Subionospheric Propagation Research Articles

A Six-Year (2014–2020) Statistical Correlation Study of VLF Terminator Time Shift with Earthquakes in Japan

In this study, we present a six-year (2014–2020) statistical analysis of VLF subionospheric propagation data at 19 VLF receivers from the VLF transmitter with call name JJI and frequency 22.2 kHz, all located in Japan. Moderate and strong earthquakes (EQs) (ML≥4.5 and depth ≤ 50 km) that occurred in the wider area around Japan during the same time period with the available VLF data are investigated. The terminator times’ (TT) shift in VLF amplitude data as a possible precursor of an EQ are statistically examined, focusing on the correlation with seismic activity. The concept of the effective EQ magnitude (Meff) is used in order to define the total EQ energy possibly affecting the midpoint of each path for each day. It is important to note that dates when geomagnetic storms or solar flares occurred as well as dates corresponding to the already known winter effect on TT statistics in the north–south direction were excluded. The cross-correlation between TT statistical anomalies and seismic activity, represented by Meff, was extracted. Maximum cross-correlation values were found for most of the cases prior to the subsequent seismic activity, indicating a link between the ionospheric anomalies and the subsequent seismicity. Finally, the wide temporal range of the cross-correlation maxima temporal locations is justified by the inhomogeneity of the lower ionosphere, coupled with the anisotropy of the preseismic effect of the impending seismicity, highlighting the complexity of the EQ preparation processes.

Whistler echo trains triggered by energetic winter lightning

Lightning generated electromagnetic impulses propagating in the magnetospheric plasma disperse into whistlers – several seconds long radio wave signals with decreasing frequency. Sometimes, multiple reflections form long echo trains containing many whistlers with increasing dispersion. On January 3, 2017, two necessary prerequisites – a pronounced lightning activity and a magnetospheric plasma duct – allowed for observations of a large number of whistler echo trains by the high-latitude station in Kannuslehto, Finland. Our investigation reveals that the duct existed for nearly eight hours. We show that causative lightning sferics arrived to the duct entry from three different winter thunderstorms: a small storm at the Norwegian coast, which produced energetic lightning capable to trigger echo trains in 50% of cases, and two large storms at unexpectedly distant locations in the Mediterranean region. Our results show that intense thunderstorms can repetitively feed electromagnetic energy into a magnetospheric duct and form whistler echo trains after subionospheric propagation over distances as large as 4000 km.

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

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.

Possible Pre-Seismic Indications Prior to Strong Earthquakes That Occurred in Southeastern Mediterranean as Observed Simultaneously by Three VLF/LF Stations Installed in Athens (Greece)

In this work, we present the analysis of VLF/LF sub-ionospheric propagation data to study anomalies possibly related to very recent strong (M > 5.5) earthquakes (EQs) that occurred in the southeastern Mediterranean in September–October 2021 and January 2022. We used the signal of one transmitter located at Negev in Israel (29.7 kHz) as received by three VLF/LF receivers (two of them using identical SW and HW) installed, at a close distance to each other, in Athens (Greece). This study employed multiple methods and techniques to analyze the reception amplitude data to identify any possible EQ-related anomalies. More specifically, first, we used both statistical and criticality analysis methods such as the “nighttime fluctuation method” (NFM), the “terminator time method” (TTM), and the “natural time” (NT) analysis method. These methods have satisfactorily been applied in the past in a series of other studies leading to interesting results. Moreover, we additionally used two more analysis techniques focusing on the signal’s amplitude characteristics. The first is the wavelet analysis of the nighttime part of the signal’s amplitude. It is based on the Morlet wavelet function, aiming to unveil the possible existence of atmospheric gravity waves (AGWs) before EQ. The second is named “long wavelength propagation capability” (LWPC), which simulates the amplitude of the signal and is based on the reflection parameters of ionosphere and by searching for increases or decreases of the electron density profile of the ionospheric D layer concerning the shifts of the minima of terminator times (TTs) in the diurnal variation of the signal. Finally, in this work, we summarize our findings and discuss possible “pre-”, “co-”, and “post-” seismic effects as observed from all the work.

