The Generation of Seismogenic Anomalous Electric Fields in the Lower Atmosphere, and Its Application to Very-High-Frequency and Very-Low-Frequency/Low-Frequency Emissions: A Review

Abstract

The purpose of this paper is, first of all, to review the previous works on the seismic (or earthquake (EQ)-related) direct current (DC) (or quasi-stationary) electric fields in the lower atmosphere, which is likely to be generated by the conductivity current flowing in the closed atmosphere–ionosphere electric circuit during the preparation phase of an EQ. The current source is electromotive force (EMF) caused by upward convective transport and the gravitational sedimentation of radon and charged aerosols injected into the atmosphere by soil gasses during the course of the intensification of seismic processes. The theoretical calculations predict that pre-EQ DC electric field enhancement in the atmosphere can reach the breakdown value at the altitudes 2–6 km, suggesting the generation of a peculiar seismic-related thundercloud. Then, we propose to apply this theoretical inference to the observational results of seismogenic VHF (very high frequency) and VLF/LF (very low frequency/low frequency) natural radio emissions. The formation of such a peculiar layer initiates numerous chaotic electrical discharges within this region, leading to the generation of VHF electromagnetic radiation. Earlier works on VHF seismogenic radiation performed in Greece have been compared with the theoretical estimates, and showed a good agreement in the frequency range and intensity. The same idea can also be applied, for the first time, to seismogenic VLF/LF lightning discharges, which is completely the same mechanism with conventional cloud-to-ground lightning discharges. In fact, such seismogenic VLF/LF lightning discharges have been observed to appear before an EQ. So, we conclude in this review that both seismogenic VHF radiation and VLF/LF lightning discharges are regarded as indirect evidence of the generation of anomalous electric fields in the lowest atmosphere due to the emanation of radioactive radon and charged aerosols during the preparation phase of EQs. Finally, we have addressed the most fundamental issue of whether VHF and VLF/LF radiation reported in earlier works is either of atmospheric origin (as proposed in this paper) or of lithospheric origin as the result of microfracturing in the EQ fault region, which has long been hypothesized. This paper will raise a question regarding this hypothesis of lithospheric origin by proposing an alternative atmospheric origin outlined in this review. Also, the data on seismogenic electromagnetic radiation and its inference on perturbations in the lower atmosphere will be suggested to be extensively integrated in future lithosphere–atmosphere–ionosphere coupling (LAIC) studies.

1. Introduction

Short-term earthquake (EQ) prediction with a lead time of about one week is still an extremely challenging and very difficult topic in geophysics as compared with medium- and long-term prediction, despite that there are a lot of demands from the general public. It had been initially considered to be a topic of seismologists, but many non-seismological (especially electromagnetic and plasma anomalies) EQ precursors have been detected, and attracted a lot attention from scientists working in multidisciplinary fields. Very significant progress has been achieved during the past few decades in the studies of short-term EQ precursors crucial for short-term EQ prediction (e.g., [1,2,3,4,5]).

Here, we will introduce several major findings related to the above short-term EQ precursors. Initially, the ground-based observations aimed at searching electromagnetic phenomena related with processes of EQ preparation started about three decades ago. The following seismogenic phenomena were discovered with a lot of hope for short-term EQ prediction: electromagnetic VLF/LF noises [6], ULF magnetic and electric emissions [7,8,9,10,11], acoustic emissions [12], amplitude and phase anomalies of subionospheric VLF/LF signals from powerful transmitters [13,14,15,16,17], ionosphere perturbations measured by the ionospheric sounding, TEC (total electron content) observation [2,18], airglow anomalies [19], and some others. Later, there was a milestone in 2004 of launching a French satellite, DEMETER, dedicated to the study of seismo-electromagnetics, and a lot of scientific outputs have been presented from this satellite observation [20]. A joint analysis of observational results led us to the conclusion that seismic activity stimulated the development of intense processes in different layers including the lithosphere, lower atmosphere, and ionosphere. The Earth’s surface seismic waves, chemically active and radioactive substances, and charged aerosols seem to act simultaneously on the lower atmosphere [1,2,3,4,5]. There is then heating of the lower atmosphere, sharp changes in its electrophysical parameters, the generation of acoustic waves, and the formation of external electric currents that occur. The acoustic action also appears on the ionosphere because of the upward propagation of infrasonic waves [21,22]. Processes in the lower atmosphere (seismic waves, atmosphere heating, and the injection of gasses) result in the generation and upward propagation of internal gravity waves (IGWs), which might perturb the ionosphere [21,22,23,24]. The formation of ULF radiation on the Earth’s surface by lithospheric sources is considered in [25,26,27,28,29,30], and the possibility of its penetration into the ionosphere is discussed by Molchanov et al. (1995) [28].

