伊豆半島東部地域の異常地殻活動に伴う全磁力変化(第三報) : 1980年伊豆半島東方沖地震

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This is a report of unsuccessful detecting of the seismomagnetic effect associated with the East off Izu Peninsula Earthquake of M 6.7 on June 29, 1980. Localities of observation points in and around the Izu Peninsula are shown in Fig. 1. Total field changes at the time of the earthquake are given in Fig. 2 for stations in the eastern part of the peninsula. At the nearest stations, i.e. HAT and YSD, we see some gap-like changes before and after the quake. This change turned out, however, to be an apparent one caused by power failure to the direct electric current railways. As shown in Fig. 3, magnetograms at YSD and HAT are greatly affected by electric car noises. Records at four stations on Izu-Oshima Island are compared with reference station KWZ and depicted in Fig. 4. The total field jump of 2 gammas or so at NOM might be caused by the resional discrepany of daily variations between the Izu-Oshima Island and the KWZ station, because we recognize no significant step-like change at other continuously recording stations. Local changes in the total field as revealed by the repeated surveys were very small-practically zero in view of the accuracy of the survey data-during the period from Mar. to July, 1980, as shown in Fig. 5. This is in harmony with the continuous observations at four temporary stations as shown in Fig. 7. The seismomagnetic effect possibly accompanying the M 6.7 earthquake is computed for a fault model based upon the studies by SHIMAZAKI (1980), KARAKAMA et al. (1980) and Hatori (1980). The solution for a vertical rectangular strike-slip fault by Sasai (1980) is employed. The model calculations indicate that we could expect no detectable total field change on land. A minor decrease in the total intensity at SGH and HAT is found during the period from Sept. to Dec, 1979 (Fig. 8), which might have some causal relation to the anomalous land uplift along the east coast during the same period. The present observation has been conducted under the cooperative project of the geomagnetic group for researches in the Izu Peninsula (RIKITAKE et al. 1980).
1981-03-31

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笹井洋一
東京都総務局総合防災部

伊豆半島東部地域の異常地殻活動に伴う全磁力変化(第三報) : 1980年伊豆半島東方沖地震

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