Permalink : http://hdl.handle.net/2115/44479

JaLCDOI : 10.14943/gbhu.73.257

Anomalous VHF-band radio-wave propagation beyond the line of sight prior to earthquakes has been observed since 2002 in Hokkaido area. These records of the propagation have been collected as variations of electric field, and they include not only EQ-echoes which are shown as anomalous upward rapid drifts associated with an impending earthquake but also other transient signals and high frequency noises. It is not appropriate to apply the Fourier transform to such rapid variations because transient local changes in time domain data are transformed into the overall feature in frequency domain. In contrast, the wavelet transform suppresses this influence of tramsform by means of expansion and contraction of a mother wavelet. In this study, we applied band-pass filtering with the wavelet transform to reduce steady noise. As a result, we could distinguish components of high frequency noise and intensity variation relating to EQ-echoes. It improves the estimate of edge intervals of EQ-echoes. Furthermore, by the frequency analysis for the EQ-echoes associated with large earthquakes in Hokkaido, we found that high frequency components were attenuating in the EQ-echo especially in case of the southern Rumoi sub-prefecture earthquake（M 6.1）of 2004. This type of EQechoes may be characterized as long-distance propagation.