The Activities of Anak Krakatau Volcano during the Years of 1992 - 1996

Abstract

1992年-1996年にかけて発生したクラカタウ火山の噴火活動について報告する。前兆となる地震活動は1992年8月17日から始まり, 11月8日の最初の爆発の前には震源の浅いB型地震(M<2)が群発し, 爆発の直前10時間には400個以上が観測された。11月13日には規模の大きい爆発により山頂の北側に新たな噴火口が形成され, そこから流出した溶岩は北海岸に達した。また, 高さ100mのスコリア・コーンが形成され, 標高300mに達した。この噴火活動は現在も継続中であるが, 以前ほど多量の噴出物を放出しているわけではない。1996年の7月には更に北側に3つの火口が形成され, 最も北側の火口から流出した多量の溶岩は北岸及び西岸に達した。4年間の活動期間中に放出された噴出物量は1200万立方メートル以上と推定される。

The 1992-1996 eruption of Anak Krakatau, Sunda Strait, South Lampung, Indonesia, began on November 8, 1992, less than fours after a swarm of the volcanic earthquakes struck beneath the volcano, which has recorded by one unit of PS-2 seismograph telemetric system.Elevated levels of seismicity were first observed on August 17, 1992, and the seismicity rapidly developed into an intense swarm of volcanic earthquakes on November 8. More than 400 volcanic earthquakes smaller than magnitude 2 were recorded in the 10 hours before the first eruption on November 8.A major explosive eruption occurred on November 13 that opened a new vent in the north part of the summit and followed by the first lava flow flowing to the north coast. The seismicity level was highly variable from November 13. It consisted of periods of intense seismicity associated with explosive activity. The initial explosive episodes were preceded and accompanied by shallow earthquakes of a low-frequency type (B-type).Tephra from the explosions spread surrounding the volcano island of Anak Krakatau and not affected the inhabitant areas of South Lampung and Banten, even the other islands around the volcano island. The eruption materials built a new cone and added the summit of the volcano as high as 100 meters, and the approximate highest peak reached 300 meters above sea level.The eruption continued until today and did not produce as much erupted material. In July 1996, the vent developed into three eruption points in a row direction to the north from the main vent. The northest vent produced a large lava flow which flowed to the west and north coasts. During the four years activity, the eruptions produced more than 12 millions cubic meters of erupted materials that was calculated from the loss materials and lavaflows.