Interferometric direction finding of over-horizon VHF transmitter signals and natural VHF radio emissions possibly associated with earthquakes

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1 RADIO SCIENCE, VOL. 44,, doi: /2008rs003884, 2009 Interferometric direction finding of over-horizon VHF transmitter signals and natural VHF radio emissions possibly associated with earthquakes Y. Yasuda, 1 Y. Ida, 1 T. Goto, 1,2 and M. Hayakawa 1 Received 30 April 2008; revised 19 November 2008; accepted 29 December 2008; published 26 March [1] The over-horizon (out-of-sight) VHF transmitter signals are found to be received before an earthquake, and also natural VHF noises are known to be detected prior to an earthquake. In order to have a better understanding of these over-horizon VHF signals and noises, an interferometric direction-finding system has been, for the first time, introduced, and some preliminary observational results have been presented here. With FM Sendai as a target VHF transmitter (at 77.1 MHz), we have carried out the observation at Chofu (Tokyo). We can receive occasionally the over-horizon VHF transmission signals in possible association with earthquakes. We have analyzed a few earthquakes since the commencement of our observation, including a large one named the Niigata Chuetsu-oki earthquake. The azimuth distribution by means of the interferometer has enabled us to correlate any burst in the temporal evolution of electric field intensity to a certain earthquake. The over-horizon VHF transmitter signals are always well above the background noise, whose azimuth is found to be relatively close to the direction of Sendai (relatively away from the epicentral direction). Then, the VHF radio noises are always simultaneously detected as an enhancement of background noise, with their azimuths being relatively close to the epicenter of the quake. The mechanisms of seismoatmospheric perturbations as seen from the over-horizon VHF signal, as well as the natural VHF noise, are discussed. Citation: Yasuda, Y., Y. Ida, T. Goto, and M. Hayakawa (2009), Interferometric direction finding of over-horizon VHF transmitter signals and natural VHF radio emissions possibly associated with earthquakes, Radio Sci., 44,, doi: /2008rs Introduction [2] There have been observed a lot of convincing evidence on electromagnetic effects associated with earthquakes (EQs) [e.g., Hayakawa and Molchanov, 2002; Molchanov and Hayakawa, 2008]. Seismic related phenomena are known to take place not only in the lithosphere, but also in the atmosphere and ionosphere [e.g., Hayakawa and Molchanov, 2002]. [3] The study of atmospheric and ionospheric perturbations associated with EQs has been based on the use of transmitter signals. Ionospheric perturbations have been 1 Department of Electronic Engineering and Research Station on Seismo Electromagnetics, University of Electro-Communications, Chofu, Japan. 2 Yokokawa Electric Company, Sagamihara, Japan. Copyright 2009 by the American Geophysical Union /09/2008RS extensively investigated by means of subionospheric VLF/LF transmitter signals [Hayakawa et al., 1996; Molchanov and Hayakawa, 1998; Hayakawa, 2007]. On the contrary, the atmospheric perturbations in possible association with EQs, is found to be investigated by means of over-horizon VHF transmitter signals. The propagation of over-horizon FM signals probably associated with impending EQs was first observed by Kushida and Kushida [1998]. They detected the signals from an over-horizon transmitter in Central Japan several days or weeks prior to the Kobe EQ. Though we do not receive any signal from a VHF transmitter out of the line of sight, we sometime receive the signals from the transmitter and we define this as being abnormal. Later, this abnormal phenomenon has been studied intensively by Fukumoto et al. [2001, 2002] for a number of EQs in Central Japan from 1 February till 30 June The FM transmitter they treated, is located in Sendai, 312 km far from the receiver in Chofu, whereas the distance of line of sight was 80 km. Though the FM signals (77.1 MHz) 1of10

