Annals of Geophysics

Preface Special Issue: Developments in Earthquake Precursors Studies

Stelios M. Potirakis — 2023-12-31

This Special Issue of Annals of Geophysics is dedicated to present recent developments in the field of earthquake (EQ) precursors. Due to their potentially catastrophic nature, EQs have early attracted the attention of humans. During the last decades, a considerable amount of scientific effort has been devoted to the study of EQ precursors, aiming at EQ forecast. Prediction of large EQs, especially in the short-term, is of outmost importance for our society, since it could prevent, or at least restrict, their disastrous consequences, both in terms of infrastructure / material damages and in terms of casualties. It is clear that EQ preparation processes are complex and multifaceted, which call for a multidisciplinary approach. Many possible precursors have been reported and studied during the last decades, which include (just to mention some): foreshock activity, pre-slip effect, surface deformation, seismic electric signals (SES), ultra-low frequency (ULF) magnetic field anomalies, fracture-induced electromagnetic emissions (FEME) (fracture-induced electromagnetic radiation, FEMR), ULF/ELF (extremely low frequency) atmospheric electromagnetic radiation, atmospheric anomalies (such as SLHF, OLR etc.) and ionospheric (lower and upper regions) anomalies, lithosphere-atmosphere-ionosphere (LAI) coupling, even abnormal animal behavior.

On the relevance of the foreshocks in forecasting seismic mainshocks

Bogdan Apostol — 2023-12-31

We analyze the usefulness of the foreshocks in forecasting seismic mainshocks. The analysis is based on possible correlations which may exist between foreshocks and mainshocks. Such correlations are expressed by a previously established time-magnitude relationship, which indicates the presence of an abrupt magnitude-decreasing sequence of correlated foreshocks in the proximity of a mainshock. By fitting this formula, we are able to derive the occurrence time of a possible mainshock. Also, we can estimate the magnitude of the mainshock, providing we know the parameters of the background seismicity of the seismic region. We report here on the application of this procedure to three Vrancea (Romania) mainshocks, the l’Aquila (Italy), Yangbi (Yunnan, China) and Izmit (Turkey) earthquakes. The limitations of the procedure are discussed. Also, a discussion is included regarding the so-called temporal variability of the Gutenberg-Richter parameter in the proximity of a mainshock, as resulting from time-magnitude and time-time correlations.

Earthquake forecast by imbalance machine learning using geophysical predictors

Tengiz Kiria — 2023-12-31

In the present paper we consider the earthquake forecast as a binary problem of machine learning on the imbalanced data base applied to five regions of Georgia. For the training we used geophysical data base collected in 2017-2021, namely, variations of statistical characteristics of geomagnetic field components, seismic activity, water level in deep boreholes and tides. In this version a new predictor – the weighted seismic activity for previous 5 days - – is added compared to the predictors’ list used in previous papers. Besides, the length of the used database is increased 3 times compared to the earlier results. As in the database the earthquakes of M > 3.5 are rare, the number of negative cases is large (there are many days without EQs of M > 3.5), meaning that there is a strong imbalance between positive and negative cases of the order of 1:20; we apply the specific methodology Matthews’ correlation coefficient (MCC) and F1 score to avoid the strong imbalance effect.

A Physical Basis of Predicting the Magnitude and Failure Time of a Forthcoming Earthquake

Jeen-Hwa Wang — 2023-12-31

Let T and M be, respectively, the precursor time of a certain precursor and the magnitude of a forthcoming earthquake. Observations may lead to a relationship of T versus M in a form of log(T)=a+bM. In this study, we will explore the intrinsic physics of the log(T)−M relationship. Based on the log(T)−M relationships of two different precursors from observed data, we propose a method of predicting the magnitude and failure time of a forthcoming earthquake. A testing example based on the log(T)−M relationships inferred from the data of presiemic radon concentration anomalies and gamma-ray emission changes observed at respective monitoring stations in Taiwan is given in this study. Results confirm a high possibility of predicting the magnitude and failure time of a forthcoming earthquake just from the observed occurrence times of two different precursors based on their log(T)−M relationships.

ELSEM-Net, a network of ground-based telemetric stations for the monitoring of fracture-induced electromagnetic emissions in Greece: Instrumentation, management and analysis of recent observations associated with strong earthquakes