An Amateur-Radio-Based Open-Source (HW/SW) VLF/LF Receiver for Lower Ionosphere Monitoring, Examples of Identified Perturbations

The ground-based monitoring of the lower ionosphere by studying the perturbations of the subionospheric propagation of very-low-frequency/low-frequency (VLF/LF) signals is important in the research of a wide variety of geophysical and Sun/space extreme phenomena. Such perturbations are identified as anomalies in the signal received from the VLF/LF transmitters operating worldwide for military purposes, time code broadcasting, etc. Especially for the study of local ionosphere-influencing phenomena, such as earthquakes, volcanoes, typhoons, etc., the monitoring of several subionospheric propagation paths is necessary. However, it is very difficult to find in the market (or reproduce) hardware (HW) for wide-band VLF/LF receivers that could receive many different transmitters, while the involved software (SW) is mainly proprietary. Aiming to provide a low-cost and easy-to-build alternative for the scientists involved in this research field, we suggest a VLF/LF receiver setup based on amateur radio open-source HW and SW. Its key components are the so-called “mini-whip” active antenna and the freeware “SpectrumLab” and “GPS2Time”. The full HW schematics and all settings of the employed SW configuration for the proposed VLF/LF receiver setup are provided in the article. To check the reliability of the proposed receiver setup, two almost identical VLF/LF radio receivers were installed in the prefecture of Attica in Greece, in June and September of 2021, respectively. Examples of ionospheric perturbations due to different phenomena (solar flares, earthquakes, and a magnetic storm) are provided to show the ability of the proposed receiver setup to provide reliable data for ionosphere-related research.

Multi‐Point Observation of Hiss Emerging From Lightning Whistlers

AbstractWe analyze continuous multi‐point measurements of electromagnetic field waveforms onboard the Cluster spacecraft in order to contribute to the discussion on sources of plasmaspheric hiss, known as a shaping agent for the Earth radiation belts. In our case study we aim at finding sources of hiss observed close to the geomagnetic equator in the outer plasmasphere on the dayside. We find hiss to be triggered from whistlers of different spectral properties. Whistlers with the lowest observed dispersion arrive to different spacecraft with time delays indicating their origin in the northern hemisphere. Positions of source lightning discharges are then found using the time coincidences with the Word Wide Lightning Location Network data from three active thunderstorm regions in Europe. We find that subionospheric propagation of lightning atmospherics is necessary to explain the observations. Geographic locations of their ionospheric exit points then determine spectral properties of resulting unducted whistlers and triggered hiss. By this well documented chain of events starting with a lightning discharge in the atmosphere we confirm that magnetospherically reflecting whistlers and hiss triggered from them are among possible sources of plasmaspheric hiss.

Spring‐Fall Asymmetry in VLF Amplitudes Recorded in the North Atlantic Region: The Fall‐Effect

AbstractA spring‐fall asymmetry is observed in daytime amplitude values of very low frequency (VLF) radio wave signals propagating over the North Atlantic during 2011–2019. We explore the processes behind this asymmetry by comparing against mesospheric mean temperatures and the semidiurnal solar tide (S2) in mesospheric winds. The solar radiation influence on VLF subionospheric propagation was removed from the daytime VLF amplitude values, isolating the fall‐effect. Similarly, the symmetric background level was removed from mesospheric mean temperatures undertaking comparable analysis. During fall, all three analyzed parameters experience significant deviation from their background levels. The VLF amplitude variation during spring is explained by the seasonal variation in solar illumination conditions, while the fall‐effect can be interpreted as a mean zonal wind reversal associated with both a S2 enhancement, and temperature reductions. Decreases in temperature can produce decreases in collision frequency, reducing VLF signal absorption, driving the observed VLF asymmetry.

Statistical and Criticality Analysis of the Lower Ionosphere Prior to the 30 October 2020 Samos (Greece) Earthquake (M6.9), Based on VLF Electromagnetic Propagation Data as Recorded by a New VLF/LF Receiver Installed in Athens (Greece).

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.

Criticality analysis of 3-year-long VLF subionospheric propagation data possibly related to significant earthquake events in Japan

Ionospheric anomalies possibly related to significant earthquakes (EQs) have been extensively investigated in recent years and could be considered promising for short-term EQ prediction. In this work, we study the subionospheric propagation data recorded during a time period of 3 years (1/1/2014–28/02/2017) by a network of 8 VLF/LF receivers deployed throughout Japan, to identify possible signatures of pre-EQ processes. Specifically, we analyzed the nighttime receiver amplitude data possibly related to any main EQ event with magnitude MW ≥ 5.5 that occurred in a wider area of Japan during the above-mentioned time period (16 main EQ events) by applying the method of critical fluctuations which is able to identify critical dynamics. Our results identified intermittency-induced criticality in the lower ionosphere before all the main seismic events that occurred in Japan during the time period of interest. Criticality was also identified prior to other global or local extreme events such as magnetic storms, solar flares and volcano eruptions, that happened during the analyzed time period, indicating that more research efforts combining the results of more analysis methods, should be devoted in the future in order to discriminate between the subionospheric perturbations related to EQs and the ones related to other extreme geophysical events.