Based on the above initial findings and following statistical analysis results, it has been found that the ionosphere (both in the upper F region and the lowest ionosphere) among various EQ precursors is extremely sensitive to pre-EQ lithospheric seismic activity, suggesting a strong coupling among the three layers (e.g., [13,14,15,16,17,18,20]), so a new concept of lithosphere–atmosphere–ionosphere coupling (LAIC) was proposed, and is now our major concern in the field of seismo-electromagnetics [1,2,3,4,5,31,32]. In order to better understand the mechanism of LAIC, Ouzounov et al. (Eds) (2018) [33] emphasized the importance of multi-parameter observations in different layers of the lithosphere, atmosphere, and ionosphere with the combined use of ground- and satellite-based measurements, even though this is very difficult because of its multidisciplinary collaboration, and several papers have been published during the past several years, focusing on the mechanism of the LAIC process [34,35,36,37,38,39,40,41,42,43]. In these LAIC studies, the information of the lower atmosphere (height less than 10 km) is always missing in nearly all previous papers [34,35,36,37,38,39,40,41,42,43] except a few [40,41], and Hayakawa and Hobara (2024) [44] have recently re-addressed the importance of observations in the lower atmosphere, upper atmosphere (such as mesosphere), and lower ionosphere, which is the region central to the studies of the LAIC process.

On the other hand, there is an important peculiar branch of seismo-electromagnetics, that is, natural electromagnetic radiation. Unfortunately, the mechanisms of the generation of seismogenic natural electromagnetic emissions in a wide frequency range from VHF, VLF/LF, to ULF (ultra-low frequency)/DC are left as the least investigated topic, though they have the longest history, being discovered many years ago as a hope for short-term EQ prediction (e.g., [45,46,47]). While the generation mechanism of natural radiation only in the lowest frequency range (DC/ULF) was extensively investigated (Varotsos, 2015 [48]; Hayakawa et al., 2023 [49]), the generation mechanism of radio noises in a higher frequency range from VLF to VHF is extremely poorly understood, though the propagation characteristics have been studied extensively during the early phase of seismo-electromagnetic studies. The elucidation of the generation of seismogenic electromagnetic radiation is focused on in this review, but through these studies, we will show that those emissions at higher frequencies will be potentially connected with the perturbations especially in the lower atmosphere. This information will be extensively utilized in future LAIC works.

Hence, we pay particular attention to the generation mechanism of natural radiation at a higher frequency range from VLF/LF to VHF, and here we propose the natural seismogenic VHF and VLF/LF radiation as indirect evidence of the formation of anomalous electric fields in the lower atmosphere due to the emanation of radon and charged aerosols during the preparation phase of EQs as initially proposed by Sorokin and his colleagues (e.g., Sorokin et al., 2015 [5]).

The present paper is organized as follows. In Section 2, we will present various phenomena taking place in the lower atmosphere over a seismic region before an EQ probably associated with the emanation of radon and related phenomena. Section 3 presents the theoretical inference on how and why the anomalous electric fields are generated in the lowest atmosphere based on the concept of the emanation of radon and charged aerosols before an EQ. This concept has been initially applied to seismogenic VHF radiation in Section 4, and, for the first time, to VLF/LF lightning discharges in Section 5. Finally, in Section 6, we address the most fundamental question of whether those VHF and VLF/LF emissions are either of atmospheric origin (of course, related to the EQ preparation process) or of lithospheric origin due to microfracturing, by conducting a critical review of earlier works. The final section, Section 7, is the conclusion.

2. Seismogenic Phenomena on the Earth’s Surface and in the Lowest Atmosphere

We introduce various phenomena taking place on the Earth’s surface and in the lower atmosphere over the EQ region. Early analyses of satellite images of the Earth’s surface in the infrared (IR) frequency range exhibited the presence of stable and unstable components of the anomalous IR radiation flux above active crust faults; this flux corresponds to an increase in the temperature of the near-Earth layer by several centigrades [50,51,52,53,54]. Simultaneously with electromagnetic and plasma phenomena in the ionosphere, there was an observed increase in the concentration of certain gasses (e.g., H₂, CO₂, and CH₄) by several orders of magnitude, an increase in atmospheric radioactivity (related to such radioactive elements as radon, radium, uranium, thorium, and actinium and their decay products), and an increase in the injection of soil aerosols [55,56,57,58,59,60,61], which may lead to different physical/chemical processes.