2 Figure 1. A photo of our interferometer directionfinding system consisting of three eight-element Yagi antennas installed on the roof of our building. from Sendai have not been detected in Chofu on normal days because it is out of the line of sight, over-horizon FM signals have been occasionally received in Chofu. They applied their direction finding based on different combinations of Yagi antennas in azimuth and in elevation to these over-horizon VHF signals, and they found that the elevation angle is smaller than 20, which was indicative of atmospheric effect (completely denying the ionospheric effect as suggested by Pilipenko et al. [2001] and Kushida and Kushida [2002]). Then the signal bearing measurement has shown that there are sometimes a lot of differences in azimuth with regards to the bearing of the future epicenter, i.e., the azimuth being closer to the land area. Fukumoto et al. [2001, 2002] further found that the cross correlation between the abnormal over-horizon FM signals and EQs exhibited a significant peak around 7 days before the EQ. Recently an experimental confirmation on this lead time has been provided by Fujiwara et al. [2004]. [4] In addition to the reception of over-horizon VHF transmitter signals, we understand that there might be generated some natural VHF radio noises during EQs. For example, Enomoto and Hashimoto [1994] and Tsutsumi et al. [1999] found natural VHF noises 4 days to 7 h before the EQs with magnitude ]4 which occurred within a radius of 60 km. Maeda [1999] observed pulsed emissions observed at the frequency of 22.2 MHz before the famous Kobe EQ. Further, Hayakawa et al. [2006] have recently found significant VHF radio noises (77 MHz) before the 2004 Niigata Chuetsu EQ. In this study, the direction-finding system by Fukumoto et al. [2002] was working, so that the direction of those VHF radio noises was estimated to be close to the epicentral direction. Further, Yonaiguchi et al. [2007a] have adopted the fractal analysis to identify the seismogenic VHF emissions from other noise sources. [5] In order to convince others that we really observe over-horizon VHF signals and natural VHF radio noises as the precursor to an EQ and also in order to have better understanding on the mechanism how the atmosphere is perturbed in possible association with an EQ and how such VHF radio noises are generated, it is highly required for us to locate those sources; that is, the place where the over-horizon VHF transmitter signal is reflected or scattered and also where such natural VHF noises are generated. Our previous direction-finding system by Fukumoto et al. [2001, 2002] had this purpose, on the basis of a combination of Yagi antennas distributed in azimuth and in elevation. This system was based on the radiation pattern of Yagi receiving antennas, so that we are afraid of having some effects including the antenna height problem (effect of ground) etc. The location accuracy should be improved, which is the reason why we have proposed the use of an interferometric direction-finding system. Some significant results from this system are presented here, in order to convince you how important this kind of interferometric observation is in our VHF study. 2. A New Interferometric System [6] Our former direction-finding system by Fukumoto et al. [2001, 2002] was based on the radiation patterns of receiving antennas (Yagi antennas) in which we estimate the azimuth of the over-horizon VHF signal by means of the radiation patterns of a few Yagi antennas distributed in azimuth, while the elevation (incident) angle is inferred with the use of radiation patterns of the same Yagi antennas directed to different elevation angles (horizontal, 45 and vertical). This direction-finding system may be affected, to some extent, by the ground level, which seems to be a problem of measurement accuracy. An interferometric direction-finding system is based on the measurement of time delays among the three antennas, instead of the measurement of radiation patterns of receiving antennas in the above system. So, the measurement accuracy is, in principle, more accurate than our former system, and this is the reason why we adopt the interferometer. Figure 1 illustrates our interferometric antenna system used. The use of three sets of Yagi antenna in Figure 1 has enabled us to determine the azimuthal and incident angles of the VHF signals with the principle of an interferometer with measuring the time delays between two pairs of antennas at a particular frequency. We have performed the preliminary experiment for the nearby known VHF transmitters, and we have found that the accuracy in estimating the azimuth is only a few degrees (2 3 ), being enough for our seismogenic study. See the work by Y. Yasuda et al. (Interferometric observation of over-horizon VHF signals, submitted to IEICE Transactions on Fundamentals of Electronics, Communications and Computer Sciences, 2009) for the technical details of this interferometric 2of10

