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In this paper we describe the Graz seismo-electromagnetic very low frequency (VLF) facility, as part of the European VLF receiver network, together with the scientific objectives and results from two years operation. After a brief technical summary of the present system – with heritage from a predecessor facility – i.e. hardware, software, operational modes and environmental influences, we discuss results from statistical data and scientific events related to terrestrial VLF propagation over Europe.

In this study, we report on Total Electron Content (TEC) and Very Low Frequency (VLF) space measurements derived from Global Positioning System (GPS) and DEMETER satellites, respectively. These measurements are associated with the earthquake (EQ) of a magnitude of 6.3, which occurred on 6 April 2009, in L'Aquila (Italy). Anomaly features are derived from the analysis of TEC and VLF observations recorded two weeks before and after the seismic event occurrence. A TEC map with an interpolated regional pixel resolution of 1° × 1° × 15 min in latitude, longitude and time was generated, allowing for the checking of a possible presence of disturbances over the L'Aquila region. This analysis is combined with the study of the time profile associated to the VLF flux density variations recorded by the Instrument Champ Electrique (ICE) experiment on-board DEMETER satellite. We discuss, on the one hand, the combination efficiency of the electronic density and the VLF electromagnetic measurements and, on the other hand, the difficulty to distinguish between global effects and regional ones related to the earthquake.

In this paper, we report the observation of a significant decrease of the vertical component of the atmospheric electrical field in the Évora region (Portugal) during the M = 4.1 Sousel earthquake of 27 March 2010. The epicentre of the earthquake was 52 km from the observation site, which falls within the theoretical earthquake preparation radius. A simple interpretation based on hypothetical radon emissions is presented, and future experiments required to elucidate these observations are outlined. To our knowledge, this is the first reported observation of a decrease of the atmospheric electrical field preceding an earthquake.

In the last years disturbances in VLF/LF radio signals related to seismic activity have been presented. The radio data were collected by receivers located on the ground or on satellites. The ground-based research implies systematic data collection by a network of receivers. Since 2000 the "Pacific VLF network", conducted by Japanese researchers, has been in operation. During 2008 a radio receiver was developed by the Italian factory Elettronika (Palo del Colle, Bari). The receiver is equipment working in VLF and LF bands. It can monitor 10 frequencies distributed in these bands and, for each of them, it saves the power level. At the beginning of 2009, five receivers were made for the realization of the "European VLF/LF Network"; two were planned for Italy and one for Greece, Turkey and Romania, respectively. In 2010 the network was enlarged to include a new receiver installed in Portugal. In this work, first the receiver and its setting up in the different places are described. Then, several disturbances in the radio signals related to the transmitters, receivers, meteorological/geomagnetic conditions are presented and described.

This paper presents a work that aims to monitor seismo-electromagnetic phenomena in the Western Part of the Eurasia-Nubia Plate Boundary. This region has a significant tectonic activity combined with relatively low electromagnetic noise levels, rendering high quality seismo-electromagnetic measurements possible. An overview of the seismicity of this region is presented and the research plan is discussed accordingly.

We investigate the VLF emissions observed by the Instrument Champ Electrique (ICE) experiment onboard the DEMETER micro-satellite. We analyze intensity level variation 10 days before and after the occurrence of l'Aquila earthquake (EQ). We found a clear decrease of the VLF received signal related to ionospheric whistler mode (mainly Chorus emission) and to signal transmitted by the DFY VLF station in Germany, few days (more than one week) before the earthquake. The VLF power spectral density decreases of more than two orders of magnitude until the EQ, and it recovers to normal levels just after the EQ occurrence. The geomagnetic activity is principally weak four days before EQ and increases again one day before l'Aquila seismic event. Our results are discussed in the frame of short- and long-terms earthquakes prediction focusing on the crucial role of the magnetic field of the Earth.