As mentioned in Section 1, an approach to study EQ precursors consists in a joint analysis of a set of possible parameters observed by multi-parameter and multi-layer instruments [34,35,36,37,38,39,40,41,42,43,44]. Such an analysis can be physically based on a model that allows a satisfactory interpretation of most of the satellite- and ground-based observations preferably as a manifestation preferably of one cause. In this case, measured parameters proved to be interrelated by certain regularities. The most essential issue of the LAIC process is the search for a chain of processes related to acting factors and the identification of a set of observed effects of a common nature (though not so simple) [34,35,36,37,38,39,40,41,42,43,44]. It is regarded that principal causes of LAIC are the (a) generation of acoustic and atmospheric gravity waves (or IGWs) and the (b) generation of electric fields in the seismic region, but we are not sure whether it can be explained only by a single mechanism or not (or a combination of a few mechanisms). A recent paper by Hayakawa and Hobara [44] has suggested the presence of two possible channels of LAIC for a particular EQ: (1) A fast channel, in which the anomalies take place nearly simultaneously (within the order of one day) both on the Earth’s surface (and lithosphere) and ionospheric heights, and (2) a slow channel, which is like a diffusion type of the effects from the lithosphere to ionosphere. That is, the anomalies in the lithosphere (and on the Earth’s surface) seem to propagate upwards to the ionosphere with a delay of a few (or several) days. In this review, we discuss only one of these influencing factors, the cause and consequences of an electric field occurring at an eve of EQs, probably closely connected with the fast LAIC channel [44]. Another channel (a) will be a topic of our future work.

3. Growth of Seismogenic Electric Fields during the EQ Preparation Phase: Breakdown Electric Fields in the Lower Atmosphere

Based on the observations in the previous section, it is likely that numerous physical phenomena appear before an EQ, with one of the most major primary agents: the emanation of radon and charged aerosols during the EQ preparation phase. Initially, in Section 3.1, we summarize the observational results (unfortunately based on few papers), and will present in Section 3.2 the theoretical inference of the generation of seismogenic abnormal electric fields in the lower atmosphere based on the radon emanation before an EQ.

3.1. Experimental Observation of Atmospheric Electric Fields

Here, we try to review the observational results of atmospheric electric fields in the lower atmosphere. Measurements of atmospheric electric fields in the lower atmosphere are extremely difficult to perform, especially as one has to use balloons in this case. So, even in the field of conventional lightning studies, the balloon experiments that measure height profiles of electric fields in the lower atmosphere (e.g., [62,63]) are rare even though the electric fields associated with thunderstorms are relatively large. Even observations of the quasi-static or DC electric field on the Earth’s surface in seismic regions are very scarce, but were carried out by some research groups [64,65,66,67,68,69,70]. Analyses of those publications show that the local electric field surges with a large amplitude reaching several kV/m are observed during the EQ preparation, but their duration is of the order of ten minutes [64,65,66,67,68,69,70]. However, there have never been observed visible electric field disturbances with the duration of several days observed simultaneously over the horizontal distance of hundreds of kilometers [64,65]. The following is a summary based on the observational facts as compared with the corresponding results within the ionosphere.

• The enhancement in seismic activity produces clear DC electric field disturbances in the lower ionosphere.
• These disturbances occupy the region with a horizontal spatial scale from hundreds to thousands of kilometers over the seismic region.
• Meanwhile, DC electric field disturbances in the lower atmosphere as inferred from indirect VHF radio observations, etc., can reach the breakdown value “from hours to 10 days” in the atmosphere at altitudes of 1 to 10 km over the EQ zone a few days before an EQ.
• The quasi-stationary electric field on the Earth’s surface does not exceed its background value simultaneously in the seismic area during several days.

3.2. Theoretical Inference

We rely on important theoretical estimations on this topic by Sorokin and his colleagues [71,72,73,74,75]. The formation of electromotive force (EMF) seems to spread in the atmosphere over a seismic region from tens to hundreds of kilometers in diameter. The external current of EMF is generated as the result of the emanation of charged aerosols transported into the atmosphere by soil gasses and subsequent processes of upward transport, gravitational sedimentation, and charge relaxation. They have taken into account the effects of atmospheric radioactivity on the external current and conductivity based on the vertical distribution of the ion product rate as the result of absorption in the atmosphere of the gamma radiation and the alpha particles from the decay of radioactive elements. To calculate the external current of the EMF and the change in conductivity of the atmosphere, it is necessary to find an equilibrium ion number density depending on the ion formation rate. The equilibrium value of ion number density is determined by its recombination processes in the air and adhesion to the aerosols. The equations for external currents of positive j_p(r,z) and negative j_n(r,z) (where z is vertically directed, and r means the radial direction) charged aerosols and for atmosphere conductivity σ(z) were obtained by Sorokin et al. (2007) [72].