Energetic Electron Precipitation Associated With Pulsating Aurora Observed by VLF Radio Propagation During the Recovery Phase of a Substorm on 27 March 2017

AbstractRecent studies suggest that electrons with energies up to several hundred keV precipitate into the atmosphere associated with pulsating aurora (PsA). It is debated the highest energy of precipitating electrons associated with PsA. Here we report for the first time that the energy extends to relativistic energies. PsA was observed by THEMIS all‐sky imagers during a substorm that occurred on 27 March 2017. Energetic electron precipitation was detected by very low frequency subionospheric propagation. We found similar time variations between the auroral intensity and perturbation of the received radio signal intensity when the PsA occurred on the radio path. The perturbation showed a short recovery time of ~2 s. The recovery time indicates relaxation from ionospheric modification due to energetic electron precipitation and depends on the stopping altitude of the electrons. The recovery time required a stopping altitude of 50–60 km and indicates that the PsA is accompanied by relativistic electron precipitation.

Jupiter Lightning‐Induced Whistler and Sferic Events With Waves and MWR During Juno Perijoves

AbstractDuring the Juno perijove explorations from 27 August 2016 through 1 September 2017, strong electromagnetic impulses induced by Jupiter lightning were detected by the Microwave Radiometer (MWR) instrument in the form of 600‐MHz sferics and recorded by the Waves instrument in the form of Jovian low‐dispersion whistlers discovered in waveform snapshots below 20 kHz. We found 71 overlapping events including sferics, while Waves waveforms were available. Eleven of these also included whistler detections by Waves. By measuring the separation distances between the MWR boresight and the whistler exit point, we estimated the distance whistlers propagate below the ionosphere before exiting to the magnetosphere, called the coupling distance, to be typically one to several thousand of kilometers with a possibility of no subionospheric propagation, which gives a new constraint on the atmospheric whistler propagation.

Seismogenic ionospheric anomalies associated with the strong Indonesian earthquake occurred on 11 April 2012 (M = 8.5)

Ionospheric perturbations in possible association with a major earthquake (EQ) (M = 8.5) which occurred in India-Oceania region are investigated by monitoring subionospheric propagation of VLF signals transmitted from the NWC transmitter (F = 19.8 kHz), Australia to a receiving station at Varanasi (geographic lat. 25.3°N, long 82.99°E), India. The EQ occurred on 11 April 2012 at 08:38:35 h UT (magnitude ≈ 8.5, depth = 10 km, and lat. = 2.3°N, long. = 93.0°E). A significant increase of few days before the EQ has been observed by using the VLF nighttime amplitude fluctuation method (fixed frequency transmitter signal). The analysis of total electron contents (TEC) derived from the global positioning system (GPS) at three different stations namely, Hyderabad (latitude 17.38°N, longitude 78.48°E), Singapore (latitude 1.37°N, longitude 103.84°E) and Port Blair (latitude 11.62°N, longitude 92.72°E) due to this EQ has also been presented. Significant perturbation in TEC data (enhancements and depletion) is noted before and after the main shock of the EQ. The possible mechanisms behind these perturbations due to EQ have also been discussed.

Unusual Electromagnetic Signatures of European North Atlantic Winter Thunderstorms

All lightning strokes generate electromagnetic pulses –atmospherics– which can travel over distances of thousands of kilometers. Night-side atmospherics show typical frequency dispersion signatures caused by sub-ionospheric propagation. Their analysis can be used to determine the distance to the source lightning, and therefore it represents a safe tool for investigation of distant thunderstorms, as well as for indirect observations of the lower ionosphere. However, such analysis has never been done on the dayside. Here we present the first results which show unusual daytime atmospherics with dispersion signatures originating from strong thunderstorms which occurred during winter months 2015 in the North Atlantic region. Using newly developed analysis techniques for 3-component electromagnetic measurements we are able to determine the source azimuth and to attribute these rare atmospherics to both positive and negative lightning strokes in northern Europe. We consistently find unusually large heights of the reflective ionospheric layer which are probably linked to low fluxes of solar X rays and which make the dayside subionospheric propagation possible. Although the atmospherics are linearly polarized, their dispersed parts exhibit left handed polarization, consistent with the anticipated continuous escape of the right-hand polarized power to the outer space in the form of whistlers.