The VLF/LF radio signals method for studying preseimic activity is applied to the Abruzzo earthquake (M=6.3, 6 April 2009). The data collected by three receivers located in Moscow (Russia), Graz (Austria) and Bari (Italy) at about 3000 km, 1000 km and 500 km from the epicenter were used. The signals received from the Sardinia (20.27 kHz) and the Sicily (45.9 kHz) transmitters, both located in Italy, were compared with those received from the Iceland (37.5 kHz), the Great Britain (19.58 kHz) and the Germany (23.4 kHz) transmitters. The propagation paths of the two Italian transmitters cross the epicentral area (seismic paths) unlike the paths of the other three signals (control paths). Using two different analyses, that are the study of the night-time signal and the research of shifts in the evening terminator times, clear anomalies were revealed 2–8 days before the occurrence of the Abruzzo earthquake in the seismic paths, while no anomalies have been found in the control paths.

On 6 April 2009 a strong (Mw=6.3) earthquake occurred in the Abruzzo region (central Italy). Since 1996, the intensity of CLT (f=189 kHz, Sicily, Italy), MCO (f=216 kHz, France) and CZE (f=270 kHz, Czech Republic) broadcast signals has been collected with a ten minutes sampling rate by a receiver operating in a place located about 13 km far from the epicenter. During March 2009, the old receiver was substituted with a new one able to measure, with one minute sampling rate, the intensity of five VLF signals and five LF signals radiated by transmitters located in different zones of Europe. The MCO and CZE transmitters mentioned above are included among them. From 31 March to 1 April the intensity of the MCO radio signal dropped and this drop was observed only in this signal. The possibility that the drop was connected to problems in the transmitter or in the receiver was investigated and excluded. So, the drop indicates a defocusing of the radiated signal. Since no particular meteorological situation along the path transmitter-receiver happened, the defocusing must be related to other causes, and a possibility is presented that it is a precursor of the Abruzzo earthquake.

The problem of detecting the occurrence of an earthquake precursor is faced in the general framework of the statistical learning theory. The aim of this work is both to build models able to detect seismic precursors from time series of different geochemical signals and to provide an estimate of number of false positives. The model we used is k-Nearest-Neighbor classifier for discriminating "no-disturbed signal", "seismic precursor" and "co-post seismic precursor" in time series relative to thirteen different hydrogeochemical parameters collected in water samples from a natural spring in Kamchachta (Russia) peninsula. The measurements collected are ion content (Na, Cl, Ca, HCO3, H3BO3), parameters (pH, Q, T) and gases (N2, CO2, CH4, O2, Ag). The classification error is measured by Leave-K-Out-Cross-Validation procedure. Our study shows that the most discriminative ions for detecting seismic precursors are Cl and Na having an error rates of 15%. Moreover, the most discriminative parameters and gases are Q and CH4 respectively, with error rate of 21%. The ions result the most informative hydrogeochemicals for detecting seismic precursors due to the peculiarities of the mechanisms involved in earthquake preparation. Finally we show that the information collected some month before the event under analysis are necessary to improve the classification accuracy.

On 22 August 2005 an earthquake with magnitude M=4.7 occurred in the Anzio (central Italy) offshore area. From 2002, a VLF-LF radio receiver is into operation in Bari (southern Italy). The intensity and the phase of the signals transmitted by GB (f=16 kHz, United Kingdom), FR (f=20.9 kHz, France), GE (f=23.4 kHz, Germany), IC (f=37.5 kHz, Island) and IT (f=54 kHz, Sicily, Italy) has been monitored with a 5 s sampling rate. The previous epicenter is near enough to some of the radio paths and the data collected were studied in order to reveal possible seismic effects. The raw analysis revealed a clear drop in the intensity of the FR radio signal on 19 August. Then the wavelet analysis was applied to the intensity and the phase data of the different radio signals. In the mentioned day an increase in the band 60–120 min was revealed in the spectra of the FR signal. Then the principal component analysis was applied and again the 19 August stood up as an anomalous day for the FR radio signal. The path of this signal, among the paths of the five radio signals collected by the Bari receiver, is the nearest to the mentioned epicentre and the anomaly revealed on 19 August appears as a precursor of the earthquake. This result confirms the possible precursor revealed by other researchers in the air Rn content in a site located 5 miles far from the epicenter.