Sorokin et al. (2005) [71] suggested that the external current of EMF depends on the vertical component of the electric field on the Earth’s surface. Such feedback is caused by the formation of a potential barrier on the ground–atmosphere boundary at the passage of upward moving charged aerosols through this boundary, and it leads to a limitation of the vertical electric field on the Earth’s surface. The upward movement is performed due to the viscosity of soil gasses flowing into the atmosphere. If, for example, a positively charged particle moves from the ground to the atmosphere, the Earth’s surface is charged negatively. The resulting downward electric field prevents particles from penetrating through the surface. At the same time, this field stimulates the going out on the surface of negatively charged particles. In the presence of such coupling, the dependence of external current magnitude on the vertical component of the electric field on the surface E_z(r,z) can be estimated. On the other hand, the critical field Ec can be estimated from the balance between viscosity, gravity, and electrostatic forces. Viscosity force connected with elevated soil gasses acts in an upward direction, while gravity force is directed downward, and electrostatic force connected with the going out of positive particles is directed downward. The electric field in the atmosphere–ionosphere layer E_z(r,z) can be finally found. Figure 1 is taken from Sorokin et al. (2012) [73] and Sorokin and Hayakawa (2013) [74]. This figure illustrates both the altitude (z) and the spatial (r) distributions of the lE_z(r,z)/E_k(z)l, for a few plausible conditions of two important atmospheric convection parameters: u₀ is mean velocity of upward aerosol motion by atmospheric convection, and H_c is the scale height of atmospheric convection velocity. In the figure, we take (a) H_c = 2 km, u₀ = 3.3 × 10⁻² m/s; (b) H_c = 5 km, u₀ = 3.2 × 10⁻² m/s; and (c) H_c = 6 km, u₀ = 3.2 × 10⁻² m/s. It is seen from these theoretical estimations that the seismogenic anomalous electric field is generated, and can reach the breakdown value E_k in the lower atmosphere at the altitude of a few kilometers to 6 km or so, as seen as red parts of Figure 1 where E_z/E_k exceeds unity. So, we can anticipate the formation of a peculiar, formed by seismic effects, capacitor (or disk)-like thundercloud at the heights (z) from a few kilometers to ~6 km or so, with the thickness of a few kilometers. The top region of this peculiar thundercloud is positively charged, while the bottom region is negatively charged, and this looks exactly like an atmospheric thundercloud formed by the conventional meteorological convection. This is the central point of this review paper.

4. Application to Seismogenic VHF Emissions

Here, we apply the basic concept in Section 3 to the seismogenic VHF radiation observed in Greece and in Japan. The initial result on seismogenic VHF radiation was reported based on the data of radar observations of distributed electric charge appearance over the EQ center 1–3 days before the Spitak EQ [45], because they showed that the properties of this charge distribution were different from those related to conventional thunderstorms, based on the main observational fact that the lifetime of such a seismogenic charged area was several hours [45], while that of the same area during conventional thunderstorms does not exceed one hour [62,63].

4.1. Application to VHF Radiation

Sorokin et al. [76] assumed that the electromagnetic emission originates from electrical discharges, which occur in the regions of the atmosphere where the DC electric field reaches the breakdown value as shown by red parts in Figure 1. According to observations, a high level of the electric field should be maintained for several days before the EQ. Significant growth of the DC electric field in the ionosphere over seismic zones has been reported by the authors of [77,78], who have shown that the magnitude of this electric field reaches up to 10 mV/m. According to the electrodynamic model of LAIC, such a field enhancement is connected with the generation of electric currents in the atmosphere–ionosphere circuit [71,72,73,74,75]. The calculation results by Sorokin et al. [72,73], as summarized in Section 3, are used for the interpretation of the observed VHF radio emission from the atmosphere over an EQ eve, because we can expect arbitrary discharges initially in such a highly charged layer in the atmosphere (see red parts in Figure 1), just like intra-cloud (IC) discharges in the conventional strokes. They suggested that the discharge is excited by a vertical component of the electric field and it is an electric dipole. They consider an electromagnetic radiation of impulsive electric dipoles, and the current source of radiation is formed as the result of random discharges. See the details of computation in Sorokin et al.’s work (2012) [76], and the back curve in Figure 2 refers to the theoretical frequency spectrum of electromagnetic radiation at a distance of 300 km calculated for a realistic set of physical parameters. This figure suggests the possible frequency ranging from 10 MHz to 100 MHz. The two vertical bars in the figure indicate the experimental data by Greek colleagues at two specific frequencies of 41 and 53 MHz [79], which are in good agreement with the theory with respect to the frequency range and intensities. Finally, due to the randomness of dipole sources, the observed VHF radiation seems to be rather incoherent, as shown in [79].

