At the end of January 2020 an intense seismic crisis occurred on Dodecanese islands. The main earthquakes (Mw= 5.6 and Mw= 5.7) happened on January 30. This seismic activity was studied using the receivers of the INFREP network. Pre-seismic anomalies on the three VLF radio signals (19.58, 20.27, 23.40 kHz) collected by the Cyprus receiver and crossing the zone of the seismic activity were identified. The analysis of daily day/night trend of these signals pointed out a clear anomaly during the night of 29 January 2020, one day before the occurrence of the main shocks of the seismic crisis.In this work we present a methodology to include the anomalies detected in the VLF radio signals into the epidemic-type aftershock sequence (ETAS) model to establish seismic correlations. The ETAS model is characterized as a self-exciting point process where each seismic event has the potential to trigger subsequent offspring events. Recognized as a benchmark method for Operational Earthquake Forecasting (OEF), it utilizes earthquake catalogues to compute the conditional intensity function, λ(x,y,t), representing the total seismicity rate. To incorporate electromagnetic precursors, our methodology modifies λ(x,y,t) by applying a multiplicative weighting factor, w(x,y,t). This factor reflects the occurrence time and location of VLF anomalies, where the spatial extent of influence is constrained by the Dobrovolsky region.The proposed methodology builds on previous studies regarding VLF anomalies and the ETAS framework, and could provide a step forward in the operational use of VLF anomalies as seismic precursors. Therefore, we present the first results of the proposed methodlogy to model and simulate earthquake occurrence for the Dodecanese earthquake sequence.

We study the sub-ionospheric VLF transmitter signals recorded by the Austrian Graz station in the year 2020. Those radio signals are known to propagate in the Earth-ionosphere waveguide between the ground and lower ionosphere. The Austrian Graz facility (geographic coordinates: 15.46°E, 47.03°N) can receive such sub-ionospheric transmitter signals, particularly those propagating above earthquake (EQ) regions in the southern part of Europe. We consider in this work the transmitter amplitude variations recorded a few weeks before the occurrence of two EQs in Croatia at a distance less than 200 km from Graz VLF facility. The selected EQs happened on 22 March 2020 and 29 December 2020, with magnitudes of Mw5.4 and Mw6.4, respectively, epicenters localized close to Zagreb (16.02°E, 45.87°N; 16.21°E, 45.42°N), and with focuses of depth smaller than 10 km. In our study we emphasize the anomaly fluctuations before/after the sunrise times, sunset times, and the cross-correlation of transmitter signals. We attempt to evaluate and to estimate the latitudinal and the longitudinal expansions of the ionospheric disturbances related to the seismic preparation areas.

Layered double hydroxides (LDHs) consist of two-dimensional, positively charged lamellar structures with the ability to host various anions in the interlayer spaces, which grants them unique properties and tunable characteristics. LDHs, a class of versatile inorganic compounds, have recently emerged as promising candidates for enhancing osseointegration. A suitable alkaline microenvironment is thought to be beneficial for stimulating osteoblasts’ differentiation (responsible for bone matrix formation) while suppressing osteoclast generation (responsible for bone matrix disintegration). LDHs are prone to adjusting their alkalinity and thus offering us the chance to study how pH affects cellular behavior. LDHs can indeed modulate the local pH, inflammatory responses, and oxidative stress levels, factors that profoundly influence the behavior of osteogenic cells and their interactions with the implant surface. Herein, we deposited Mg–Fe LDH films on titanium substrates for dental implants. The modified Ti substrates was more alkaline in comparison to the bare ones, with a pH higher than 8 after hydrolysis in an aqueous environment.