In our earlier papers we have found the effect of VLF transmitter signal depression over epicenters of the large earthquakes from observation on the French DEMETER satellite that can be considered as new method of global diagnostics of seismic influence on the ionosphere. At present paper we investigate a possibility VLF signal-ionospheric turbulence interaction using additional characteristic of VLF signal-spectrum broadening. This characteristic is important for estimation of the interaction type: linear or nonlinear scattering. Our main results are the following: – There are two zones of increased spectrum broadening, which are centered near magnetic latitudes Φ=±10° and Φ=±40°. Basing on the previous case study research and ground ionosonde registrations, probably it is evidence of nonlinear (active) scattering of VLF signal on the ionospheric turbulence. However occurrence rate of spectrum broadening in the middle-latitude area is higher than in the near-equatorial zone (~15–20% in comparison with ~100% in former area) that is probably coincides with the rate of ionospheric turbulence. – From two years statistics of observation in the selected 3 low-latitude regions and 1 middle-latitude region inside reception area of VLF signal from NWC transmitter we find a correlation of spectrum broadening neither with ion-cyclotron noise (f=150–500 Hz), which possibly means poor representation of the turbulence by the noise due to its mixture with natural ELF emission (which correlates with whistler), nor with magnetic storm activity. – We find rather evident correlation of ion-cyclotron frequency noise, VLF signal depression and weak correlation of spectrum broadening with seismicity in the middle-latitude region over Japan. But in the low-latitude regions we do not find such a correlation. Statistical decrease of VLF signal supports our previous case study results. However rather weak spectrum broadening-seismicity statistical correlation means probably that passive scattering prevails upon nonlinear (active) one.

The results of the monitoring of three VLF/LF signals collected in Petropavlovsk station (Kamchatka, Russia) and one VLF signal collected on board of the DEMETER French satellite are presented. Two periods of the seismic activity occurred in the Japan-Kamchatka area during November–December 2004 and July–September 2005 were investigated and the earthquakes with M≥6.0 in the Japan-Kamchatka area, located inside one or more of the third Fresnel zones of the three radio paths were considered. The ground data were analysed using residual signal of phase dP or of amplitude dA, defined as the difference between the signal and the average of few quiet days (±5 days) immediately preceding or following the current day. Also the satellite data were processed by a method based on the difference between the real signal and the reference one, but in order to obtain this last signal it was necessary to construct previously a model of the signal distribution over the selected area. The method consists: (a) in averaging all the data available in the considered region over a period characterized by low level seismicity, regardless of the global disturbances, in particular, of the magnetic activity; (b) in computing a polynomial expression for the surface as a function of the longitude and the latitude. The model well describes the real data in condition of their completeness and in absence of magnetic storms or seismic forcing. In the quoted periods of seismic activity clear anomalies both in the ground and in satellite data were revealed. The influence of the geomagnetic activity cannot to be excluded, but the seismic forcing seems more probable.

In February 2002, a receiver was put into operation at the Department of Physics of Bari University (Southern Italy) to record VLF-LF radio signals. The intensity and the phase of the signals transmitted by GB (f=16 kHz, United Kingdom), FR (f=20.9 kHz, France), GE (f=23.4 kHz, Germany), IC (f=37.5 kHz, Island) and IT (f=54 kHz, Sicily, Italy) has been monitored with a 5 s sampling rate. The intensity raw data averaged over 10 min, from February 2002 to April 2006, have been analysed. Several decreases of the electric field intensity of the radio signals with a duration of some days were revealed, generally occurring in not concomitant periods. The GE signal decreases systematically in winter and summer each year and so, it could be supposed that such decreases are related to the transmitter. On the contrary, all the other decreases pointed out are sporadic and the previous justification does not seem realistic. On the time occurrence of these decreases, the geomagnetic activity, the meteorological conditions in the receiver area and the regional seismic activity were investigated. The main result is that, generally, a pre or post seismic effect seems to give the most convincing justification. The effect seems to appear for earthquakes with magnitude M≥4.3, when the epicentres are within the third Fresnel zone of the radio signals or near enough to some radio path.

