Abstract
Abstract. The Schumann resonance phenomenon has been monitored at Nakatsugawa (near Nagoya) in Japan since the beginning of 1999, and due to the occurance of a severe earthquake (so-called Chi-chi earthquake) on 21 September 1999 in Taiwan we have examined our Schumann resonance data at Nakatsugawa during the entire year of 1999. We have found a very anomalous effect in the Schumann resonance, possibly associated with two large land earthquakes (one is the Chi-chi earthquake and another one on 2 November 1999 (Chia-yi earthquake) with a magnitude again greater than 6.0). Conspicuous effects are observed for the larger Chi-chi earthquake, so that we summarize the characteristics for this event. The anomaly is characterized mainly by the unusual increase in amplitude of the fourth Schumann resonance mode and a significant frequency shift of its peak frequency (~1.0Hz) from the conventional value on the By magnetic field component which is sensitive to the waves propagating in the NS meridian plane. Anomalous Schumann resonance signals appeared from about one week to a few days before the main shock. Secondly, the goniometric estimation of the arrival angle of the anomalous signal is found to coincide with the Taiwan azimuth (the unresolved dual direction indicates toward South America). Also, the pulsed signals, such as the Q-bursts, were simultaneously observed with the "carrier" frequency around the peak frequency of the fourth Schumann resonance mode. The anomaly for the second event for the Chia-yi earthquake on 2 November had much in common. But, most likely due to a small magnitude, the anomaly appears one day before and lasts until one day after the main shock, with the enhancement at the fourth Schumann resonance mode being smaller in amplitude than the case of the Chi-chi earthquake. Yet, the other characteristics, including the goniometric direction finding result, frequency shift, etc., are nearly the same. Although the emphasis of the present study is made on experimental aspects, a possible generation mechanism for this anomaly is discussed in terms of the ELF radio wave scattered by a conducting disturbance, which is likely to take place in the middle atmosphere over Taiwan. Model computations show that the South American thunderstorms (Amazon basin) play the leading role in maintaining radio signals, leading to the anomaly in the Schumann resonance.
Highlights
Electromagnetic phenomena associated with seismic activity have been extensively discussed, e.g. see the comprehensive monographs on these subjects by Hayakawa and Fujinawa (1994), Hayakawa (1999), and Hayakawa and Molchanov (2002)
The Schmann resonance (SR) resonance takes place in the Earth-ionosphere cavity driven by electromagnetic radiations from lightning discharges, which are concentrated in the tropical region (Nickolaenko and Hayakawa, 2002)
We can summarize the anomalous SR behaviors observed at Nakatsugawa prior to two large earthquakes which took place in Taiwan in September and November, 1999
Summary
Electromagnetic phenomena associated with seismic activity have been extensively discussed, e.g. see the comprehensive monographs on these subjects by Hayakawa and Fujinawa (1994), Hayakawa (1999), and Hayakawa and Molchanov (2002). Hayakawa et al, 2004; Hattori, 2004; Hayakawa and Hattori, 2004) Another is the active (or radio) probing of seismo-perturbations taking place in the atmosphere and ionosphere by means of radio transmitter signals; that is, these seimo-atmospheric and ionospheric perturbations are detected in the form of propagation anomalies of transmitter signals in different frequency ranges from VLF/LF to VHF. The waveform data from each channel are digitized with a sampling frequency of 100 Hz, and are saved on a hard disk every six hours The details of this ULF/ELF observing system has already been extensively described in Ohta et al (2001). We show briefly some ፧፧ᎌᎸ details of the SR phenomena for the last earthquake
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