3 Table 1. EQs Selected for Our Study Name Date Epicenter a Magnitude Depth (km) Offshore Ibaraki prefecture EQ 23 Jun N, E Niigata Chuetsu-oki EQ 16 Jul N, E July 2007 EQ 21 Jul N, E a Given in geographic coordinates. system, but some important points are mentioned here. The information on signal waveforms is utilized in this new system, we have increased S/N ratio with the use of digital filters, and we use the software computations of direction of arrival. 3. Observation With the Interferometer and Some Observational Results [7] We have started the extensive interferometric observation with a target of the VHF transmitter (FM Sendai) working at the frequency of 77.1 MHz since June This frequency is also used in the area of Tokyo as Open University, so that the reception of FM Sendai is only possible during midnight, because Open University is not working at night. During the period since June 2007, we had three significant EQs around or near our observing station (our university in Chofu). Table 1 indicates the characteristics of those three EQs (including name, place, magnitude, and depth), and Figure 2 illustrates the relative locations of the transmitter and a receiver in Chofu. We will present the results from the interferometric direction finding for those EQs one by one EQ on 23 June 2007 (Offshore of Ibaraki Prefecture) [8] The EQ epicenter was located offshore of the Ibaraki prefecture as seen from Table 1. The magnitude was not so big, 4.2 and the depth is 53 km. However, the EQ intensity was rather big, the biggest being about 4 in Ibaraki prefecture. The temporal evolution of electric field intensity at 77.1 MHz observed at Chofu is illustrated in Figure 3, in which you find a different time internal for each day, reflecting the different duration of FM Sendai observation on different days. Figure 3 indicates some precursory signatures on 17, 18, 19 and 20 June (that is, about 7 4 days before the earthquake). The electric field intensity is found to be in a range from 1.5 mv/m to 4 mv/m, with the background intensity of 1 mv/m. In addition to the interferometric observation, we monitor the sound of the signal in order to check whether the signal received is attributed to FM Sendai or not. [9] The elevation angle of all VHF signals inferred from this interferometer is very small, around 10, so that we will pay more attention to the azimuth information. Figure 4 illustrates the interferometric direction-finding results (only in the azimuth) (about every 1 min) as a function of VHF signal intensity during the whole period from 17 to 20 June Figure 4 suggests a few important findings. First, when the observed electric field intensity is rather enhanced above mv/m, the occurrence distribution in azimuth is extremely concentrated to a value + a few degrees (+ in the ordinate in Figure 4 means east of the path from Chofu (Rx) to the FM Sendai (Tx), while - means west of the path). In these intensity ranges, we have confirmed by means of aural monitoring of the received signals that the observed Figure 2. Relative locations of the VHF transmitter (FM Sendai) (Tx) at Sendai and the receiver (Rx) in Chofu. 3of10

4 Figure 3. Chofu. Temporal evolution of electric field intensity (77.1 MHz) in June 2007 observed at signals are all definitely over-horizon signals from the transmitter of FM Sendai. Whereas, the occurrence distribution in azimuth is found to be widely distributed in a wide range from +30 (30 east of the path) to 0 (nearly along the great circle path) (though some azimuths are rarely distributed in a range from 0 to 30 ) in the electric field intensity region up to 1.5 mv/m. The aural monitoring for the signals in this intensity region has indicated that these signals are not the transmitter signals, but they are presumably natural electromagnetic noise. In the medium field strength region in a range from 1.5 to 2.0 mv/m, the occurrence distribution in azimuth is a kind of combination of two distributions below 1.5 mv and above 2.0 mv. That is, we have found from the aural monitoring that some signals are FM Sendai, and others, not FM Sendai, but natural noises. [10] Figure 5 illustrates the polar diagram of the occurrence distribution in azimuth of the VHF signals observed for this event, on the basis of Figure 4. The whole plot of azimuthal directions for all signals in the whole range of electric field, is plotted. It is seen from Figure 5 that we notice two main maxima (or peaks) in the azimuth distribution. The left peak whose main azimuth is close to the great circle path, is found to be composed of higher-intensity signals, i.e., over-horizon VHF signals, while the right peak in Figure 5 is composed of lower-intensity signals (i.e., natural VHF radio emissions). It seems that natural VHF radio noises are Figure 4. Occurrence distribution in azimuth as a function of electric field intensity during the signal enhancement on 3 days from 17 to 20 June. 4of10

