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タイトル: 伊豆半島東部地域の異常地殻活動に伴う全磁力変化(第4報) : 1981年伊豆半島東北部の異常変化
その他のタイトル: Changes in the Geomagnetic Total Force Intensity Associated with the Anomalous Crustal Activity in the Eastern Part of the Izu Peninsula (4) : Anomalous Change Observed in the North-Eastern Area in 1981
著者: 笹井, 洋一
石川, 良宣
著者(別言語): Sasai, Yoichi
Ishikawa, yoshinobu
発行日: 1986年3月31日
出版者: 東京大学地震研究所
掲載誌情報: 東京大学地震研究所彙報. 第57冊第4号, 1986.3.31, pp. 739-757
抄録: 伊豆半島東部地域のプロトン磁力計観測網によって,1981年7月から10月にかけて,伊豆半島北東部に,全磁力の減少と回復が観測された.この異常変化は,伊東市付近を中心に半径10km以内にある,初島,浮橋,吉田で見出され,最大の減少量は5nTに達する.一方,1980年と1981年の冬期に,鹿野山の全磁力値が数nTの一時的増加を示した.このため,鹿野山を基準としている東伊豆の全磁力地点差に,見掛けの減少ピークを生じた.この変動は,季節的変化にも見えるが,過去において冬期に目立った変化は認められていない.地震予知研究計画地磁気グループの公表データに基づき,1980年6月から1982年5月迄の,関東・東海地域11点における全磁力変化を調べた.離島(八丈島,伊豆大島野増)や海岸近傍(御前崎)の観測点に,極めて大きな変動が見られる.これは海流のダイナモ作用による磁場変化である可能性が強い.伊豆半島の観測点では,1980年中期以降,周辺地域に比べて全磁力が減少していることが判った.伊豆半島北東部の異常変化の原因として,①地殼隆起に伴うピエゾ磁気効果,②海流のダイナモ作用の漏洩電流,③界面動電現象の磁場発生効果,が考えられ,これ等について簡単な吟味を行なった.
The Eastern-Izu magnetometer array (Fig. 1) detected a transient decrease in the total intensity in the north-eastern area of the Izu Peninsula during the period from July to October, 1981. The anomaly was found within an area with a 10 km radius, centered on Ito city, where the crustal uplift is still continuing. The total field change amounted to 5 nT at three stations (HAT, UKH, YSD), while a minor decrease was observed at one of the neighbouring stations SGH (Fig. 2(c)). The total field differences showed spurious bay-like variations from Dec, 1980, to Feb., 1981, and from Dec, 1981, to Jan., 1982, owing to transient increase of the F component at reference station KNZ. This may be a kind of seasonal change, although no dominant variations in winter were noticed before 1980 at KNZ. The total field variations at 11 standard observatories in the Kanto and Tokai districts were plotted in Fig. 7(a), (b) and (c) on the basis of data published by the Geomagnetic Research Group for Earthquake Prediction, Japan. Some island stations (HJJ, NOM) and some near the coast (OMZ) showed a large amount of disturbances, which might be brought about by the dynamo action of ocean currents. In order to interpret the transient anomaly on Izu, the piezomagnetic field associated with a macroscopic model of crustal dilatancy was investigated, which is the multiple Mogi model as proposed by Hagiwara (1977). The model consists of microcracks clustered in the upper crust with the Gaussian distribution : each microcack is simulated by a point force source of the center of dilatation. According to the present model, the total field decreases upon the upheaval. Even a few cm uplift can produce a few nT decrease in the F component, if we are allowed to assume an intense crustal magnetization (5.0×10-3emu) and stress sensitivity (2.0 × 10-4 bar-1) in the eastern Izu region concerned. No definite evidence is, however,obtained that the magnetic change did accompany any significant uplift and subsidence. An alternative explanation is that the observed anomaly might be caused by the dynamo action of ocean flow. Again, we have no evidence of unusual ocean flow off the east coast of the Izu Peninsula. The third physically plausible interpretation is that due to the electrokinetic-magnetic effect. Unfortunately, we have no self-potential data in the eastern Izu region. The electrokinetic effect is less promising because no anomaly in the ground-water level is reported by official and volunteer well-observers in the peninsula.
URI: http://hdl.handle.net/2261/12865
ISSN: 00408972


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