Philopimin Malkotsis — 2023-12-31

The hELlenic Seismo-ElectroMagnetics Network (ELSEM-Net, http://elsem-net.uniwa.gr) is a network of telemetric stations spanning all over Greece for the monitoring of fracture-induced electromagnetic emissions (FEME), often also referred to as electromagnetic radiation (FEMR). At the laboratory scale, it has been proved that FEME/FEMR are produced by opening cracks in a wide frequency spectrum ranging from the MHz to the kHz bands as the general (macroscopic) fracture is approaching, rendering them precursors of general fracture that permit the monitoring of the gradual damage of stressed materials. FEME/FEMR is also observed at the geophysical scale, preceding strong earthquakes (EQs) with epicenters on land or near the coast-line. Based on the idea that MHz-kHz FEME/FEMR should also permit the monitoring of the gradual damage of stressed materials in the Earth’s crust, as it happens in the laboratory experiments, ELSEM-Net was gradually developed from 1992 to 1998 and currently comprises 11 stations installed in various locations in Greece. Here we present the instrumentation that has been specifically designed for ELSEM-Net, both hardware and firmware/software, as well as a web-based system for the management of the field stations, the recorded data, and the automated preliminary analysis results. Finally, we present the analysis of selected observations associated with recent strong earthquakes that hit Greece, using a most recently introduced time series analysis method. Our presentation aims to communicate in detail the experimental infrastructure behind our almost 30 years of research in geophysical scale FEME/FEMR so that other interested research groups around the world can take advantage of it and have the opportunity to install similar stations/networks in other locations of the world for the study of pre-EQ processes associated with natural or man-induced seismicity.

Earthquakes and related anomalous electromagnetic radiation

Manana K Kachakhidze — 2023-12-31

According to the presented work, VLF/LF electromagnetic emissions might be declared as the main precursor of earthquakes since based on these very emissions, it might predict (M ≥ 5) inland earthquakes. As for ULF radiations, it governs some processes going on in the lithosphere-atmosphere-ionosphere coupling (LAIC) system. By these points, VLF/LF/ULF electromagnetic emissions have to consider more universal fields than other geophysical field anomalies during the earthquake preparation period up to aftershocks extinction.

Schumann resonance anomalies associated with two (M~7) Tohoku offshore earthquakes in the spring of 2021

Masashi Hayakawa — 2023-12-31

ABSTRACT. We describe the Schumann resonance (SR) anomalies associated with two earthquakes (EQs) observed in Japan in the spring of 2021. SR is the global electromagnetic phenomenon observed in the ELF (extremely low frequency) band, and its resonance peaks are observed in the power spectra on natural radio noise at frequencies of 8, 14, 20, Hz, etc. The natural source of ELF radiation is the global lightning activity occurring in the Earth-ionosphere cavity. The anomalies were observed for the first time in Japan for the EQs in Taiwan when the distance between the observatory and the EQ epicenter was a few Mm (1 Mm = 1000 km). Recently, a new SR anomaly was addressed, related to nearby (a few hundred km) EQs (Hayakawa et al., 2019, 2020a,b). This paper presents the SR anomalies observed in the vicinity of Nagoya-city for two relatively close (~500 km) successive EQs with magnitude around 7 that occurred offshore the Tohoku area in Japan. The anomaly is characterized by the noticeable simultaneous increase or decrease in the amplitudes of three SR modes. This SR unusual behavior was observed prior to and after each of the two EQs that occurred in February and March, 2021. Observational data were interpreted in the model of seismogenic perturbations of the lower ionospheric conductivity profile. Model computations imply the full-wave solution of the ELF electromagnetic problem in the form of the Riccati equation and the 2D (two dimensional) telegraph equations. We show that observed disturbances in the SR power spectra might be attributed to two types of seismogenic modifications in the lower ionospheric profile: the compression or the expansion of the vertical profiles of mesospheric conductivity over the EQ epicenter.

Double resonance in seismo-lithosphere-atmosphere-ionosphere coupling

Chieh-Hung Chen — 2023-12-31

nvestigations into causal mechanisms behind anomalous pre-earthquake phenomena are considered a promising way of earthquake prediction. Numerous promising channels for seismo-lithosphere-atmosphere-ionosphere coupling have been proposed; however, predicting earthquakes remains a great challenge in the scientific society. Short-period ground vibrations exhibiting frequency characteristics similar to natural frequencies caused by strata failure resonance have recently been detected using tiltmeters embedded in magnetometers prior to earthquakes. These vibrations originate from regions near the epicentres of forthcoming earthquakes and can be simultaneously detected by broadband seismometers and ground-based global navigation satellite system (GNSS) receivers. Unlike the total electron contents (TECs) obtained from orbiting satellites, the vibrations and the identifiable TEC perturbations in data from geostationary satellites of the BeiDou Navigation System share frequencies prior to earthquakes. However, the causal relationship between the vibrations and TEC perturbations remains unclear due to a gap in data observations between the lithosphere and ionosphere. To address this issue, an instrumental array was established to monitor vibrations and perturbations in the lithosphere, atmosphere, and ionosphere. Observational data from the array partially fill the gap, and analytical results show that ground vibrations, air pressure, magnetic fields, and TEC data shared a common frequency of approximately 5 × 10–3 Hz (5 mHz) before major earthquakes. This suggests that the resonant ground vibrations trigger atmospheric resonance before earthquakes. Therefore, the double resonance (crustal and atmospheric resonance) model is a new explanation for the observed anomalies in multiple geophysical parameters in the lithosphere, atmosphere, and ionosphere. Retrieving resonant signals from multiple sources of observational data is a significant challenge, but once this issue is overcome, double resonance may contribute to practical earthquake prediction.