4.2. Seismogenic VHF Emissions in Greece

The night sky glow at distances of 100–200 km from the epicenter of an impending powerful EQ (M = 7.3) in China was reported in [80]. Williams (1989) [81] noted that the seismic-related airglow can reach altitudes above 1–2 km at a distance of about 140 km from the EQ epicenter. The first regular observations of pre-EQ anomalies in VHF electromagnetic radiation were carried out on Crete Island for three years starting from 1992. The observations were made simultaneously at four sites in two frequency bands of around 41 and 53 MHz [82,83]. These frequencies have been chosen for electromagnetic monitoring on Crete Island, referring to the highest signal-to-noise ratio in this area, but this frequency choice was later found to be appropriate for the observation of seismogenic VHF radiation as shown in Figure 2. A pre-EQ VHF radiation effect has been found both for ground and under-the-sea EQs. Based on these observational data, Ruzhin and Nomicos (2007) [79] argued that the VHF radiation source in the studied events was in the atmosphere at altitudes of several kilometers over the EQ center. Unfortunately, they have not provided any statistical results, but their results have been confirmed by the Japanese observations in the following subsection.

4.3. Seismogenic VHF Emissions as Observed in Japan

Yamada et al. (2002) [84] confirmed the previous Greek conclusion that VHF radiation sources are located in the atmosphere at altitudes over several kilometers. VHF radiation (52.1–52.5 MHz) related to an EQ was detected as the result of long-term observations from July 1999. The distance to epicenters was several hundreds of kilometers, and therefore it was possible to register radiation if its source was located in the atmosphere at an altitude of several kilometers. Yonaiguchi et al. (2007) [85,86] investigated the fractal characteristics of VHF radiation at a frequency of 49.5 MHz, which occurred on the eve of an EQ (M = 7.2) on 16 August 2005. The observations were made at three stations in the nearby zone located at the distances of dozens of kilometers from the EQ epicenter. It was assumed that the occurrence of such radiation depends on geological characteristics of the ground.

Hayakawa et al. (2006) [87] found the strong VHF radio noise at 77.1 MHz to be a precursor to the 2004 Mid Niigata prefecture EQ (23 October). On 15–18 October, they detected the VHF radio noises, and their direction finding indicated that those noises are coming close to the direction of the EQ epicenter. Yasuda et al. (2009) [88] developed an interferometric VHF system and found that the VHF radio noises are always simultaneously detected with the over-the-horizon VHF transmitter signal (77.1 MHz), with their azimuths being relatively close to the epicenter of the EQ.

These Japanese results can be interpreted in the same way as in the previous paragraph in terms of the atmospheric origin due to the abnormal enhancement in electric fields in the lower atmosphere probably due to the radon emanation before an EQ.

5. Application to Seismogenic VLF/LF Lightning Discharges

5.1. Application to VLF/LF Lightning Discharges

Here, we want to apply the theoretical inference of Figure 1 in Section 3, in which we have generated a few possible layers in the height range from a few kilometers to 6 km or with the electric field exceeding the breakdown value due to the emanation of radon and charged aerosols before an EQ. This situation of the formation of a seismic-related thundercloud is quite similar to the conventional atmospheric lightning discharges [68] as shown in Figure 3a. We will repeat the essential findings in Section 3; that is, at the altitude where the electric field exceeds the breakdown value (see red parts in Figure 1), a peculiar specific thundercloud (with a height range of 1–2 km) will generate with positive charges on the top and negative charges on the bottom, as shown in Figure 3b. Discharges within this special thundercloud serve as the source of VHF radiation discussed in Section 3. On the other hand, Figure 3b illustrates a possible picture of this peculiar capacitor or a disk-like seismogenic thundercloud. Its lower height and narrow thickness remind us of its similarity to a conventional thundercloud in the winter storms as observed in the Hokuriku area of Japan [89,90] rather than a summer thundercloud (Figure 3a). Hence, it is reasonable for us to expect the cloud-to-ground (CG) discharge in this structure. In the leader–return stroke sequence, the descending leader creates a conductive channel between the cloud charge source and ground surface, and deposits negative charges in this channel. The following return stroke traverses that path, moving from the ground to the cloud charge source, and neutralizes the negative leader charge. Thus, both the leader process and the return stroke transport effectively negative charges from the cloud to the ground. As imagined from the conventional lightning, the speed of the return stroke, averaged over the visual channel, is typically one-third the speed of light. The first return stroke current measured on the ground rises to an initial peak in some microseconds and decays to the half-peak value in some tens of microseconds. This is the source of VLF/LF lightning discharges, exactly like a conventional CG discharge.