The analysis of the VLF signals radiated by ground transmitters and received on board of the French DEMETER satellite, reveals a drop of the signals (scattering spot) connected with the occurrence of large earthquakes. The extension of the "scattering spots" zone is large enough (1000–5000 km) and, probably, it increases with the magnitude of the "relative" earthquake. A possible model to explain the phenomenology, based on the acoustic gravity waves and the ionosphere turbulence, is proposed. The method of diagnostics applied to this study has the advantage to be a global one due to the world wide location of the powerful VLF transmitters and of the satellite reception. However, a specific disadvantage exists because the method requires rather a long time period of analysis due to the large longitudinal displacements among the successive satellite orbits. At the moment, at least, one month seems to be necessary.

The energy released by the earthquakes occurred in the seismogenetic area of the southern Kamchatka (Russia) from January 1977 to December 2004, reveals an increase in the peak energy in the period 1992–1999. This increase is related to the occurrence of seven earthquakes with magnitude ranging from 6.9 to 7.7; the first of these earthquakes happened on 2 March 1992 with M=7.1. For many years, hydro-geochemical data have been collected with a mean sampling rate of three days, in the form of the most common ions and gases in the water of deep wells and natural springs of a network operating in the south area of the Kamchatka, where the capital city Petropavlovsk is located. The collected data were analysed and differences in the trend and in the spectral content of some hydro-geochemical parameters at the springs were pointed out before and after the occurrence of the March 1992 earthquake, indicating clear post-seismic effects. Then an evident increase in the Carbon Dioxide and in the Hydrogen content appeared practically at each measurement site during the two-four years preceding the earthquake, so that long term precursors can be claimed. Finally, an anomalous decrease in the Carbon Dioxide content at the springs was revealed two/three months prior the earthquake, as a middle term precursor. The earthquake on March 1992 was the event nearest (90–120 km) to the measurement sites that happened in last thirty years.

On February 1996, a receiver able to measure the electric field strength of LF radio broadcastings, with a sampling frequency of ten minutes, was put into operation in a site (AS) located in central Italy. One of the broadcasting stations selected is MCO (f=216 kHz), located in southeast France, 518 km far from the receiver. The MCO data collected since February 1996 up to September 2004 were examined and, at first, the night time data and the day time data (in winter and summer) were separated. Then, the wavelet analysis on the night and day time data was applied. The main result of the analysis was the appearance of a very clear anomaly during summer (July–August) 1998, at day time and at night time. The anomaly is a strong exaltation of the signal components with period in the 25–40 days range. Theoretical calculations of electric field strength were made and the only way to justify this anomaly seems to be the occurrence of an increase of the ground wave propagation mode of the radio signal. Such an increase could have been produced by an increase of the ground conductivity and by modifications of some parameter of the troposphere, mainly the refractive index. On 15 August 1998 a seismic sequence started with 17 earthquakes (M=2.2–4.6) on the Reatini mountains, a seismogenic zone located 30 km far from the AS receiver along the path MCO-AS. In this paper, the possibility that the previous radio anomaly can be a precursor of this seismic sequence is proposed.

A signal transmitted by Japan Time Standard LF station (40 kHz, Fukushima prefecture) and recorded in Petropavlovsk-Kamchatski (Russia) is analyzed during a time interval from 1 July 2004 till 24 January 2005. 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. There were three series of earthquakes with M=5.6–7.1 in a zone of sensitivity of our wave path during two months. Nighttime 'bay-like' phase and amplitude anomalies of the LF signal are observed several days before and during every series of earthquakes. 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 period of about 25 min.