A model of TEC perturbations possibly related to seismic activity

Valery Sorokin — 2023-12-31

For application of observed anomalies of the total electron content (TEC) in the ionosphere for short-term earthquake prediction, it is necessary to understand the physical nature of such anomalies during the preparation of a strong seismic event. The physical mechanism of occurrence of such anomalies is considered in order to distinguish the TEC disturbance of a seismic origin and to clear the processes in the lithosphere resulted in an appearance of the TEC anomaly in the ionosphere. It is supposed that the observed TEC anomalies arise as a result of occurrence of additional electric field in the ionosphere demonstrated by numerous satellite data before the impending earthquakes. At the same time, an injection of charged aerosols into the atmosphere in the epicentral earthquake area was detected. As a result, an electromotive force arises in the surface layer of the atmosphere, which initiates a perturbation of the electric current in the global circuit and the electric field appearance in the ionosphere. We have shown that the TEC disturbance arises as a result of the heating of the ionosphere by electric current and the plasma drift in the electric field of this current. The spatial distribution of TEC arises as a result of the combined action of these two factors, and its nature depends on the relationship between them. Based on the developed model the numerical study of the spatial distribution of TEC in the ionosphere is possible for a given horizontal distribution of the concentration of charged aerosols in the atmosphere near the Earth’s surface.

Space-based GNSS radio signals to investigate ionospheric plasma changes preceding the 2016 Al Hoceima-Morocco earthquake, Mw=6.4

Abdennasser Tachema — 2023-12-31

This paper examines abrupt variations in ionospheric electron density preceding the Al Hoceima earthquake (January 25th, 2016, Mw = 6.4, Northern Morocco). The observed anomalous behavior in the F2-ionospheric region, at about 350 km altitude, raises the possibility of a connection with impending seismic activity of moderate-to-great magnitude, supporting the hypothesis that Total Electron Content (TEC) variations could serve as potential earthquake precursors. For this purpose, we have exploited one of the main ionospheric keys, TEC, through a network of over one hundred dual-frequency Global Positioning System/Global Navigation Satellite Systems (GPS/GNSS) receivers. Through calculation algorithms based on spherical harmonic analysis of GPS/GNSS Observation-Navigation data, we were able to produce local ionospheric maps to restore the GPS-TEC signal and investigate potential ionospheric disturbances associated with this shallow-focus and strong earthquake.
Since the ionospheric TEC is a function of the variability and dynamics of the Earth’s ionosphere, mainly time-space and solar-geomagnetic activities, we had to consider each of these disturbing factors separately. In fact, the seismic zone of Al Hoceima (Morocco, North Africa), at about 35°N latitude, belongs to the region of low geographical latitudes. In such regions the variations of the ionospheric layer are slight. Moreover, during the earthquake preparation period, space weather conditions exhibited a calm state characterized by low solar activity and the absence of geomagnetic storms. The adequate effects of these physical conditions allow us, through wavelet transform, to emerge solely signatures of earthquake-related ionospheric disturbances. Based on the seismo-ionospheric combination model, we have highlighted some ionospheric electron density irregularities that decreased abnormally, near the epicenter several days prior to the 2016 Al Hoceima main event. From these findings, we can state that such research provides a promising approach for predicting earthquakes through large fluctuations in the ionized shell of the Earth’s atmosphere, thereby offering the prospect of a life-saving seismic alarm.

Lower Ionospheric variations during the intense tectonic activity in the broader area of Greece on October of 2020

Michael E. Contadakis — 2023-12-31

In this paper, we investigate the Lower ionospheric variations from TEC observations during the intense seismic activity of October 2020 in the area of Greece. The data were analysed using both, statistical analysis of TEC variations in order to detect uneven gross variations and Discrete Fourier analysis in order to investigate the TEC turbulence. The results of this investigation indicate that the High-Frequency limit fo of the ionospheric turbulence content, increases as aproaching the occurrence time of the earthquake, pointing to the earthquake epicenter, in accordance with our previous investigations. We conclude that the Lithosphere Atmosphere Ionosphere Coupling, LAIC, mechanism through acoustic or gravity waves could explain this phenomenology. In addition, the statistical analysis shows an excess greater than 3𝜎 from the mean TEC values, one and seven days before the earthquake. Since no major disturbance of the geomagnetic field occured during these days, we conclude that we probably observed precursory ionospheric variations in accordance to analogous findings from the variation of VH/VHF electromagnetic wave propagrations over strong earthquake areas.

