Seismogenic Effects in the ELF Schumann Resonance Band

Abstract

The purpose of this paper is to review the seismogenic effects in the ELF (extremely low frequency) Schumann resonance (SR) band. Two interesting phenomena have been recently observed in this ELF SR band. The first is the enhancement of the SR third or fourth harmonic prior to a large earthquake (EQ), and the second is the excitation of SR-like line emissions possibly related with large EQs. The first phenomenon was initially found as the enhancement of the SR fourth harmonic in Japan, in possible association with the famous 1999 Chi-chi EQ in Taiwan. This was further statistically confirmed on the basis of data during several years and was additionally confirmed by further observations at a different place in Japan. The second SR-like line emissions have been detected in Japan in possible association with two huge EQs in Japan. The frequencies of those line emissions are likely to be shifted from the conventional SR harmonics by a significant amount of 1∼2 Hz, but their temporal variation seems to suggest a close relationship with the conventional SRs. Together with the explanation of the observational evidence of those two phenomena, we provide the plausible theoretical explanation of those phenomena. The first anomaly is interpreted in terms of the wave interference due to the significant difference in propagation length between the direct signal from the American lightning source and that scattered by the seismo-ionospheric perturbations above the epicenters of EQs in Taiwan. The second phenomenon is recently interpreted in terms of the ground detection of gyrotropic waves in the ionospheric dynamo region, being excited by the seismogenic noises from below in the ELF SR band.