VLF modal interference distance and nighttime D region VLF reflection height for west‐east and east‐west propagation paths to Fiji

AbstractVery low frequency (VLF) signals from navigational transmitters propagate through the Earth‐ionosphere waveguide formed by the Earth and the lower conducting ionosphere and show the pronounced minima during solar terminator transition between transmitter and receiver. Pronounced amplitude minima observed on 19.8 kHz (NWC transmitter) and 24.8 kHz (NLK transmitter) signals recorded at Suva (18.149°S, 178.446°E), Fiji, during 2013–2014, have been used to estimate the VLF modal interference distance (DMS) and nighttime D region VLF reflection height (hN). The NWC transmitter signal propagates mostly in west‐east direction, and the NLK transmitter follows a transequatorial path propagating significantly in the east‐west direction. The values of DMS calculated using midpath terminator speed are 2103 ± 172 km and 2507 ± 373 km for these paths having west‐east and east‐west components of VLF subionospheric propagation, respectively, which agree with previously published results and within 10% with theoretical values. We have also compared the DMS estimated using a terminator time method with that calculated using terminator speed for a particular day and found both the values to be consistent. The hN values were found to be maximum during winter of Southern Hemisphere for NWC signal and winter of Northern Hemisphere for NLK signal VLF propagation paths to Suva. The hN also shows significant day‐to‐day and seasonal variabilities with a maximum of about 10 km and 23 km for NWC and NLK signal propagation paths, respectively, which could be due to the atmospheric gravity waves associated with solar terminator transition, as well as meteorological factors such as strong lightnings.

Subionospheric propagation and peak currents of preliminary breakdown pulses before negative cloud‐to‐ground lightning discharges

AbstractWe analyze broadband electromagnetic measurements of pulse sequences occurring prior to first return strokes of negative cloud‐to‐ground lightning flashes. Signals generated by lightning discharges were recorded close to the thunderstorm by a magnetic field receiver and traveled up to 600 km to three distant electric field receivers. We found that amplitudes of observed preliminary breakdown pulses, as well as amplitudes of the corresponding return strokes, are attenuated approximately by 2 dB/100 km when propagating in the Earth‐ionosphere waveguide over mountainous terrain. Propagation simulations show that there is a significant contribution of the sky wave signals in the waveforms observed beyond 500 km from their source. The estimated peak currents of the largest preliminary breakdown pulses reach over 60 kA. Such current pulses propagating through in‐cloud lightning leader channels in a strong electric field may be able to initiate terrestrial gamma ray flashes.

Estimating the VLF modal interference distance using the South America VLF Network (SAVNET)

Pronounced amplitude minima are observed during the subionospheric propagation of VLF waves at times (Terminator Times) when the Terminator Line crosses given locations along the propagation path. The distance between such two successive minima is called the modal interference distance D, which is related to nighttime mode propagation in the Earth-ionosphere waveguide. Therefore, the temporal behavior of the distance D can bring information on the dynamics of the nighttime lower ionosphere and on the presence of external forcing agents, including those associated with seismic activity. In this paper we present a methodology to estimate D based on the measure and analysis of the pronounced VLF amplitude minima. We have used a long-term database of almost 5 years from three different VLF propagation paths from the South America VLF Network. We emphasize that the accuracy of the determination of the distance D achieved by our method is better than those obtained in earlier studies. The reason for that is the use of a long-term continuous database, from different parallel propagation paths mainly oriented along the west-to-east direction. We discuss typical properties of the obtained distance D, as the simultaneous occurrence of amplitude minima for parallel propagation paths, anomalous values of D at locations where the Terminator Line is close to the receiver, and the derivation of the undisturbed nighttime ionospheric height at hN ~ 88 km.

Very low frequency subionospheric remote sensing of thunderstorm‐driven acoustic waves in the lower ionosphere

AbstractWe present observations of narrowband subionospheric VLF transmitter signals on 20 March 2001, exhibiting coherent fluctuations of over 1 dB peak to peak. Spectral analysis shows that the fluctuations have periods of 1–4min and are largely coherent. The subionospheric propagation path of the signal from Puerto Rico to Colorado passes over two regions of convective and lightning activity, as observed by GOES satellite imagery and National Lightning Detection Network lightning data. We suggest that these fluctuations are evidence of acoustic waves launched by the convective activity below, observed in the 80–90 km altitude range to which nighttime VLF subionospheric remote sensing is sensitive. These observations show that VLF subionospheric remote sensing may provide a unique, 24h remote sensing technique for acoustic and gravity wave activity. We reproduce this event in simulations using a fluid model of gravity and acoustic wave propagation to calculate the ionospheric disturbance, followed by an electromagnetic propagation model to calculate the perturbation amplitude at the location of the VLF receiver. Simulation results show that a very large and coherent convective source is required to produce these amplitude perturbations.

