On the association of anomalies in subionospheric VLF propagation at Kasugai with earthquakes in the Tokai area, Japan

Abstract

The subionospheric VLF (Very Low Frequency) signals (amplitude and phase) transmitted from NWC VLF transmitter (f=19.8kHz) in Australia, have been monitored at Kasugai (near Nagoya) in order to find out the ionospheric perturbations associated with earthquakes. The period of seven months from January to July 1998 when the observation was stable was chosen for the analysis. The terminator times (the times when a minimum appears in the diurnal variation of amplitude and/or phase around sunset and sunrise) are used to identify the VLF subionospheric propagation anomaly, in order to make correlation with the earthquakes in the Tokai area (relatively close to the great circle path (i.e. the path connecting the transmitter and receiver)) with magnitude (on the Richter scale) greater than 3.5. It is then found to be likely that significant anomalies (i.e. the associated ionospheric perturbations) have been observed as a precursor to a few isolated earthquakes when their depth is small and their magnitude is more enhanced.

オーストラリアのNWC VLF(超長波)送信機(f=19.8kHz)から伝送された下部電離層VLF信号(振幅および位相)を春日井(名古屋近郊)でモニタし、地震に伴って発生する電離層擾乱の検出を試みた。解析に当たっては、観測が安定していた1998年1月?7月の7か月の期間を選んだ。ターミネータ時間(日没および日の出近辺の振幅や位相の日変化に最小値が出現する時間)を用いてVLFの電離層下部伝搬異常を同定し、東海地方(大円経路、すなわち送信機と受信機を結ぶ経路に比較的近い)のマグニチュード(Richterスケールで)3.5以上の地震との相関関係を調べた。その結果、深さが浅くてマグニチュードが比較的大きな数個の孤立地震に対する先行現象として、有意な伝搬異常(すなわち、地震に伴う電離層擾乱)が観測されたらしいことが明らかになった。