Starting from 1992 geochemical data are being collected with a mean sampling frequency of three days in the form of the pH value and of the most common ions and gases in the groundwater in one deep well located in Petropavlovsk, the capital city of Kamchatka (Russia). On 1 January 1996 a strong eruption started from the Karymsky volcano, that is located about 100km far from the well, in the north-northeastern direction. At the same time, a large earthquake (M=6.9) occurred in the Karymsky area. On 5 December 1997 a very large earthquake (M=7.7) occurred offshore, at a distance of 350km from the well and towards the same direction. The analysis of the geochemical data shows clear variations in the raw temporal trends on both cases. For the first event, a clear premonitory phase appeared; for the second one, some pre-seismic variations could be revealed but permanent modifications of the chemistry of the water subsequent to the earthquake are very clear. In both cases the feature of the geochemical variations is consistent with an afflux of new water in the aquifer connected with the well and with an escape of the Carbon dioxide gas from the ground in different directions. A schematic model able to justify such a phenomenology and the connections of the geochemical variations with the previous tectonic activities is proposed.

We present a general concept of mechanisms of preseismic phenomena in the atmosphere and ionosphere. After short review of observational results we conclude: 1. Upward migration of fluid substrate matter (bubble) can lead to ousting of the hot water/gas near the ground surface and cause an earthquake (EQ) itself in the strength-weakened area; 2. Thus, time and place of the bubble appearance could be random values, but EQ, geochemistry anomaly and foreshocks (seismic, SA and ULF electromagnetic ones) are casually connected; 3. Atmospheric perturbation of temperature and density could follow preseismic hot water/gas release resulting in generation of atmospheric gravity waves (AGW) with periods in a range of 6–60min; 4. Seismo-induced AGW could lead to modification of the ionospheric turbulence and to the change of over-horizon radio-wave propagation in the atmosphere, perturbation of LF waves in the lower ionosphere and ULF emission depression at the ground.

At the beginning of 2002 an OmniPAL receiver was put into operation at the Department of Physics of Bari University (Southern Italy). The electric field strength of five VLF-LF signals transmitted from United Kingdom (f=16kHz), France (f=20.9kHz), Germany (f=23.4kHz), Iceland (f=37.5kHz) and Italy (f=54kHz) has been monitoring with a 5s sampling frequency. In a first step we reduced the amount of the data taking one datum each 10min (mean of the ±5min raw data) and then we smoothed these data by a running adjacent averaging over 7 days. Analysing the trends we obtained, we revealed at first in the signal from the Italian transmitter two clear intensity decreases in April 2002 and in August-September 2002. At these times we observed earthquakes with M=4.3 and M=5.6 respectively near the transmitter-receiver path and a precursory effect in the previous decreases appeared. Then, we noted that all of the five radio trends in the time interval March 2002-February 2003 are more disturbed than in other periods; in particular an evident simultaneous decrease appears in January-February 2003. We propose that these disturbances are related to general excitation of the margin between the African and European plates. In a second step we examined the terminator time (evening) changes for the Italian transmitter in July-September 2002, and we found significant deviations from the mean value at the end of August, which is supportive for some precursory ionospheric signature of earthquakes.

