Sagardweep Biswas

Earlier studies have recorded seismic effects in sub-ionospheric Very Low Frequency (VLF) signal, predominantly examining a single VLF propagation path associated with a specific earthquake. This research expands on these findings by taking a more advanced setup. Using Japan's VLF network with eight stations nationwide and a single transmitter signal from JJI (22.2 kHz), our study examines four earthquakes, each with a magnitude over 6, happening near VLF network locations. Chosen earthquakes are Iwaki on July 11, 2014, Aomori prefecture on August 10, 2014, Ogasawara on May 30, 2015, and Makurazaki on November 13, 2015. Three are shallow and close to land, but deliberately included are one deep oceanic quake (Ogasawara) at 664 km depth and one occurring under pre-geomagnetic storm conditions six days before (Makurazaki). We examine nighttime VLF signal distortion during the earthquake events and investigate the presence of atmospheric gravity waves (AGW). This study supports the notion that deep oceanic earthquakes can perturb the ionosphere at considerable distances from their epicenter. Furthermore, we investigate the simultaneous ionospheric impact due to geomagnetic storms and the earthquake. Through the examination of seismic effects preceding the storm occurrence, we finds the likelihood of seismogenic effects that are not contaminated by other factors. The response of the initial seismogenic perturbation varies at different distances from the epicenter. Surprisingly, nearby receivers do not experience early disturbances. This indicates how challenging it is to forecast the location of earthquake.

We present the atmospheric anomalies instigated through seismogenic sources by a multichannel observation using ground‐ and satellite‐based systems. This study emphasizes the seismic event which happened on the east coast of Japan, near the Fukushima Prefecture on November 21, 2016 (in UTC), with a magnitude of 6.9 and a depth of 11.4 km. We mainly focus on the atmospheric and ionospheric irregularities via acoustic and electromagnetic channels originating from earthquakes in the process of the lithosphere, atmosphere, and ionospheric coupling (LAIC) mechanism. In the acoustic channel, we study the seismogenic atmospheric gravity wave (AGW) which perturbs the local lower atmosphere. The observation of nighttime fluctuations in the very low frequency (VLF) signals and total electron content (TEC) is used to investigate the atmospheric perturbation through the electromagnetic channel. For the ground‐based observations, a VLF signal network consisting of 5 receivers in Japan is used to study by recording the VLF amplitude transmitted from the Japanese transmitter JJI (22.2 kHz). VLF nighttime fluctuation is used to check the unusualities due to the earthquake. Preseismic wavelike structures having periods of AGW are observed in the nighttime signal. Direct investigation of such AGWs is done by computing the potential energy related to AGW from the sounding of the atmosphere using broadband emission radiometry (SABER) temperature profiles mounted on the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite. Ionospheric TEC inspection is done by using a ground‐based global navigation satellite system (GNSS) receiver from the International GNSS Survey (IGS) station MIZU in Japan and observing anomalies in diurnal TEC around 6 and 10 days prior to the earthquake. We also obtain the wavelike structure of AGW from the small‐scale fluctuation of TEC using wavelet analysis. All the parameters are found to be preseismic for this earthquake; the acoustics channel gives more consistent outcomes than the electromagnetic channel.

A VLF (10 – 47.5 kHz) radio receiver with call sign UWA has recently been installed at the University of West Attica in Athens (Greece) and is continuously monitoring the lower ionosphere by means of the receptions from many transmitters, in order to identify any possible pre-seismic signatures or other precursors associated with extreme geophysical and space phenomena. In this study, we examine the case of three very recent strong mainshocks with magnitude Mw ≥ 5.5 that happened in September and October of 2021 in the southeastern Mediterranean. The VLF data used in this work correspond to the recordings of one specific transmitter with the call sign “ISR” which is located in Negev (Israel). The borders of the 5th Fresnel zone of the corresponding sub-ionospheric propagation path (ISR-UWA) are close in distance with the epicenters of the two earthquakes (EQ), while the third one is located within the 5th Fresnel zone of the specific path. In this work, we computed the morlet wavelet scalogram of the nighttime amplitude signal in order to check for any embedded wave-like structures, which would indicate the existence of Atmospheric Gravity Waves (AGW) before each one of the examined EQs. In our investigation, we also checked for any other global extreme phenomena, such as geomagnetic storms and solar flares, which may have occurred close in time with the examined EQs and could have a contaminating impact on the obtained results. Our results revealed wave-like structures in the amplitude of the signal a few days before the occurrence of these three EQs.

