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International audience; The paper presents the results of a joint analysis of ground-based and satellite observations of very low-frequency and low-frequency (VLF/LF) signals during periods of strong seismic activity in the region of Kuril Islands and Japan in 2004–2010. Ground and satellite data was processed using a method based on the difference between the real signal in nighttime and that of a model. The results of the analysis show a good correlation between ground-based and satellite data for several cases of strong (M ⩾ 6.8) earthquakes.

Since 1996, the electric field strength of the broadcasting station CZE (Czech Republic, f = 270 kHz) has been sampled each ten minutes, by a receiver (AS) located in central Italy, 818 km far from the transmitter. Here, we present the results obtained by a detailed analysis applied on the data recorded from February 1996 up to September 2004. At first, we separated the day time data and the night time data in the radio signals; then, in the day time data we separated the data collected in winter from the data collected in summer. Finally, we applied the wavelet analysis on the previous trends. The first result was the appearance of a very clear anomaly during February–March 1998, at winter day time and at night time. This result confirms an anomaly revealed previously in the same data but analysed with a different approach. The anomaly was related to a strong ( M = 5.1–6.0) seismic sequence occurred in a zone (Slovenia) lying in the middle of the transmitter–receiver path. The present result reinforces the hypothesis of the occurrence of some disturbances in the ionosphere during the preparatory phase of the Slovenia seismic sequence. The second result came from the wavelet analysis applied to the summer day time data and it was the appearance of a very clear anomaly during August–September 1997. On September 26 the Umbria–Marche (central Italy) seismic sequence started with two earthquakes with magnitude M = 5.6 and M = 5.9 and the seismic activity lasted for more than six months. We consider the August–September 1997 radio anomaly as a precursor of the previous earthquakes and a possible explanation model is proposed.

Variations of the LF signal from observations in 6 receiving stations during 2000–2004 are analyzed. Five stations are located in Japan (Kochi, Moshiri, Kasugai, Shimizu and Chiba) and one station is in Russia (Petropavlovsk–Kamchatski). All the stations are equipped with the identical OMNIPAL receivers. JJY (40 kHz) transmitter is situated in Fukushima prefecture (Japan). Sensitivity of the phase and amplitude of the LF signal to ionosphere and atmosphere perturbations associated with variations of the magnetic and meteorological conditions and solar activity is studied. Correlation of phase and amplitude variations of the LF signal with D st -index, outer-zone particles (protons and electrons) with high-pitch angle is established for all the stations. It is found that LF anomalies are typical for the main phase of magnetic storm if the sudden commencement happens in the night time. The similarity of phase and amplitude anomalies to the structure of the simultaneously observed Pi3 geomagnetic pulsations is found for all the stations. Correlation between variations of atmosphere pressure and amplitude of the LF signal is established. Correlation between variations of humidity for the stations located on the coast of Japan and amplitude of the LF signal is found.

The variations of phase and amplitude of LF signal propagating from the transmitter JJY (40 kHz, Fukushima prefecture, Japan) are analyzed. The receiver is situated in Petropavlovsk-Kamchatski (Russia). The time interval from July1, 2004 till January 24, 2005 is included in the examination. This interval is characterized by quiet seismic conditions up to the beginning of November, but rather strong seismic activity occurs in November and December not far from Hokkaido (Japan) and in the region of northern Kuril Islands. Seismic activity in a zone of sensitivity of our wave paths is determined by three series of earthquakes with M = 5.6–7.1. Nighttime “bay-like” phase and amplitude anomalies of the LF signal begin some days before the first earthquake and continue till the last earthquake of this series ( M = 6.2, November 11, 2004). Most strong and obvious anomalies appear in the amplitude of the LF signal. After that, during about one fortnight a period of seismic calm is observed. Then there is a following anomaly of the LF signal before two earthquakes ( M = 7.1, 28.11.04 and M = 6.8, 6.12.04). The next LF anomaly is observed before the earthquakes on December, 18 and 21 ( M = 5.8 and M = 5.6). During the whole period of seismic activity a significant shift in terminator times is also evident. The spectrum of LF seismo-induced anomalies shows a clear increase for the periods of 10–25 min.

Air temperature, surface temperature, retrieved from satellite data, and well observations on the Kamchatka peninsula, Far East, Russia were jointly analysed. Air temperature indicates correlation with seismic activity. Satellite observations showed the presence of thermal anomalies on the earth surface in the basin of the Kamchatka river. Thermal anomalies reaction on three strong earthquakes were recorded. Water temperature, outflow and hydrogen ion exponent (pH) changes were observed as a response to seismic events. Joint analysis indicates similarity both satellite and ground observations related to earthquakes. A model of litho–atmo–ionospheric coupling in seismic processes is proposed.

The probability of 2–3 h variation in electron concentration at different regions of ionosphere 1–3 days before earthquakes at distances R 4.5 the number of variations with time scales of 2–3 h in the lower F-layer of the ionosphere f300 (real altitude of about 250 km; virtual altitude of 300 km) increases for 10 earthquakes, decreases for 4 and it does not changes for 2, on 16 cases we analyzed. Using the hourly data of 10 ionospheric stations collected for 20–30 years it was revealed that the number of disturbances in the foF2-frequency (real altitudes of about 300–350 km corresponding to the maximal electron density of F2 layer) with time scales of 2–3 h does not change before earthquakes with M⩽5.5 and slightly decreases (on 9%) before more strong earthquakes (M>5.5). Possible models able to justify the phenomenology we observed are presented.