Since 2009, several VLF/LF radio receivers have been installed throughout Europe in order to realize a European radio network for studying the radio precursors of earthquakes, called the INFREP network. The current network has nine VLF/LF receiving stations, two in Romania and Greece, one in Italy, Austria, Portugal, Cyprus, and Serbia. The receivers can measure with 1 min sampling rate the intensity of 10 radio signals in the band VLF (10-50 kHz) and LF (150-300 kHz). The scope of existing transmitters is manifold, e.g. they are used for radio broadcast (LF), for radio-navigation or time signals and mainly for military purposes in the VLF range. At the end of January 2020 an intense seismic crisis occurred in Dodecanese Islands; the main event (Mw= 6.0) occurred on January 30. This seismic activity occurred in the "sensitive" area of the INFREP network. The analysis of the data collected by INFREP receivers has revealed clear anomalies in three VLF signals appearing some days before the main earthquake. The anomalies appear in the trends collected by the Cyprus receiver and the epicenter is inside the 5th Fresnel ellipses defined by transmitters-receiver. Here we report the data analysis and we present in detail the anomalies. The possibility that they are precursors of the quoted earthquake seems significant. Biagi, P.F., Colella, R., Schiavulli, L., Ermini, A., Boudjada, M., Eichelberger, H., Schwingenschuh, K., Katzis, K., Contadakis, M.E., Skeberis, C., Moldovan, I.A. and Bezzeghoud, M. (2019) The INFREP Network: Present Situation and Recent Results. Open Journal of Earthquake Research,8, 101-115. https://doi.org/10.4236/ojer.2019.82007

We analyze the radio wave propagations of VLF/LF transmitter signals along subionospheric paths using two different reception systems localized in the Graz seismo-electromagnetic facility (15.43E,47.06N). Those systems allow the simultaneous detection of more than fifteen transmitter signals emitting in the northern (i.e. France, Germany and United Kingdom) and southern (i.e. Italy and Turkey) parts of Europe. In this work, we investigate the transmitter radio wave propagations associated with two earthquakes (EQs) which occurred, at two occasions, in nearly the same Croatian regions (Geo. Long.=16°E; Geo. Lat.=45°N). The first and second EQs happened, respectively, on March 22 and December 29, 2020, with magnitudes Mw equal to 5.4 and 6.4. The use of two complementary reception systems, i.e. INFREP (Biagi et al., Open Journal of Earthquake Research, 8, 2019) and UltraMSK (Schwingenschuh et al., Nat. Hazards Earth Syst. Sci., 11, 2011), and the proximity to the epicenters lead us to characterize the behavior of the transmitter signal amplitudes particularly above the Croatian seismic regions. We analyze the amplitude variation for a given transmitter frequency starting few weeks before the earthquakes occurrences. We discuss the observed anomalies in the transmitter signals which may be considered as precursors due to the ionospheric disturbances of the transmitter ray paths above the earthquakes preparation zones.

Since 2009, several radio receivers have been installed throughout Europe in order to realize the INFREP European radio network for studying the VLF (10-50 kHz) and LF (150-300 kHz) radio precursors of earthquakes. Precursors can be related to “anomalies” in the night-time behavior of VLF signals. A suitable method of analysis is the use of the Wavelet spectra. Using the “Morlet function”, the Wavelet transform of a time signal is a complex series that can be usefully represented by its square amplitude, i.e. considering the so-called Wavelet power spectrum. The power spectrum is a 2D diagram that, once properly normalized with respect to the power of the white noise, gives information on the strength and precise time of occurrence of the various Fourier components, which are present in the original time series. The main difference between the Wavelet power spectra and the Fourier power spectra for the time series is that the former identifies the frequency content along the operational time, which cannot be done with the latter. Anomalies are identified as regions of the Wavelet spectrogram characterized by a sudden increase in the power strength. On January 30, 2020 an earthquake with Mw= 6.0 occurred in Dodecanese Islands. The results of the Wavelet analysis carried out on data collected some INFREP receivers is compared with the trends of the raw data. The time series from January 24, 2020 till January 31, 2000 was analyzed. The Wavelet spectrogram shows a peak corresponding to a period of 1 day on the days before January 30. This anomaly was found for signals transmitted at the frequencies 19,58 kHz, 20, 27 kHz, 23,40 kHz with an energy in the peak increasing from 19,58 kHz to 23,40 kHz. In particular, the signal at the frequency 19,58 kHz, shows a peak on January 29, while the frequencies 20,27 kHz and 23,40 kHz are characterized by a peak starting on January 28 and continuing to January 29. The results presented in this work shows the perspective use of the Wavelet spectrum analysis as an operational tool for the detection of anomalies in VLF and LF signal potentially related to EQ precursors.