In this work we present the statistical and criticality analysis of the very low frequency (VLF) sub-ionospheric propagation data recorded by a VLF/LF radio receiver which has recently been established at the University of West Attica in Athens (Greece). We investigate a very recent, strong (M6.9), and shallow earthquake (EQ) that occurred on 30 October 2020, very close to the northern coast of the island of Samos (Greece). We focus on the reception data from two VLF transmitters, located in Turkey and Israel, on the basis that the EQ’s epicenter was located within or very close to the 5th Fresnel zone, respectively, of the corresponding sub-ionospheric propagation path. Firstly, we employed in our study the conventional analyses known as the nighttime fluctuation method (NFM) and the terminator time method (TTM), aiming to reveal any statistical anomalies prior to the EQ’s occurrence. These analyses revealed statistical anomalies in the studied sub-ionospheric propagation paths within ~2 weeks and a few days before the EQ’s occurrence. Secondly, we performed criticality analysis using two well-established complex systems’ time series analysis methods—the natural time (NT) analysis method, and the method of critical fluctuations (MCF). The NT analysis method was applied to the VLF propagation quantities of the NFM, revealing criticality indications over a period of ~2 weeks prior to the Samos EQ, whereas MCF was applied to the raw receiver amplitude data, uncovering the time excerpts of the analyzed time series that present criticality which were closest before the Samos EQ. Interestingly, power-law indications were also found shortly after the EQ’s occurrence. However, it is shown that these do not correspond to criticality related to EQ preparation processes. Finally, it is noted that no other complex space-sourced or geophysical phenomenon that could disturb the lower ionosphere did occur during the studied time period or close after, corroborating the view that our results prior to the Samos EQ are likely related to this mainshock.

A new VLF/LF (10 - 47.5 kHz) radio receiver has recently been installed in the University of West Attica, in Athens (Greece), and has been operating in trial mode since April 2020 for the study of sub-ionospheric propagation variations, mainly aiming at the identification of possible earthquake (EQ) precursors or signatures of other extreme geophysical phenomena. The receiver is monitoring signals from a number of transmitters. Most of them are located in Europe, while some are located in Asia, Australia and North America. The recorded data (amplitude and phase) from this receiver are sampled at a rate of 1 sample per second. In this paper we present information about the new VLF/LF receiver as well as preliminary results concerning a very recent, strong (Mw = 6.7), shallow (focal depth = 12 km), EQ that occurred in Greece (epicenter located in the Aegean Sea, off-coast of the Samos island, close to the Greece-Turkey borders) on 30/10/2020, hereafter referred to as Samos’ EQ. The subionospheric propagation data associated with two specific transmitters were analyzed. Τhe first transmitter, with call sign TBB, is located in Denizköy (Turkey) and the location of Samos’ EQ epicenter is within of 5th Fresnel zone of the corresponding propagation path. The second transmitter, with call sign ISR, is located in Negev (Israel) and the location of Samos’ EQ epicenter is in close distance to the borders of the 5th Fresnel zone, so that, considering the magnitude of the specific EQ, the corresponding propagation path could possibly be disturbed. In this paper we present the analysis of the receiver’s amplitude data by means of the Terminator Time Method (TTM) in order to reveal any possible pre-seismic anomaly in the lower ionosphere. Our preliminary results show that there are indications for disturbance of the lower ionosphere a few days before the EQ occurrence.