On the statistical correlation of over-horizon VHF signals with meteorological radio ducting and seismicity

With the use of a complete 1 year (2005) data on over-horizon VHF signals (transmitted in Sendai and received in Chofu), we have investigated the statistical significance on the correlation of VHF signal characteristics (amplitude and variance) with meteorological radio duct effect and seismicity (earthquakes). It is found that the meteorological radio duct effect is rather predominant in July-September. On the other hand, over-horizon VHF signals are found to be correlated with earthquakes in other seasons. Especially, when we narrow our analysis area just around the middle region of the great-circle path, we have always significant correlation between VHF signal characteristics (average, median intensity or variance) with seismicity in any months of the year, with exceeding the confidence level.