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Title: Analysis of Topographic Change at Mount Sakurajima, South Kyushu, Japan, using JERS-1 SAR Interferometry
Other Titles: JERS-1干渉SARを用いた桜島火山の地形変化解析
Authors: TOMIYAMA, Nobuhiro
KOIKE, Katsuaki  kyouindb  KAKEN_id
IGUCHI, Masato  kyouindb  KAKEN_id
OMURA, Makoto
Author's alias: 冨山, 信弘
小池, 克明
大村, 誠
井口, 正人
Keywords: 活火山
Active volcano
SAR interferometry
Issue Date: Mar-2011
Publisher: 日本情報地質学会
Journal title: 情報地質 = Geological data processing
Volume: 22
Issue: 1
Start page: 17
End page: 24
Abstract: Mount Sakurajima, situated in the southern Kyusyu district of southwest Japan, is known as one of the most active volcanoes in Japan. It has erupted intermittently, ejecting large volumes of volcanic materials including lava, scoria, and ash. We have adopted a SAR interferometry method using JERS-1 images to monitor volcanic activity and estimate accompanying topographic changes in order to mitigate hazard from the inevitable eruptions and debris flows. A map expressing the phase-difference distribution for pairs of images recorded on two acquisition dates is produced for this purpose. This map, obtained by subtracting orbital and topographic fringes from the initial interferogram, is capable of quantifying vertical displacements at each point on the SAR image. Considering the acquisition-date intervals for the images used, three phase-difference maps are generated for the period from January 1996 to March 1997. These show that relatively large displacements occurring along the valleys on the northern slopes of the volcano are common to the three pairs. Because the volcanic activity was low during the period, the displacements may be caused chiefly by the erosion of the surface materials. Differential SAR interferometry is thus thought to be capable of estimating temporal fluctuations of topography during arbitrary periods, detecting regions that are readily aggraded or eroded, and helping to determine the position and dimensions of debris barriers.
火山体の地形変化の把握は, 火山活動や広域応力場に起因した地殻変動の特徴の抽出において重要であり, 火山噴出物量の見積もり, および火山噴火後の降雨による流出量の推定という火山防災の分野にも貢献できる. LANDSATやSPOTに代表される光学センサによる観測は, 火山噴出物の分布範囲の特定に利用できるが, 地形や地形変化の詳細, かつ定量的な評価が困難である. そこで, 全天候型である合成開口レーダ(SAR)データに注目し, 活動が継続的な桜島火山を対象に選んで, JERS-1衛星データの干渉処理により火山体の地形変化の抽出を行った. 1996年1月~1997年3月の比較的連続する3つのデータペアで良質の干渉画像が得られたので, 解析はこの期間に限定した. 1996年の火山活動は穏やかであり, しかも変位量の大きな部分は侵食谷の位置に対応しているので, 干渉SARによって推定される地形変化は主に侵食に起因すると考えられた. 3ペアでの変位量を加算した結果から, 対象期間では侵食が優勢であり, 北岳の北側で20 cm以上の侵食量が推定でき, 侵食量の大きい部分が南北方向に伸びていることが見出された. また, 南西側の斜面では火山噴出物の供給が大きいので, 侵食量は相対的に小さいことなども明らかになった.
DOI(Published Version): 10.6010/geoinformatics.22.17
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