We examined the groundwater Argon content data sampled from 1988 to 2001 at two wells in Kamchatka (Russia) and anomalous increases appeared clearly during June-July 1996. On 21 June, a shallow (1km) earthquake with M=7.1 occurred at a distance less than 250km from the wells and so the previous increases could be related to this earthquake and, in particular, could be considered premonitory anomalies. In order to support this raw interpretation, we analysed the data collected in details. At first we smoothed out the high frequency fluctuations arising from the errors in a single measurement. Next we considered the known external effects on the water of a well that are the slow tectonic re-adjustment processes, the meteorology and the gravity tides and we separated these effects applying band-pass filters to the Argon content raw trends. Then we identified the largest fluctuations in these trends applying the 3 σ criterion and we found three anomalies in a case and two anomalies in other case. Comparing the time occurrence of the anomalies at the two wells we found out that a coincidence exists only in the case of the premonitory anomalies we are studying. The simultaneous appearance of well definite anomalies in the residual trends of the same parameter at two different sites supports their meaning and the possibility that they are related to some large scale effect, as the occurrence of a strong earthquake. But, other earthquakes similar to the June 1996 event took place during the Argon content measurements time and no anomaly appeared in this content. In the past, some of the authors of this paper studied the Helium content data collected in three natural springs of the Caucasus during seven years. A very similar result, that is the simultaneous appearance of clear premonitory anomalies only on the occasion of a strong (M=7.0) but shallow (2–4km) earthquake, was obtained. The correspondence with the case of the Caucasus validates the interpretation of the Kamchatkian anomalies as precursors.

During February–March 1998, we observed a significant increase (6–8 dB) in the electric field of the CZE (f = 270 kHz, Czech Republic) broadcasting station at a site named AS and located in central Italy. On 13 March 1998 an earthquake (M = 5.2) occurred in Slovenia, starting a strong seismic crisis (M = 6.0 on 12 April, M = 5.1 on 6 May). The distances of the epicentres from the radio receiver were over 400 km, but the epicentres are located in a zone that is in the middle of the CZE-AS path. Previously, we advanced the hypothesis that the increase of radio-signal electric field detected could be a precursor of the Slovenia seismic crisis. At the purpose to precise the connection between this radio anomaly and the preparatory phase of the earthquakes, we analysed in detail the data collected. At first we carried out a FFT on the data and clear harmonic components with period 0.5d, 1d, 14d, 28d and 365d were revealed. These periods are characteristic of gravity tides and therefore, it seems reasonable to consider the previous harmonics in the electric field of the radio signal as components related to tides of the atmosphere, assuming as tides of the atmosphere only the linear action of gravity tides on atmosphere. Then, we carried out band pass filters on our data and we discovered that the February–March 1998 increase is mainly related to an exalting of the harmonics with period 0.5d, 1d, 14d and 28d, i.e. of the (semi)monthly and (semi)diurnal atmospheric tidal components in the radio signal. Supposing that this exalting is produced by a corresponding exalting of the atmospheric tides we propose a model able to explain the production of such an effect during the preparatory phase of the Slovenia earthquakes.

For many years flow-rate, temperature, ions and gases content data have been collected from a natural spring located in the Koryakskiy volcano area (Kamchatka, Russia). We have investigated the correlations between the hydrogeochemical data and the areal seismicity represented by the ks values (ks is a function of magnitude and hypocentral distance) of the earthquakes. At first we smoothed the raw hydrogeochemical data using a semi-triangle weight function. Then we compared the trends of each smoothed hydrogeochemical parameter with the ks trend using a running cross-correlation function with a maximum lag of ± 30 days and the main result was that, sometimes, we found 0.7–0.4 cross-correlation coefficients with no lag for flow rate and with + (10 – 15) days lags for some ion and gas contents. The correlation is positive, i.e. flow rate and ion and gas contents increase when ks increases. This phenomenology could be explained by an underground water pumping produced by some earthquake. We advance the hypothesis that this pumping could be the response of the viscoelastic underground medium of the Koryakskiy volcano to seismic waves. So, sometimes, the supply of elastic energy of the earthquakes may provide the trigger to a catastrophic nucleation of bubbles of this material producing a new melt with a lower density which will tend to expand and cause a pressure increase. This pressure produces a more intensive circulation of underground water and an anomalous increase of the flow rate and subsequently anomalous increases in groundwater ions and gases content.