Additionally, we will provide some speculations on seismogenic ULF/ELF atmospheric emissions (impulsive noises), which are the least understood topic in seismo-electromagnetics (e.g., [91,92,93]). In the case of a seismogenic thundercloud of Figure 3b, we have illustrated the occurrence of −CG lightning discharges, but it may be possible for us to expect the occurrence of +CG discharges on some occasions. It is only when we have +CG discharges that recent lightning physics indicates that we can anticipate the generation of TLEs (transient luminous events) such as sprites in the mesosphere [94,95], in which the radiation caused by the sprite current (in other words, “cloud-to-ionosphere discharge (CID)” current) manifests itself as 1–2 ms pulses that follow a lightning return stroke by a few milliseconds to a few hundred milliseconds [95]. This radiation is called an ELF transient (or a Q-burst) in lightning studies [96]. The configuration and charge distribution of our seismogenic thundercloud in Figure 3b are quite similar to those of winter thunderclouds [89], and the occurrence rate of +CG lightning discharges for winter thunderclouds is known to amount up to about 50% [63,89]. On the contrary, summer lightning as shown in Figure 3a has a major distinction from winter lightning, with higher cloud heights (~10 km) and the corresponding much smaller occurrence (~10%) of +CG [63]. Finally, we speculate that this kind of situation of +CG happens before an EQ, and leading to the observation of our “seismogenic ULF/ELF transients (or seismogenic Q-bursts)” [49]. We speculate that this situation might be related to our seismogenic ULF/ELF radiation as observed as a regular precursor to EQs [91,92,93].

5.2. Seismogenic VLF/LF Lightning Discharges

The most convincing evidence on the presence of VLF/LF lightning discharges has been summarized by Liu et al. (2015) [97], and Figure 4 is taken from the paper by Tsai et al. (2006) [98] for the 1999 Chi-chi EQ. This figure illustrates the temporal (daily) evolution of CG lightning discharges 15 days before and after the 1999 Chi-chi EQ, because a network of lightning detectors has been continuously monitoring the lightning discharges in Taiwan. It is found that the lightning occurrence increases significantly on 17 September 1999, 4 days before the EQ. This EQ happened near the Chelungpu fault as shown in the inset of Figure 4, so we can expect a notable emanation of radon and other substances around the relevant faults. They tried to count the number of the lightning discharges within a certain area around the EQ epicenter, and this enabled them to distinguish between the seismogenic lightning and the conventional atmospheric lightning, because the position of seismogenic lightning seems to be stationary in space as noted by Asada et al. (2005) [99], while the conventional lightning is likely to move spatially. Liu et al. (2015) [97] extended the above case to an extensive statistical study on M > 5.0 EQs in Taiwan during the 12 yr period of 1993–2004. Their conclusion is that (1) lightning activities tend to appear a few days (especially 17–19 days) before M > 6.0 shallow (depth less than 20 km) land EQs, and (2) the size of the lightning active area is proportional to the EQ magnitude. A majority of observational results by Liu et al. (2015) [97] can be reasonably explained by the mechanism in the previous section, Section 5.1, having the atmospheric origin.

VLF/LF noise studies were performed many years ago by many Japanese colleagues, and these will be discussed in the next section, because the VLF/LF noises could not be clearly attributed to a definite origin (i.e., atmospheric origin or lithospheric origin).

6. VHF and VLF/LF Emissions: Lithospheric or Atmospheric Effects?

Based on the above discussions, we support the key idea of an atmospheric effect of seismogenic anomalous electric field enhancement due to the emanation of radon and charged aerosols before an EQ. However, there has been another hypothesis that VHF and VLF/LF radiation is of lithospheric origin by Enomoto et al. (1994) [100], Eftaxias et al. (2002) [101], and others since early days. In this paper, we want to raise a strong question to this old supposition of this lithospheric origin (or the generation of seismogenic electric fields or electric currents in the lithosphere) by proposing an alternative hypothesis of atmospheric origin in this paper.

It seems that there is no consensus on the physical mechanism of VLF-VHF emissions (Fujinawa et al., 1997 [102], and references therein; Gershenzon and Bambakidis, 2001 [103], and references therein). Many authors suggest that electromagnetic waves are directly emitted from the focal region of the lithosphere (e.g., [104,105,106]). Eftaxias et al. (2002) [101] remind us that the time evolution of the precursory electromagnetic emissions with an EQ in Greece reveals similarities to that observed in laboratory acoustic and radio emissions during different stages of the failure preparation process in rocks. If we consider that the same dynamics govern the large-scale EQ and the laboratory scale sample rheological structure, the results of this analysis lead to the suggestion that the recorded anomalies are probably emitted from the focal area during the microfracturing process. Several laboratory experiments (e.g., [100,107]) have shown that exo-electrons and charged particles are emitted from rocks during microfracturing. Such emissions of charges from stressed rocks around the focal area accompanying microfracturing can act as an electric current system in a conductor with a length of the order of the subsequent fracture generated by the EQ (Biagi, 1999 [61]). In general, the radiation pattern of an antenna can be effectively excited, only by certain frequencies corresponding to the characteristic length scales of the antenna, i.e., k L~1, where k denotes the radiation wavenumber and L represents the fault scale length. The corresponding spectral content of the pre-seismic radio activity has finally been extended to lower frequencies (i.e., a few kHz), whereas at intermediate stages (L < 10 km) of the fault’s creation, the expected electromagnetic activity is located in higher frequencies. The fact that the 3 and 10 kHz pre-seismic emissions were detected at the end of the VHF pre-seismic emissions supports the above-proposed scenario.