5 Figure 5. Occurrence distribution in azimuth. Two significant peaks are present: one (over-horizon VHF transmitter signal) is close to Sendai, and the other (natural noises) is close to the epicentral direction. generated relatively close to the epicentral direction, while the over-horizon VHF transmitter signals are scattered away from the epicentral direction Niigata Chuetsu-oki EQ (16 July 2007) [11] This is a rather huge EQ with magnitude of 6.8, and Figure 6 illustrates the temporal evolution of electric field intensity at 77.1 MHz at Chofu for this EQ (to be more exact, before this EQ). We observe a significant enhancement on 9 and 10 July (8 and 7 days before this EQ) and an additional enhancement on 14 July (3 days before this EQ). The similar analysis as done in Figure 4, has been performed by using separately the data for the above two peaks with sufficient signal strength, and the corresponding result only for the former peak on 9 and 10 July, is shown in Figure 6. Because the second peak on 14 July will be found to be related with the next EQ to be treated in section 3.3. Nearly the same characteristics of the relation of signal azimuth with signal intensity as for the previous event, has been confirmed in Figure 7. That is, the signals with a higher intensity than 1.5 mv/m are found to be exactly the over-horizon VHF transmitter signal on the basis of the aural monitoring. Also, the azimuthal direction is a few degrees east of the great circle path, and the concentration in azimuth is very sharp. Whereas, the occurrence distribution in azimuth of the VHF signals at lower amplitudes (less than 1 mv/m) is widely scattered, but the main part is distributed to the west of the path ( ). This signal is found to be natural electromagnetic emissions. In between (E = 1 mv/m 10.5mV/m) the noise is a combination of the over-horizon VHF transmitter signal and natural noise, as judged by the aural monitoring. Figure 8 illustrates the polar map of the azimuth of observed signals for this event on the basis of the interferometric system. The over-horizon signals are observed nearly in the direction of great circle path, but the natural VHF noises are seen to be propagated relatively close to the epicentral direction EQ on 21 July 2007 [12] Let us look at Figure 6. When we monitor only the intensity, it is difficult for us to distinguish the precursory VHF signature to this EQ. However, the use of our newly developed interferometer has enabled us to distinguish between the peak on 9 and 10 July and another peak on 14 July The first intense peak is already confirmed Figure 6. Temporal evolution of electric field intensity in July 2007 (5 16 July) at Chofu. Two peaks are found: one on 9 and 10 July and the other on 14 July. 5of10

6 Figure 7. Occurrence distribution in azimuth for the signals observed at Chofu on 9 and 10 July as a function of electric field intensity. to be associated with the former EQ in section 3.2. The second peak on 14 July in Figure 6 is estimated to be a precursor to this 21 July EQ because of the following completely different azimuthal characteristics. The date of 14 July is 7 days before this EQ. Below we are ready to show that this anomaly on 14 July is the signature of this EQ. We show, in Figure 9, the azimuth characteristics of VHF signal intensity. Again for a higher intensity than 1.5 mv/m, the azimuth of the noises is found to be sharply concentrated to a few degrees east of the path. Also, these noises are definitely identified to be over-horizon VHF transmitter signal by means of aural monitoring. While, the signal below 1.0 mv/m are found to have a scattered azimuth distribution, but the main direction is about east of the path. These noises are found to be natural noise. In a range between 1.0 mv/ m and 1.4 mv/m, there exists a combination of two different kinds (over-horizon transmitter signal and natural noises). [13] Figure 10 illustrates the polar plot of azimuthal directions of VHF signals