Anomalous ELF phenomena in the Schumann resonance band as observed at Moshiri (Japan) in possible association with an earthquake in Taiwan

Abstract

Abstract. The ELF observation at Moshiri (geographic coordinates: 44.29° N, 142.21° E) in Hokkaido, Japan, was used to find anomalous phenomena in the Schumann resonance band, possibly associated with a large earthquake (magnitude of 7.8) in Taiwan on 26 December 2006. The Schumann resonance signal (fundamental (n=1), 8 Hz; 2nd harmonic, 14 Hz, 3rd harmonic, 20 Hz, 4th, 26 Hz etc.) is known to be supported by electromagnetic radiation from the global thunderstorms, and the anomaly in this paper is characterized by an increase in intensity at frequencies from the third to fourth Schumann resonance modes mainly in the BEW component with a minor corresponding increase in the BNS component also. Spectral modification takes place only in the interval of 21:00 UT±1 h, which corresponds to the global lightning activity concentrated in America. While distortions were absent in other lightning-active UT intervals, in particular, around 08:00 UT±1 h (Asian thunderstorms) and around 15±1 h (African lightning activity). The anomaly occurred on 23 December three days prior to the main shock. The results observed were explained in terms of ELF radio wave perturbation caused by the lower ionospheric depression around the earthquake epicenter. The difference in the path lengths between the direct radio wave from an active global thunderstorm center and the wave scattered from the non-uniformity above Taiwan causes interference at higher resonance modes, which is successful in explaining the observational data.