Among radio signals, low frequency (LF) radio signals lie in the band between 30–300 kHz. Monitoring equipment with the ability to measure the electric strength of such signals at field sites, were designed and assembled in Italy. From 1993 onwards, the electric field strength of the MCO (216 kHz, France) broadcasting station has been collecting measurements at two sites in central Italy that were chosen according to very low noise levels. At the end of 1996, radio signals from the CLT (189 kHz, Italy) and CZE (270 kHz, Czech Republic) broadcasting stations were included in the measurements. Meteorological data from central Italy were also collected over the same time period in order to study the influence of weather conditions on the experimental measurements. During the monitoring period, we observed some evident attenuation of the electric field strength in some of the radio signals at some of the receivers. The duration of the attenuation observed was several days, so it could possibly be related to particular meteorological conditions. On the other hand, this phenomenon might represent precursors of moderate (3.0 < M < 3.5) earthquakes that occurred near the receivers (within 50 km) along the transmitter-receiver path. In this case, it is possible that the pre-seismic processes could have produced irregularities in the troposphere, such as ducts, reflecting layers and scattering zones, so that some local troposphere defocusing of the radio signals might have occurred. These observations were related only to moderate earthquakes and in these cases, suitable meteorological conditions were probably needed to observe the effect. Between February – March 1998, we observed at one measuring site, a significant increase in the CZE electric field strength. Unfortunately, we could not use the data of the other receiver in this case, due to frequent interruptions in the data set. The increase might have been a precursor of the strong seismic sequence (M = 5.0–6.0) that occurred during March – May 1998 in Slovenia at a location over 400 km from the receiver, but lying in the middle of the transmitter-receiver path. In this case, it is possible that an ionospheric disturbance, produced by the pre-seismic processes, might have occurred.

For many years, ion and gas content data have been collected from the groundwater of three deep wells in the southern area of the Kamchatka peninsula, Russia. In the last ten years, five earthquakes with M > 6.5 have occurred within 250 km of the wells. In a previous study, we investigated the possibility that the hydrogeochemical time series contained precursors. The technique used was to assume that each signal with an amplitude of three times the standard deviation is an irregularity and we then defined anomalies as irregularities occurring simultaneously in the data for more than one parameter at each well. Using this method, we identified 11 anomalies with 8 of them being possible successes and 3 being failures as earthquake precursors. Precursors were obtained for all five earthquakes that we considered. In this paper, we allow for the cross-correlation found between the gas data sets and in some cases, between the ion data sets. No cross-correlation has been found between gas and ion content data. Any correlation undermines the idea that an anomaly might be identified from irregularities appearing simultaneously on different parameters at each site. To refine the technique, we re-examine the hydrogeochemical data and define as anomalies those irregularities occurring simultaneously only in the data of two or more uncorrelated parameters. We then restricted the analysis to the cases of just the gas content data and the ion content data. In the first case, we found 6 successes and 2 failures, and in the second case, we found only 3 successes. In the first case, the precursors appear only for three of the five earthquakes we considered, and in the second case, only for two, but these are the earthquakes nearest to the wells. Interestingly, it shows that when a strict set of rules for defining an anomaly is used, the method produces only successes and when less restrictive rules are used, earthquakes further from the well are implicated, but at the cost of false alarms being introduced.

We report the information that in the days of the radio anomaly presented in the paper Biagi et al. (2009) an interruption of the broadcasting from the transmitter (RMC, France) happened. It remains unclear if the action resulted in a complete power off of the system, or in a reduction in the radiated power, and if this has affected France only, or every direction. Should a complete power off have occurred, the proposed pre-seismic defocusing is inexistent. Our doubts on this action are reported.

In the present work it is shown that, in accordance with the observations of the vertical sounding station "Tashkent", the critical foE-frequency of the daytime E-layer increases about one day before winter-earthquakes with magnitudes M > 5 and depths of the epicentre of h < 60 km, which appeared at distances of R < 2000 km from the station. The reliability of the result is larger than 99 %. The phenomenon is not observed for summer-earthquakes. It seems to be determined by the atmospheric wind system. Further, the variations of the foE-frequency are compared with possible simultaneous variations of the critical frequency foF2 of the F2-layer. First results show that only very large changes of the ionisation density in the E-layer influence the ionisation density in the F-region. Therefore, no synchronous growth of the foE- and foF2-frequencies 1–2 days before seismic shocks could be observed.

