Interferometric direction finding of over‐horizon VHF transmitter signals and natural VHF radio emissions possibly associated with earthquakes

Abstract

The over‐horizon (out‐of‐sight) VHF transmitter signals are found to be received before an earthquake, and also natural VHF noises are known to be detected prior to an earthquake. In order to have a better understanding of these over‐horizon VHF signals and noises, an interferometric direction‐finding system has been, for the first time, introduced, and some preliminary observational results have been presented here. With FM Sendai as a target VHF transmitter (at 77.1 MHz), we have carried out the observation at Chofu (Tokyo). We can receive occasionally the over‐horizon VHF transmission signals in possible association with earthquakes. We have analyzed a few earthquakes since the commencement of our observation, including a large one named the Niigata Chuetsu‐oki earthquake. The azimuth distribution by means of the interferometer has enabled us to correlate any burst in the temporal evolution of electric field intensity to a certain earthquake. The over‐horizon VHF transmitter signals are always well above the background noise, whose azimuth is found to be relatively close to the direction of Sendai (relatively away from the epicentral direction). Then, the VHF radio noises are always simultaneously detected as an enhancement of background noise, with their azimuths being relatively close to the epicenter of the quake. The mechanisms of seismoatmospheric perturbations as seen from the over‐horizon VHF signal, as well as the natural VHF noise, are discussed.