PALSAR/InSARにより得られた小笠原硫黄島の2006年火山活動活発化に伴う地殻変動:速報

Abstract

2006年8月頃から火山活動の活発化が観測された小笠原硫黄島を調査するため、陸域観測技術衛星「だいち(ALOS: Advanced Land Observing Satellite)」のPALSAR(Phased Array-type L-band Synthetic Aperture Radar)データを用いたSAR(Synthetic Aperture Radar)干渉法の適用を行った。元山が隆起を開始する直前においては、従来から継続的に進行していた収縮変形パターンは鈍化していたように見える。また、隆起に転じてから11月までの期間においては、島全体が隆起するような地殻変動が生じていたことを示唆する結果が得られた。このことは、11月までの期間に比較的深部に位置する力源の膨張に起因する地殻変動が卓越していたと考えられる。もっとも火山活動が活発であった11月以降においては、ディセンディング軌道とアセンディング軌道からの2方向からの観測が行われ、これらのデータから地殻変動の準上下成分と準東西成分の2成分を求めた。島の南端付近における隆起量は8cm程度であるが、元山に近づくにつれて隆起量は大きくなり、島の北端付近では約40cmの隆起が検出された。西海岸付近における南北に伸びる断層帯においては、拡大かつ西落ちの上下変位を伴う地殻変動が検出された。特に、断層帯の東端に位置する阿蘇台断層において、変形が集中していることが特徴である。また、元山と千鳥ヶ原の境界付近に位置する東西に伸びる断層帯においても、元山側が隆起する方向の急激な変形が見られ、元山がブロック状に隆起しているように見える。元山内部においては、中央部における収縮変形パターンや北端、南東海岸域における隆起が重畳している。特に、北端域においては東進、南東岸域においては西進を伴っていることが特徴である。これらの地殻変動パターンの空間波長は比較的短いことから、比較的浅部に位置する力源に起因するものと推測される。

Vertical movement at a GPS (Global Positioning System) site located in the Motoyama district of Iwo-jima volcano changed from continuous subsidence since 2003 to uplift in mid-August of 2006. Increased seismicity was also observed. Observations by PALSAR (Phased Array-type L-band Synthetic Aperture Radar) (synthetic aperture radar sensor) aboard the ALOS (Advanced Land Observing Satellite: Daichi) satellite were conducted to investigate the volcanic activity. In this study, we are attempting to detect crustal deformation associated with the increase in volcanic activity by SAR interferometry using PALSAR data. We report the preliminary results in this paper. Just before the increase of volcanic activity, a slant-range change representing a subsidence of several centimeters was detected throughout the Motoyama district, but it seemed to differ from the contraction pattern that had been dominant in the area. The contraction in Motoyama may have slowed down just before the increase in volcanic activity. Three months after the increase in volcanic activity, a slant-range change suggesting that the whole island had uplifted was detected. It suggests that crustal deformation induced by a relatively deep source was prominent in this period. When the volcanic activity was highest, the uplift accelerated and seismicity increased further. PALSAR observations were conducted from ascending and descending orbits in this period, and a two-dimensional displacement field could be obtained from these data. The obtained result is characterized by an uplift pattern suggesting inclination of the whole island with uplifts in the south end of the island of 8 cm and uplifts in the north end of 40 cm. In the north-south fault zone in the west coast area, crustal deformation with an expansion and an east-up pattern was detected. Concentrated deformation was detected along the Asodai Fault located in the east end of the fault zone. Additionally, a steep gradient of vertical movement detected along the fault zone located in the south end of Motoyama district extended to the east and west. This crustal deformation pattern suggests a blockwise uplift of Motoyama district. In the interior of Motoyama district, a contraction pattern in the center area and an uplift pattern in the north and the southwest area were superposed. Uplift patterns in the north are was accompanied by eastward motion and uplift in the southeast area was accompanied by westward motion. Since the wavelengths of these deformation patterns were relatively short, crustal deformation induced by a relatively shallow source must have dominated in this period. The detailed crustal deformation detected in this analysis is expected to be important information for revealing mechanisms of the Iwo-jima volcano.