2000年鳥取県西部地震および2001年芸予地震に関連したVHF帯電磁現象の観測

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タイトル別名
  • Observation of Co-seismic Electromagnetic Phenomena in VHF Associated with the Tottori-ken Seibu Earthquake in 2000 and the Geiyo Earthquake in 2001
  • 2000ネン トットリケン セイブ ジシン オヨビ 2001ネン ゲイヨ ジシン ニ カンレン シタ VHFタイ デンジ ゲンショウ ノ カンソク

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Many observations have been made in order to clarify the relationship between electromagnetic (EM) phenomena and earthquake (EQ) activities. We have been observing EM waves in east, west, south and north directions by a PLL type synthesized FM tuner from 76 MHz to 90 MHz in VHF band. This frequency band is strictly administrated for FM broadcastings in Japan and hardly influenced by artificial or urban noises compared with other lower frequency bands. In observing EM phenomena at FM broadcasting frequency band, an observation method that distinguishes seismic EM waves from FM broadcasting waves is required. Therefore, we have developed a novel dual frequency observation method in which the observation is made at two different frequencies of fn and fr. During the observation, the Tottori-ken Seibu EQ (M=7.3, d=11 km) occurred on October 6, 2000. Co-seismic broadband EM waves associated with the Tottori-ken Seibu EQ were detected by the observation system located at Hiroshima City University (HCU) 130 km from the epicenter. The observed EM waves rose up to the peak level right after the EQ, then slowly returned to a normal level about three hours later. Following the Tottori-ken Seibu EQ, we had the Geiyo EQ (M=6.4, d=51 km) on March 24, 2001. Our observation system detected two kinds of EM phenomena associated with the Geiyo EQ at HCU 45 km from the epicenter. One EM phenomenon detected in VHF band appeared after the earthquake. The observed EM waves followed similar variation to that of the Tottori-ken Seibu EQ, which had a sharp rise followed by a slower decline lasting about three hours. EM levels of both fn and fr frequency channels were similarly fluctuated. It is therefore suggested that the detected EM waves had broadband spectra. The maximum peak level of -98 dBm was observed in the west direction that was not toward the epicenter. The peak levels of EM waves were detected at 15 : 28 : 20 (JST) after the occurrence time of the Geiyo EQ, at 15 : 27 : 54 (JST). The time when the detected EM waves rose up to the peak level corresponded to the time of seismic swing arrival at the HCU observatory. The other EM phenomenon detected in VHF band disappeared at the time of earthquake. Amplitude fluctuations of about 2 dB were observed since one day before the Geiyo EQ, then, the fluctuations disappeared at the occurrence of the earthquake. These fluctuations were mainly detected from the east and south directions, which coincide with the direction toward the epicenter.

収録刊行物

  • 地震　第２輯

    地震　第２輯 55 (2), 107-118, 2002

    公益社団法人 日本地震学会

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