A statistical study on the effect of earthquakes on the ionosphere, based on the subionospheric LF propagation data in Japan

Abstract

Abstract.A superimposed epoch analysis has been undertaken, in order to find the correlation of the ionospheric perturbations with seismic activity. We take the wave path from the Japanese LF transmitter (frequency=40 kHz) and an observing station of Kochi (wave path length of 770 km), and a much longer period (of five years) than before, is considered. This subionospheric LF propagation can be called "an integrated measurement" in the sense that any earthquakes in the LF sensitive area just around the great-circle path can influence the observed LF signals, so that we define the "effective magnitude" (Meff) by integrating the total energy from different earthquakes in the sensitive area on a current day and by converting it back into magnitude. A superimposed epoch analysis for the effective magnitude greater than 6.0 has yielded that the ionosphere is definitely disturbed in terms of both amplitude and dispersion, and that these perturbations tend to take place prior to an earthquake. The statistical z-test has also been performed, which has indicated that the amplitude is definitely depleted 2-6 days before the earthquake day and also that the dispersion is very much enhanced during the same period. This statistical study has given strong support to the existence of seismo-ionospheric perturbations for high seismic activity.

On the statistical correlation between the ionospheric perturbations as detected by subionospheric VLF/LF propagation anomalies and earthquakes

“…We follow our conventional way of analysis in VLF/LF data (Rozhnoi et al, 2004;Maekawa et al, 2006;Hayakawa, 2007;Molchanov and Hayakawa, 2008), and we show the analysis procedure briefly. In order to study short-term variations (such as EQ precursors), we want to remove the longterm (e.g., seasonal) variation from the data.…”

“…Then, we pay attention to the local nighttime (UT=11:00 a.m.-07:00 p.m.; LT=08:00 p.m. to 04:00 a.m.). Two physical parameters are dealt with as in Maekawa et al (2006); (1) trend (or average amplitude at night) and (2) nighttime fluctuation (abbreviated as N.F.). As for (1) trend, we estimate the average (mean) value of the amplitude (dA>0 and dA<0) during the whole night (LT=08:00 p.m. to 04:00 a.m.).…”

“…As is already shown in Hayakawa (2007) and Molchanov and Hayakawa (2008), we have a Japanese VLF/LF network consisting of seven observing stations within Japan and also one station in Kamchatka (Russia). The details of our VLF/LF receiving system have already been described in Maekawa et al (2006) and Hayakawa (2007). At each station we normally observe simultaneously several VLF/LF transmitter signals (normally two Japanese transmitters (one is JJY (40 kHz, in Fukushima) and JJI (22.2 kHz, at Ebino, Kyushu)), so that we have many combinations of the transmitter and receiver.…”

“…According to our previous two papers on the statistical study on the correlation between VLF/LF anomalies and EQs (Rozhnoi et al, 2004;Maekawa et al, 2006), the correlation is significant only when EQ magnitude is, at least, 5.5. So that, we impose the following criteria on the selection of EQs for our analysis.…”

Anomalous radon emanation linked to preseismic electromagnetic phenomena

“…Among them, ionospheric perturbations have attracted many researchers and have been investigated for earthquake predictions Pulinets and Boyarchuk, 2004). Gokhberg et al (1989) and pointed out the precursory ionospheric anomalies associated with large earthquakes, and Maekawa et al (2006) emphasized the verification of the existence from a statistical analysis of the subionospheric LF propaCorrespondence to: Y. Omori (big-forest@dges.tohoku.ac.jp) gation data (Kamogawa, 2007). Since lithosphere does not have a direct connection with the ionosphere, problems related to seismo-ionospheric disturbances are treated by considering the interaction among the lithosphere, atmosphere and the ionosphere, i.e., the LAI coupling Kamogawa, 2006).…”