007 Volume 52 Issue 6 Pages 335-340
    Published: December 28, 2007
    Released on J-STAGE: March 20, 2017
    JOURNAL FREE ACCESS
    New geothermal activities occurred at the Yoshioka hot spring of Aso Volcano from June 2006. We found a new fumarole named “a3” in a new steaming ground called “A”. The maximum temperature of the steams from it exceeded 98 degrees centigrade. In August, a vigorous fumarole “b1” and new steaming grounds “B” and “C” were formed. Vapor fluxes from these fumaroles obviously increased in October, and the daily mean heat and water discharge rates of the fumarole “b1” were estimated approximately to be 15-30MW and 6.7-13kg/s (580-1,100ton/day), respectively. Especially, “b1” ejected over 10 tons of ash on 16 October. After November 2006, the temperatures of the steaming grounds fell, and heat and the water discharge rates of the fumarole “b1” decreased to be about 4.6MW and 2.0kg/s (170ton/day), respectively. The average heat discharge rate from “b1” is more than tens times higher than that of existing natural fumaroles in the Yoshioka, Yunotani, Jigoku and Tarutama hot springs in the usual period. We infer that the vigorous geothermal events were caused by the temporal increase of vapor flux from the deep geothermal hot water.
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