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29. 東伊豆地域の光波測量結果と歪の蓄積・解放過程

By 照之 加藤, 滋夫 松本, 安廣 平田 and 茂 渡辺


地震研究所の光波移動班は最近の伊豆半島の地殻活動による地殻歪の変化を監視する目的で1976年から光波測量を東伊豆地域において実施してきた.東伊豆基線網では1983年2月までに8回の測量が行われ,7期間について辺長変化が求められた.この結果を用いて歪解析を行ったところ,以下に示すような,この地域の地殻活動との興味ある相関が認められた.(1)歪変化から求められた歪の主軸は,各期間共,一方が伸び,他方が縮みを示す.このことは地震のメカニズムや活断層が横ずれ型であることと調和的である.(2)期間毎の主歪のパターンは(a)北西一南東縮み・北東一南西伸び,と(b)北西-南東伸び・北東一南西縮み,の2種類に大別され,これらは地震活動の低い時期と高い時期にそれぞれ対応する.これは,フィリピン海プレートが北西進して本州と衝突していることを考え合わせ,地震活動の低い時期には地殻内に歪が蓄積し,ある限界値をこえて地震活動が活発化すると歪が解放され,主歪のパターンが反転すると考えればよく説明できる.We have conducted geodimeter surveys eight times at the Higashi-Izu base line network, eastern part of Izu peninsula, from February, 1976 to February, 1983. By analysing the observed distance changes for four base lines (2.6-6.9km), the following features are suggested; (1) The two principal strain axes have appeared with mutually opposite signs, suggesting that the axis of intermediate principal strain is vertical. From this we may expect the strike-slip mechanism for earthquakes in this area. This account is consistent with the observed mechanism of earthquakes and the strike-slip active faults. (2) Temporal reversal of pattern of principal axes, i.e. contraction to extension and vice versa, correlates well with the sequence of energy release rate by earthquakes in this area. That is, when a NW-SE extension is observed, the energy release rate is generally higher and vice versa. This is readily understood from consideration that this area is subject to the NW-SE contraction due to convergence of the Philippine Sea plate against the Honshu block, and that the reversal of patterns of principal strains is attributed to release of the accumulated strain energy by crustal activities

Topics: 453
Publisher: 地震研究所
Year: 1984
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Provided by: UT Repository
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