著者

森谷 武男 茂木 透 高田 真秀 山本 勲

出版者

北海道大学大学院理学研究院自然史科学部門(地球物理学) = Department of Natural History Sciences (Geophysics), Graduate School of Science, Hokkaido University

雑誌

北海道大学地球物理学研究報告 (ISSN:04393503)

巻号頁・発行日

vol.72, pp.269-285, 2009-03-15

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A new observation system established in Hokkaido, northern Japan to confirm a suspected relationship between anomalous radio-wave propagation and impending earthquakes has been documenting anomalous VHF-band radiowave propagation beyond the line of sight prior to earthquakes since December, 2002. During such events, radio waves transmitted from an FM radio station were scattered, such that they could be received by an observation station beyond the transmitting station's line of sight. A linear relationship was established between the logarithm of the total duration time of the anomalous transmissions (Te) and the magnitude (M) or maximum seismic intensity (I) of the impending earthquake for M4- to M5-class earthquakes that occurred at depths of about 50 km beneath the Hidaka Mountains in Hokkaido, Japan in June 2004 and March 2008 as reported in the previous paper (Moriya et al., 2005). Similar linear relationships are also valid for earthquakes that occurred at other depths. Te is longer for shallower earthquakes and shorter for deeper ones. Numerous parameters seem to affect Te, including hypocenter depths and epicentral surface conditions (i.e., sea versus land). This relationship is important because it means that pre-seismic, anomalous transmission of VHF-band waves may be useful in predicting the size of an impending earthquake. To avoid misidentification of FM stations that have identical frequencies, three 64 MHz band transmitters were established, each with a unique frequency. Earthquakes that occurred in and around eastern Hokkaido scattered waves from FM-band and 64 MHz-band stations and provided quantitative relationships between Te and M, and between Te and I. Using the interferometer at the TES observation site, the incident azimuth of the scattering waves from the Hiroo station was measured. Prior to two earthquakes that occurred beneath almost the same part of the Tokachi region at depths of 86 km and magnitudes of M 4.9 and 4.0, the interferometer yielded incident azimuths of S18W and S34W. The true azimuths from TES to the hypocenters of the two earthquakes were S35W and S38W, respectively. These two measurements, therefore, suggest that anomalous transmission of VHF waves is caused by scattering at the epicenters of impending earthquakes.