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The observation of broadband co-seismic electromagnetic waves in VHF band

Observations in electromagnetic fields have been considered as one of the most promising candidates for short-term earthquake prediction. We have been interested in VHF band where there are few artificial and urban sources of noise compared with other lower frequency bands. Our observation method, the synthesized tuner method, with a PLL type FM tuner which has a high frequency stability and sensitivity from 76 MHz to 90 MHz. In this paper, we describe the observation system configuration and a newly proposed the dual frequency observation method in order to realize more accurate observations based on the synthesized tuner method. Our observation method can detect up to about -120 dBm (0.001 pW) of the received level. Therefore it can observe galactic noise of about -114 dBm. Moreover, this paper shows that our system in Hiroshima City University successfully observed the naturally radiated VHF wave corresponding to the Tottori-ken Seibu Earthquake (magnitude 7.1, depth 10 km) on October 6 in 2000.