Signals of two Japanese transmitters (22.2 kHz and 40 kHz) recorded on the ground VLF/LF station in Petropavlovsk-Kamchatsky and on board the DEMETER French satellite have been analyzed during a seismic activity in Japan in May–June 2008. The period of analysis was from 18 April to 27 June. During this time two rather large earthquakes occurred in the north part of Honshu Island – 7 May (M=6.8) and 13 June (M=6.9). The ground and satellite data were processed by a method based on the difference between the real signal in nighttime and the model one. For ground observations a clear decrease in both signals has been found several days before the first earthquake. For the second earthquake anomalies were detected only in JJI signal. The epicenters of earthquakes were in reliable reception zone of 40 kHz signal on board the DEMETER. Signal enhancement above the seismic active region and significant signal intensity depletion in the magnetically conjugate area has been found for satellite observation before the first earthquake. Anomalies in satellite data coincide in time with those in the ground-based observation.

The statistical analysis of the characteristic frequency foF2 of the Earth's ionosphere averaged over mid-day hours – from 11:00 till 17:00 h LT – is carried out. Disturbances of foF2 connected to earthquakes are considered on the background of seasonal, geomagnetic, 11-years and 27-days solar variations. A special normalized parameter F is introduced, which represents the almost seasonal-independent part of foF2. Days with high solar (Wolf number >100) and geomagnetic (∑Kp>30) activity are excluded from the analysis. Events with magnitude M>5, distance from the sounding station RhF. It is found that F increases about 3–6 days before the earthquakes and then decreases one day – two days before the shock. The decreased values of F continue to exist two-three days after events with M>5.5. The obtained phenomenon depends on the magnitude of the earthquake. For events with M>5.5, the reliability of the effect is larger than 95%. For data of more than 80 earthquakes in the vicinity of Petropavlovsk-Kamchatsky and more than 200 earthquakes in the vicinity of Tokyo analogous results are obtained.

On the background of seasonal and helio-geomagnetic disturbances, disturbances of the ionization density near noon connected to earthquakes are investigated. The study is performed for some tens of earthquakes with magnitudes M>5 and depths hR=exp(M)+150 km. The analysis is performed using data registered by the three sounding stations every hour during 30 years. Especially methods of the statistical analysis are applied to search for regularities in the lithospheric-ionospheric links before and after earthquakes. The growth of the mean ionization density N at the F-layer electron density maximum Fof2 five-three days before earthquakes, and the decrease of N approaching the eruption and during a few days after the earthquake is investigated in dependence on both the magnitude of the earthquake M and the ionization density N near the F-layer maximum. It is found that some days before earthquakes the decrease of the ionization density ΔN is proportional to the values of M and N. After the earthquakes, ΔN depends much weaker on the magnitude M, and it is not influenced by the ionization density N. The variations of the ionization density three days before earthquakes do not depend on N and M (within the same order of approximation).

We analyze variations of the LF subionospheric signal amplitude and phase from JJY transmitter in Japan (F=40 kHz) received in Petropavlovsk-Kamchatsky station during seismically quiet and active periods including also periods of magnetic storms. After 20 s averaging, the frequency range of the analysis is 0.28–15 mHz that corresponds to the period range from 1 to 60 min. Changes in spectra of the LF signal perturbations are found several days before and after three large earthquakes, which happened in November 2004 (M=7.1), August 2005 (M=7.2) and November 2006 (M=8.2) inside the Fresnel zone of the Japan-Kamchatka wavepath. Comparing the perturbed and background spectra we have found the evident increase in spectral range 10–25 min that is in the compliance with theoretical estimations on lithosphere-ionosphere coupling by the Atmospheric Gravity Waves (T>6 min). Similar changes are not found for the periods of magnetic storms.