A study of ionospheric response to regional seismic activity by VLF radio sounding

• Published in: Physics and chemistry of the earth. Parts A/B/C 2004, Vol.29 (4), p.627-637
• Main Authors: Shvets, A.V., Hayakawa, M., Molchanov, O.A., Ando, Y.
• Format: Article
• Language: English
• Subjects: Earthquake prediction; Ionospheric perturbations; VLF subionospheric propagation
• ISSN: 1474-7065; 1873-5193
• DOI: 10.1016/j.pce.2003.08.063

Electric field strength nighttime fluctuations in subionospheric signals of very low frequency (VLF) on Omega, Tsushima-Chofu (Tokyo) and NWC, Australia-Chofu propagation paths recorded during March–August, 1997 were studied for searching for correlation with seismic activity. Wave-like anomalies in VLF Omega signal with periods of a few hours were observed 1–3 days before or on the day of moderately strong earthquakes of magnitudes 5–6.1. Analysis of the correlation between regional seismic activity around Tokyo and dispersion of VLF fluctuations revealed that maximal correlation was observed during pre-seismic periods for shallow earthquakes (focal depth less than 40 km) that occurred at distances up to 350–400 km from receiving place. Spectral analysis of the earthquake catalogue has shown rhythmical variations of seismic activity integrated over large areas with periods of 4–14 days. Similar periodic components are observed in fluctuations of VLF signals propagating over the NWC-Chofu path. For both VLF propagation paths, a 27-day periodicity was found in fluctuations of signals that can be referred to as tidal or solar rotation influence.