Lower ionosphere monitoring by the South America VLF Network (SAVNET): C region occurrence and atmospheric temperature variability

AbstractDaily profiles of phase measurements as observed on fixed VLF paths generally show a transient phase advance, followed by a phase delay, for about 90 min after sunrise hours. This is indicative of a reflecting ionospheric C region developing along the terminator line at an altitude below the normal D region. The suggested occurrence of a C region is consistent with rocket measurements made in the 1960s, showing a maximum of the electron density between 64 and 68 km, and by radio sounding in the 1980s. In order to correctly describe the properties of the phase effect associated with the presence of a C region, it is important to understand the subionospheric propagation characteristics of the VLF paths. In this paper, we analyze the variations presented by the temporal properties of the VLF narrowband phase effect and determined a parameter associated with the appearance of the C region at sunrise hours observed by receivers from the South America VLF Network. Periodic patterns emerge from the parameter curves. Two distinct temporal behavior regimes can be identified: one exhibiting slow variations between March and October, and another one exhibiting faster variations between October and March. Solar illumination conditions and the geometrical configuration of the VLF paths relative to the sunrise terminator partly explain the slow variation regime. During periods of faster variations, we have observed good association with atmospheric temperature variability found in the measurements of the Thermosphere Ionosphere Mesosphere Energetics and Dynamics and Sounding of the Atmosphere using Broadband Emission Radiometry satellite instrument, which we assume to be related to the winter anomaly atmospheric phenomenon. However, when comparing the parameter time series with temperature curves, no direct one‐to‐one correspondence was found for transient events.

Subionospheric VLF perturbations observed at low latitude associated with earthquake from Indonesia region

Subionospheric propagation from a Very Low Frequency (VLF) transmitter (VTX, 18.2kHz) received at a low latitude station Suva, Fiji over a Transmitter–Receiver Great Circle Path (TRGCP) length of 11,400km has been utilized to identify any possible ionospheric perturbations associated with the earthquakes that occurred in the Indonesia region during the period December 2006–October 2010. Out of five earthquakes that occurred with their epicenter in the fifth Fresnel zone, only an earthquake on 18 December 2006, in the North Sumatra region, has shown convincing evidence of lower ionospheric perturbations on the VTX transmission. The magnitude of this earthquake was 5.8 measured on the Richter scale and occurred at a depth of 53km with its epicenter located 45km off the TRGCP. The VLF amplitude data for this earthquake was analyzed using (1) terminator time (TT), (2) average nighttime and daytime amplitude variation, and (3) nighttime fluctuation (NF) methods. The results show that the sunrise TTs deviated considerably in the period 14–22 December 2006 measuring up to ~20min on the day of the earthquake. The results also show that the average nighttime as well as the average daytime signal amplitudes decreased by about 5dB and 3dB, respectively, during the period of the earthquake. The NF method revealed a decline in the trend at least 2 days before the earthquake though not exceeding the 2σ criteria and enhancements in the NF exceeding 2σ mark, however, the normalized values of the trend, NF and dispersion did not reveal an increase above the 2σ marks as reported by previous researchers. This could be due to the very long path length and the prevalence of lightning activity along the TRGCP in the Asia-Oceania Region.

On the Relative Effect of Magnitude and Depth of Earthquakes in the Generation of Seismo-Ionospheric Perturbations at Middle Latitudes as Based on the Analysis of Subionospheric Propagation Data of JJY (40 kHz)-Kamchatka Path

The relative importance of magnitude and depth of an earthquake (EQ) in the generation of seismo-ionospheric perturbations at middle latitudes is investigated by using the EQs near the propagation path from the Japanese LF transmitter, JJY (at Fukushima) to a receiving station at Petropavsk-Kamchatsky (PTK) in Russia during a three-year period of 2005-2007. It is then found that the depth (down to 100km) is an extremely unimportant factor as compared with the magnitude in inducing seismo-ionospheric perturbations at middle latitudes. This result for sea EQs in the Izu-Bonin and Kurile-Kamchatka arcs is found to be in sharp contrast with our previous result for Japanese EQs mainly of the fault-type. We try to interpret this difference in the context of the lithosphere-atmosphere-ionosphere coupling mechanism.