This work presents an application of the Perceptually Important Points (PIP) technique for the analysis of VLF time series. The aim of the analysis is to detect anomalies with respect to the normal variations of the data trends. Such anomalies could reveal possible radio precursors of the earthquake. Since 2009, several radio receivers have been installed throughout Europe in order to realize the INFREP European radio network for studying the VLF (10–50 kHz) and LF (150–300 kHz) radio precursors of earthquakes. The time series used for experiments was collected during the Dodecanese islands earthquakes ( $\text{MW}=5.6$ and $\text{MW}=5.7$ ) occurred on January 30, 2020.

In this work we analyse variations in VLF/LF radio signal amplitudes recorded by the INFREP network in the period 16 November – 6 December, 2019 characterized by very intensive seismic activities in the Balkan peninsula, Crete, and Adriatic, Aegean and Black seas. Namely, 38 earthquakes with magnitude greater than 4.0 occurred in this area during the noticed period; the most intensive of them occurred on 26 and 27 November: three events in Albania (Mw= 6.4, 5.3, 5.1), one in Crete (Mw= 6), one in Bosnia and Herzegovina (Mw= 5.4) and two in Adriatic sea (Mw= 5.4, 5.3). We study both long- and short- term variations that are already recorded in earlier studies. The long-term variations relate to changes in the amplitude intensities in periods of several days and their existence is shown in many previous studies. The recent analyses also indicate short-term variations in signal amplitude noises started about several tents of minutes before the earthquake (Nina et al. 2020). In this work, we analyse different areas using INFREP network, which allow us to study local changes in the atmosphere. In order to examine possible precursors we considered longer time started and ended 10 days before and after the most intensive of the considered earthquakes, respectively. This research is supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia, under the projects 176002 and III44002. References Nina, A., S. Pulinets, P. F. Biagi, G. Nico, S. T. Mitrović, M. Radovanović and L. Č. Popović. Science of the Total Environment 710 (2020) 136406

In this work we use Very Low Frequency (VLF) radio signals, having a frequency in the bands 20-80 kHz, to study the VLF signal propagation in the atmosphere quite undisturbed conditions by selecting the signals recorded during night. As a good approximation, we can model the propagation of VLF radio signals as characterized by a ground-wave and a sky-wave propagation mode. The first one generates a radio signal that propagates in the channel ground-troposphere, while the second one generates a signal which propagates using the lower ionosphere as a reflector. The VLF receivers of the INFREP (European Network of Electromagnetic Radiation) network are used. These receivers have been installed since 2009 mainly in southern and central Europe and currently the INFREP network consists of 9 receivers. A 1-minute sampling interval is used to record the amplitude of VLF signals. Long time-series of VLF signals propagating during night are extracted from recorded signals to study possible seasonal effects due to temporal variations in the physical properties of troposphere. A graph theory approach is used to investigate the spatial correlation of the aforementioned effects at different receivers. A multivariate analysis is also applied to identify common temporal changes observed at VLF receivers. This work was supported by the Ministero dell'Istruzione, dell'Università e della Ricerca (MIUR), Italy, under the project OT4CLIMA. This research is supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia, under the projects 176002 and III44002.