We are not in a position to deny the opinion by Eftaxias et al. (2018) [108] that electromagnetic radiation takes place in and around the focal regions of EQs, in different frequency ranges from VHF to VLF/LF depending on the development of fault lengths, but the relevance of such a process to the observational facts observed by either a borehole or an aerial (in the air) antenna is considered to be a completely different issue (Tsarev and Sasaki, 1999 [109]). On the presumption that VHF radiation is generated in the lithosphere, Enomoto and Hashimoto (1994) [100] tried to detect these HF (high frequency)/VHF emissions by measuring the vertical electric field with the use of a borehole antenna. They found that these HF/VHF radio noises appeared from 4 days to 7 h before EQs with M > 4 within 60 km from their observing site. When thinking of the severe attenuation in the lithospheric propagation (see Singh et al.’s work, 2000, 2003 [110,111]), it seems impossible for us to consider that this radiation can propagate in the lithosphere over 60 km, but there is no problem when those emissions are generated over an EQ epicenter, and followed by propagation in the atmosphere, and detected at the site due to the skin effect at the observing site. Further, the observations in Greece are measured by an aerial antenna; they can be observed only when an EQ happens very close to their observation site and we assume that the VHF source is located very close to the Earth’s surface.

Here, we move on to the VLF/LF noise. We have proposed in Section 5 that a similar kind of discussion can be applied to VLF/LF radiation. At VLF/LF, we have to cite the most important paper by Singh et al. (2003) [111] because their observation is very unique in the sense of the simultaneous use of a borehole and an aerial antenna. They found three types of observed data in general, i.e., (1) noise bursts recorded only by an aerial antenna, (2) noise bursts recorded only by a borehole antenna, and (3) noise bursts recorded by both borehole and aerial antennas. The first and second kinds of noise bursts were observed on nine and five occasions, respectively, in the month of March, 1999, during the period of simultaneous operations of the two antennas. From a close examination of the variations in the occurrence number of noises observed by the borehole antenna in relation to EQs, they found a positive correlation between the two. Further, the occurrence number of the noise bursts by the borehole antenna is larger than that observed by the aerial antenna in March when a seismic swarm activity started with a devasting main shock of Ms = 6.6, 400 km north of the observing station of Agra. Further, they showed that smaller-magnitude EQs do not generate strong signals to be picked up by the borehole antenna. So, the paper suggests a possibility that the signals generated at the source are propagated through seismic faults in a manner similar to that in the Earth–ionosphere waveguide [112,113]. Such a possibility of propagation through seismic faults was suggested by Yoshino and Tomizawa (1998) [114], Kingsley (1989) [115], Yoshino (1991) [116], and Tsarev and Sasaki (1999) [109]. The spectrograms of noise bursts may indicate that the lithospheric VLF/LF bursts are incoherent (just like magnetospheric/ionospheric hiss emissions [117]).

Finally, we come to a conclusion that there exist two types of seismogenic VLF/LF noise bursts: (1) impulsive VLF/LF noise (just like lightning discharges) as suggested in this review paper, and (2) noise-type VLF/LF emission just like incoherent noise as an increase in the background noise level.

In the above context, we will revisit earlier VLF/LF studies by different workers. In their pioneering paper, Gokhberg et al. (1982) [6] demonstrated an increase in the background noise level observed at Sugadaira, Japan, at LF (81 kHz) before an EQ (M = 7) in the Osaka region (epicentral distance of about 200 km), which is the second (2) type and seems to be of lithospheric origin. So, it is impossible for us to suppose the propagation in the lithosphere with an extremely high attenuation of ~13 dB/km [110]; then, we can suppose that those LF noises emerge into the atmosphere, followed by the Earth–ionosphere waveguide (with extremely low attenuation). Further, based on observations at seven observing stations in the Kanto (Tokyo) district at the frequencies of 82 kHz, 1.5 kHz, and 36 Hz during the period of 1985–1990, Yoshino et al. (1992) [118] detected 29 cases, and indicated that VLF radio noises tend to be observed in a wider area for larger-magnitude EQs. Also, they mentioned that impulsive noises are also observed in addition to the noise-like emissions. Hata et al. (1998) [119] revealed the 225 Hz electromagnetic radiation generated close to the sea surface during seismic activity in the zone of an underwater fault. Next, Fujinawa and Takahashi (1994) [47] established a network of borehole antennas in the Tokyo district, and they observed that VLF pulses (type (1)) increased before an EQ. Later, Oike and Ogawa (1986) [120] and Oike and Yamada (1994) [46] established a station in Kyoto with an aerial ball antenna, and they succeeded in detecting anomalies of impulsive noises before an EQ. Recently, Asada et al. (2003) [99] made an attempt of wide-band (1–10 kHz) observation, together with the use of goniometer systems, and VLF impulsive noises seem to increase before an EQ, and their location remained stable and fixed in space, being in good agreement with the results by Liu et al. (2015) [97]. These are all of the type (2) impulsive noise in favor of our hypothesis in this review paper.