Lower-ionosphere anomalies prior to strong earthquakes that occurred in north-central mainland Greece on March 2021 as revealed by multi-method analysis of VLF sub-ionospheric propagation data

Dimitrios Z. Politis — 2023-12-31

In this work we present the multi-method analysis of very low frequency (VLF) data, acquired by the radio receiver with call name UWA, located in Athens (Greece), in the University of West Attica, focusing on two strong ( ) earthquakes (EQs) that occurred in north-central mainland Greece sequentially, on 3 and 4 March 2021, with very close epicenters. Specifically, we used the data acquired from seven VLF transmitters located in Europe, North/North-West to UWA, and their propagation paths include the specific EQs epicenters. We analyzed these data using multiple analysis methods in order to investigate for possible EQ-related anomalies, taking also into account all the other possibly ionosphere-influencing extreme events that occurred during the studied period. Especially, we applied the “nighttime fluctuation method” (NFM), as well as, the “terminator time method” (TTM) in order to reveal any statistical anomaly in the nighttime amplitude recordings of VLF sub-ionospheric propagation data within 15 days before each one examined EQs. Also, we calculated the scalogram (wavelet power spectrum over time) using Morlet mother wavelet of the same nighttime data searching for possible imprints of wave-like structures during the same time period. In terms of criticality analysis, first we applied the “natural time” (NT) analysis method to the daily-valued NFM VLF propagation quantities, and subsequently applied the “method of critical fluctuations” (MCF) to the raw nighttime amplitude VLF recordings, to check for any criticality signatures up to two weeks before the examined EQs. Taking into account all the above-mentioned analysis results, we conclude that there are multiple indications that the lower ionosphere was indeed disturbed due to the preparation processes of the above-mentioned EQs, offering different types of seismogenic indications.

Multi-parametric study of seismogenic anomalies during the 2021 Crete earthquake (M=6.0)

Sudipta Sasmal — 2023-12-31

It is well established that pre- and co-seismic irregularities in the earth’s atmosphere highly depend on a set of parameters. According to the Lithosphere-Atmosphere-Ionosphere Coupling (LAIC) mechanism, these parameters are associated with various channels (thermal, chemical, acoustic, electromagnetic, etc.) through which an earthquake (EQ) preparation process can have its anomalous phenomena. In this research, we perform a multi-parametric observation using various channels during a strong EQ (M = 6.0) on September 27, 2021, on Crete Island in Greece. We investigate the acoustic and electromagnetic channel using ground and satellite-based observation. We present the Atmospheric Gravity Wave (AGW) in the acoustic channel using the temperature profile computed from the SABER/TIMED instrument. In the electromagnetic channel, ionospheric Total Electron Content (TEC) is recorded by the GNSS-IGS station DYNG in Greece. This TEC information is also used in the acoustic channel anomaly by computing the wave-like structures in the small-scale fluctuation of the TEC profile. We compute energetic (30 to 100 keV) electron precipitation in the inner radiation belt from the NOAA satellite. We also investigate the outcomes of the SWARM satellite to compute the magnetic field and electron density profile. All the parameters show significant seismogenic anomalies (mostly enhancement) before the EQ. To understand each parameter’s temporal and spatial sensitivity, we present a comparison using the anomalies in each parameter.

Sudden modulation in the UHF wireless signals probably caused by the activation of pre‑earthquake processes. Case studies for the Balkans (SE Europe)

Dimitar Ouzounov — 2023-12-31

We study the atmospheric variations of broadband wireless signal propagation intensity correlated with pre‑earthquake processes. We have maintained ground observations in the VHF range of 1.8 and 3.5 GHz in Bulgaria (Southeast Europe), close to the border with Greece and Northern Macedonia, since 2012. The signal source is 1.8 GHz –LTE broadcasting cellular communication signal and the receivers are digital HF to SHF antennae collecting the data via mobile internet. Our observations revealed phenomena associated with a natural enhancement of the intensity of the signals (no change in the transmitting level) days/hours before the seismic events, even far from the observation region. We are presenting the results for four significant earthquakes in the area: (1) M5.6 on May 22, 2012, in Bulgaria, (2) M6.9 on May 24, 2014, in the Aegean Sea, Greece, (3) M6.5 on Nov 17, 2015, Lefkada, Greece, and (4) M6.3 of May 12, 2017, in Western Türkiye. Some changes in the atmospheric boundary layer (ABL), triggered by an intensification of radon and other released gases, could lead to a change in lower atmospheric conductivity. Although the intensity modulation was observed far (> 200 km) from the epicenter areas, the anomalies were always inside the estimates of the Dobrovolsky‑Bowman area of preparation. We examined the possible correlation between magnitude and the spatial size of the earthquake preparation zone in the Lithosphere‑Atmosphere-Ionosphere coupling (LAIC) framework.