We report on VLF/LF transmitter signals observed in the year 2018 during the minimum of solar activity. Those signals were recorded in Graz (Austria) using INFREP (Biagi et al., Nat. Hazards Earth Syst. Sci., 11, 2011) and UltraMSK (Schwingenschuh et al., Nat. Hazards Earth Syst. Sci., 11, 2011) systems. This leads us to record fourteen transmitter signals in the frequency range between 19 kHz and up to 270 kHz. Six transmitter channels are common to both systems and are localized in Great-Britain (Anthorn, GBZ, 19.58kHz), Italy (Tavolara, ICV, 20.27kHz), Germany (Rhauderfehn, 23.4kHz,) and Island (Keflavik, NRK, 37.5kHz). Others are mainly LF broadcasting transmitters from Romania (Brasov, 153kHz), Luxembourg (Felsberg-Berus, 183kHz), Algeria (Berkaoui, 198kHz), Monte-Carlo (Roumoules, 216kHz) and Tchecki (Lualualei, 270kHz). In the year 2018, the solar activity decreased reaching its minimum in the end of 2019. We emphasize in this work on three aspects: (a) C-flares related to the solar activity, (b) Kp-index linked to the geomagnetic activity, and (c) seismic events in the southern part of Europe, i.e. Greece and Italy. The dominant patterns observed on almost all transmitters are due to the solar flares. Geomagnetic activity is found to depend on the seasonal effect and mainly observed few weeks before and after the summer solstice in the northern hemisphere. Few earthquakes occurred in the southern part of Europe, in Greece (6 events) and in Italy (2 events) with a magnitude of 5.5 Mw and depths less than 10 km. We discuss the disturbances of VLF/LF transmitter signals prior to EQs occurrences, and their links to external effects. Our results are compared to recent investigations of Zhang et al. (Radio Sci., 52, 2017) and Rozhnoi et al. (Ann. Geophys., 37, 2019) concerning, respectively, the spatial distribution of VLF transmitter signals recorded by Demeter satellite, and the solar X-flare effects on VLF/LF transmitter signals.

On October 12, 2013, an earthquake with M-w = 6.5 occurred in the southern Hellenic Arc, approximately 20 km off the west coast of Crete. The main shock, the focal depth of which is on the order of 40 km, was followed by aftershocks felt in the nearby cities and villages, although the aftershock sequence was poor. The epicentre was located at approximately 60 km from a radio receiver in Crete (CRE), which belongs to the European VLF/LF Radio Network. Several days before the earthquake, a clear disturbance occurred in one of the ten radio signals that the CRE receiver sampled. The disturbance, which can be considered an anomaly, appeared in the 216 kHz radio signal radiated by the Radio Monte Carlo (MCO) transmitter. The radio path MCO-CRE crossed directly over the epicentre area of the aforementioned earthquake. In this work, we present a detailed analysis of the MCO signal anomaly using spectral tools. We also investigate the behaviour of other radio signals sampled by the CRE receiver and consider other possible causes of disturbances on the MCO radio signal. We conclude that the disturbance in the MCO radio signal is a convincingly possible precursor of the earthquake in Crete. Emission of electromagnetic waves with a frequency band that includes 216 kHz from the focal zone of the earthquake can provide a satisfactory explanation of the radio anomaly. (C) 2015 Elsevier Ltd. All rights reserved.

Since 2009 a network of VLF (20-60 kHz) and LF (150-300 kHz) radio receivers has been put into operation in Europe in order to study earthquakes precursors. At the moment the network consists of ten receivers three of which are located in Italy, two in Greece and one in Portugal, Romania, Malta, Cyprus and Turkey. The data (sampling rate of 1min) are downloaded automatically at the end of each day and are collected at the Department of Physics of the University of Bari (Italy) that is the central node of the network. A detailed study of the radio data collected in the radio network from July 2009 to September 2011 was performed, using different methods of analysis. In total 27 cases suitable for analyzing were found and successes, i.e. radio anomalies preceding the subsequent earthquake (Mw 5.0) and clearly related to the event, were obtained in 70% of the cases; but increasing the value of the Mw threshold for the earthquakes this percentage seems to increase. Among the different methods of analysis the Wavelet spectra appear to be the most sensitive ones. At the moment a system able to apply on the radio data the Wavelet analysis automatically at the end of each day is being developed. On May 20, 2012 an earthquake with Mw=6.1 occurred in north Italy (Emilia region); the epicenter is located inside the “sensitive” area of the network. The results obtained in such occasion are presented.