Here, we suggest future steps for the better understanding of seismogenic natural noises. First of all, an investigation is required of the noise characteristics (i.e., coherent noise or incoherent noise?) and its frequency spectra. Further, critical analyses should be used (such as finding the power spectral scaling exponent, fractal analysis, etc.) [121,122,123,124,125].

Finally, we comment on the satellite observations on seismogenic natural VLF noises. Many observations of electromagnetic waves associated with the seismic activity have been reported using data from numerous satellites (e.g., [126,127,128,129,130,131,132]). Parrot [20] made an extensive review based on the DEMETER observations, and he presented some examples of electromagnetic radiation associated with EQs in addition to their major topic of seismogenic ionospheric perturbations. Further, there is a recent paper by Lv et al. (2023) [133] based on the observation onboard the Chinese seismo-electromagnetic satellite CSES. Their statistical analysis of the shallow strong EQs (M > 6.0, depth below 30 km) that occurred in mainland China from 2019 to 2022 showed that although most of the ELF events are typical downward propagating ionospheric hiss waves [117], there are certain events including the upward propagation emissions. These emissions might be noise-like emissions as discussed above.

7. Conclusions and Outlook

The major purpose of this review is to better understand the mechanism of the generation of seismogenic natural electromagnetic radiation from VHF to VLF/LF, which is the least studied topic in seismo-electromagnetics. In this paper, we have presented these radio emissions as an indirect indicator of abnormal electric fields in the lower atmosphere. The following is a summary of the present review with our own original suggestions.

• The fundamental idea of this review is to accept the theoretical inference on the generation of anomalous DC electric fields in the lower atmosphere due to the emanation of radon and charged aerosols during the EQ preparation phase. That is, we expect the generation of a peculiar seismogenic thundercloud at the altitudes of a few to 6 km or so, in which the DC electric field exceeds the breakdown value. This layer has a height width of a few kilometers, with positive charges on the top and negative charges in the bottom of the layer.
• Seismogenic VHF electromagnetic emissions can be explained in terms of chaotic random discharges in such a highly charged layer, just like intra-cloud (IC) discharges in conventional lightning.
• The theoretical estimation of incoherent VHF radiation was performed on the frequency spectrum and intensity, which is found to be in good agreement with the experimental observations in Greece and in Japan.
• The same concept should be applied, for the first time, to the seismogenic VLF/LF lightning strokes, which are the discharges between the cloud charge source and the ground (CG discharges).
• This inference is supported by a good agreement with the observational results for Taiwan EQs; e.g., the VLF/LF radiation from lightning increased a few days prior to the 1999 Chi-chi EQ.
• The observation of seismogenic VHF radiation and VLF/LF discharges can be considered as indirect evidence of such anomalous enhancement in electric fields in the lowest atmosphere driven by the emanation of radon and charged aerosols during the EQ preparation phase.
• It seems that no consensus is reached on the generation mechanism of those VHF and VLF/LF noise bursts. The final issue is whether the observed VHF and VLF/LF noise bursts were of atmospheric origin as proposed in this review or if they were of the lithospheric origin hypothesized for a long time. The extensive review of earlier works has indicated that VLF/LF noises can be split into two types: (1) the impulsive noise, just like VLF/LF lightning discharges explainable by our hypothesis, and (2) the noise burst caused by an increase in continuous background noise, which is highly likely to be of lithospheric origin.
• Unfortunately, there have been no reports published so far of seismogenic electromagnetic radiation (either in VHF or in VLF/LF) that is simultaneous with observations of radon and charged aerosols, so we cannot prove the validity of our hypothesis proposed in this review. This will be part of a future work.
• Future areas of studies are suggested to clarify the generation mechanism of seismogenic natural electromagnetic radio emission: (i) studies of noise characteristics (coherent or incoherent?), (ii) frequency spectra, (iii) an application of a critical analysis to the noise data.
• When we prove that a considerable number of seismogenic natural emissions (either in VHF or in VLF/LF) are of atmospheric origin as proposed in this paper, they will be effectively integrated as a useful indicator of atmospheric perturbations in future LAIC studies.