In 2008, a radio receiver that works in very low frequency (VLF; 20-60 kHz) and LF (150-300 kHz) bands was developed by an Italian factory. The receiver can monitor 10 frequencies distributed in these bands, with the measurement for each of them of the electric field intensity. Since 2009, to date, six of these radio receivers have been installed throughout Europe to establish a ‘European VLF/LF Network’. At present, two of these are into operation in Italy, and the remaining four are located in Greece, Turkey, Portugal and Romania. For the present study, the LF radio data collected over about two years were analysed. At first, the day-time data and the night-time data were separated for each radio signal. Taking into account that the LF signals are characterized by ground-wave and sky-wave propagation modes, the day-time data are related to the ground wave and the night-time data to the sky wave. In this framework, the effects of solar activity and storm activity were defined in the different trends. Then, the earthquakes with M ≥5.0 that occurred over the same period were selected, as those located in a 300-km radius around each receiver/transmitter and within the 5th Fresnel zone related to each transmitter-receiver path. Where possible, the wavelet analysis was applied on the time series of the radio signal intensity, and some anomalies related to previous earthquakes were revealed. Except for some doubt in one case, success appears to have been obtained in all of the cases related to the 300 km circles in for the ground waves and the sky waves. For the Fresnel cases, success in two cases and one failure were seen in analysing the sky waves. The failure occurred in August/September, and might be related to the disturbed conditions of the ionosphere in summer.

Since 2009 a network of VLF (20 - 60 kHz) and LF (150 - 300 kHz) radio receivers is operating in Europe in order to study the disturbances produced by the earthquakes on the propagation of these signals. In 2011 the network was formed by nine receivers, of which three are located in Italy and one is in Austria, Greece, Portugal, Romania, Russia and Turkey. On May 19, 2001 an earthquake (Mw = 5.7) occurred in western Turkey, that is inside the “sensitive” area of the network. The radio data collected during April-May 2011 were studied using the Wavelet spectra, the Principal Component Analysis and the Standard Deviation trends as different methods of analysis. Evident anomalies were revealed both in the signals broadcasted by the TRT transmitter (180 kHz) located near Ankara and in a VLF signal coming from a transmitter located in Western Europe and collected by the receiver TUR of the network located in eastern Turkey. Evident precursor phases were pointed out. Some differences in the efficiency of the three analysis methods were revealed.

During 2008 a radio receiver was developed by the Italian factory Elettronika. The receiver is an equipment work- ing in VLF (15-60 kHz) and LF (150-300 kHz) bands. It can monitor 10 frequencies distributed in these bands and, for each of them, saves the electric field intensity. During 2009 six receivers were installed for the realization of the “European VLF/LF network”. Actually, two of them are into operation in Italy and one in Greece, Turkey, Por- tugal and Romania, respectively; a sampling rate of 1 minute is used. The LF radio data collected from July 2009 to December 2010 have been analysed. At first, for each radio signal, the day time data and the night time ones were separated. Taking into account that the LF signals are characterized by the ground wave and the sky wave propagation modes, the day data are related to the ground wave and the night data to the sky wave. In a first analysis the effect of the solar activity and of the thunderstorm activity was pointed out in the different trends. Then the wavelet analysis was applied on the same trends. Some anomalies probably related to earthquakes occurred nearby some transmitter-receiver path with M>5 were revealed.

On February 1996, a receiver able to measure the electric field strength of LF radio- signals, with a sampling frequency of ten minutes, was put into operation in a site (AS) located in the Apennines (Central Italy). On the basis of the best reception, the LF broadcasting stations - MCO (France, f=216 kHz) and CZE (Czech Republic, f=270 kHz) - were selected. The receiver is located 518 km far from the MCO broadcasting station and 818 km far from the CZE broadcasting station. Here, we present the result obtained by the analysis of the MCO data. LF radio wave reception is characterised by ground-wave and sky-wave propagation modes. The sky-wave signal is greatly variable between day and night and, at day time, between winter and summer. The ground-wave, for distances lesser than 500-1000 km, provides a stable signal that can be significant one. At first, we calculated the theoretical electric field strength of the ground wave for the MCO radio signal, using the GRWAVE software package and we found a value of 0.54 mV/m. For the same signal we calculated the theoretical electric field strength of the sky wave at night time and at day time during winter and summer, using the wave hop approach and we found the values: 3.1 mV/m (night time), 0.50 mV/m (winter day time) and 0.02 (summer day time).Then, on the MCO data we collected since February 1996 up to September 2004, we separated the night time data and the day time data (in winter and summer). Finally, we applied the wavelet analysis on the night and day time data. 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 components with period in the 25-40d range. From the previous calculations, the only way to justify this anomaly seems to assume the occurrence of an increase of the ground wave propagation mode. Such an increase could be connected with an increase of the ground conductivity or with modifications of some parameter of the troposphere. At August 15, 1998 a seismic sequence (M=3.0-4.6) started on the Reatini mountains, that is a seismogenic zone located 30 km far from the AS receiver, exactly along the path MCO-AS. We present the possibility that the previous radio anomaly is a precursor of this seismic activity

Since 1988 the argon content in underground water has been measured at the Morosnaya well, in the Kamchatka peninsula, with a sampling frequency of three days. In the same well other gases and ions dissolved in water are measured, together with flow rate, pH and temperature. The most active seismogenetic area in Kamchatka is that located offshore, along the south-eastern coast of the peninsula. The strongest earthquakes in this area occurred on March 2, 1992 (M47.1), November 13, 1993 (M47.0) and June 21, 1996 (M47.1), within a distance of 200 km from the well. The focal depth of the earthquakes of 1992 and 1993 was 20 and 40 km, respectively. The earthquake which occurred in 1996 was very shallow; a focal depth of few kilometres was estimated. No anomalies in the argon or other dissolved gas concentration were observed on the occasion of 1992 and 1993 earthquakes; on the other hand, a very clear preseismic anomaly appeared in the concentration of argon and nitrogen on the occasion of the 1996 earthquake. The behaviour of the ion content was opposite; no anomaly on the occasion of the last earthquake and clear preseismic anomalies on the occasion of the two former earthquakes appeared. A possible explanation of the quoted behaviour of dissolved gases and ions in groundwater according to the different focal depth of the subsequent earthquakes is presented.

In this paper we present the results of characterising time series of the argon content of groundwater recorded in the Kamchatka area of Russia. The problems of correlating anomalies in the argon data with seismic activity are explored. A new statistical technique for relating anomalies to geophysical observations based on Markov Chain Monte Carlo modelling methods is outlined.

The Spitak and Karymsky earthquakes occurred with M46.9 in Armenia and in Kamchatka (Russia), respectively. As regards the Spitak earthquake, we analysed the groundwater helium content data collected by three Georgian and one Armenian measurement sites; as regards the Karymsky earthquake, we analysed the groundwater helium content data collected by two measurement sites in Kamchatka. The first analysis has pointed out that precursory anomalies appeared in the northern area with respect to the Spitak epicentre; on the contrary, only co-post seismic anomalies were revealed in the southern area. As regards the Karymsky earthquake, no pre-co-post seismic variation in the groundwater helium content was revealed at both the measurement sites. The analysis of other hydrogeochemical parameters, collected in these sites, revealed that one site does not show any anomaly; on the contrary, at the other measurement site clear preseimic anomalies appeared in some hydrogeochemical parameters. A possible explanation of the quoted results is presented.

Prototype instrumentation, able to automatically measure groundwater radon content variations, is presented. The equipment is made of stainless steel and has spherical valves with automatic and pneumatic control. The deemanation of the gases from the water is obtained by evacuating a suitable expansion chamber. The instrumentation can make discrete sampling ranging from 1 per hour to 1 per 99 hours. The equipment was tested in the laboratory: the efficiency was measured by means of a266Ra solution. A mean value of (0.65±0.07) count/s/Bq was obtained. A calibration test was carried out by comparing countings from the automatic equipment with those obtained by the standard laboratory cell. Results of an operational check over a period of approximately one year indicate that variations in radon at the calibration site are attributable more to meteorological than to tectonic causes.