received at 77.1 MHz. Two peaks (or two different kinds of noises) are clearly identified. That is, we can notice two preferred directions in Figure 10. The right one (natural noises) is directed close to the epicentral direction, while the azimuth for the left maximum (over-horizon FM Sendai) is close to the great circle path. 4. Summary and Discussion [14] In order to have further understanding of overhorizon VHF transmitter signals and VHF natural electromagnetic noises, we have developed, for the first time, an interferometric system which enables us to estimate the direction of arrival (azimuth and incident angle) of those received waves. The details can be found in a technical paper by Yasuda et al. (submitted manuscript, 2009). [15] The present paper presents the first results from this interferometric VHF direction-finding system. Three Figure 8. Polar plot of the occurrence number of azimuths of the signals observed at Chofu on 9 and 10 July. Shown are two peaks: one dominant one (overhorizon FM Sendai) and another relatively close to the epicentral direction. 6of10

7 Figure 9. Occurrence distribution in azimuth for the signals observed in Chofu on 14 July as a function of electric field intensity. tentative event studies have been performed in this paper, including one huge EQ of Niigata Chuetsu-oki EQ on 16 July 2007 and two moderate ones. [16] The most important advantage of the use of our interferometer is that we can distinguish the noise bursts easily by studying the azimuth data of the directionfinding results. For example, when we observe only the electric field intensity changes as in Figure 6, we cannot say anything about whether one peak on 9 and 10 July and another one on 14 July are associated with one particular EQ or each burst corresponds to a different EQ. With the additional information on the azimuths of these two peaks, we have succeeded in identifying the first peak as being related to the Niigata Chuetsu-oki EQ and the second peak as related to the 21 July EQ taken place at a different position. [17] First we summarize the characteristics of overhorizon VHF transmitter signals on the basis of the three event studies. [18] 1. When we observe an enhancement in VHF electric field intensity, the signals with a higher intensity are identified as over-horizon VHF transmitter signals on the basis of the aural monitoring. [19] 2. The interferometric direction finding for those signals with a higher intensity indicates that the elevation angle is just around 10 and the azimuthal angle is seen to deviate a lot from the epicentral direction. For example, the azimuths for all three EQ studied in this paper, are close to the great circle path (from Chofu to the transmitter at Sendai). [20] 3. Those anomalies take place about one week before the EQs. [21] Next, we summarize the observational properties of natural VHF radio noises associated with EQs as follows. [22] 1. When we observe the above over-horizon VHF transmitter signals with relatively high intensity, we simultaneously detect the VHF electromagnetic radio noises with smaller intensity just above the background level. [23] 2. Correspondingly these natural noises are found to take place again about one week before the EQ. [24] 3. The interferometric result indicates that the elevation angle of these noises is very small (]10 ) Figure 10. Polar plot of signal azimuth for the peak on 14 July. There are two peaks as in Figure 8. 7of10

8 and their azimuthal direction is found to be considerably close to the epicentral direction, which is very different from the result for the over-horizon VHF transmitter signal. [25] We now discuss these new findings by our new interferometric observation with reference to previous works. First of all, our interferometric observations have indicated small elevation angles of rather intense overhorizon VHF transmitter signals, which is consistent with the result by our former simpler direction-finding system of Fukumoto et al. [2001, 2002]. This is strongly indicating that the region of scattering (or reflection) for over-horizon VHF transmitter signals, is definitely located in the atmosphere (not in the ionosphere). And, the occurrence time of reception of such over-horizon VHF transmitter signals is found to be about one week before the EQ, which is consistent with previous works by Fukumoto et al. [2001, 2002], Kushida and Kushida [2002], and Fujiwara et al. [2004] based on statistical data. The former result by Fukumoto et al. [2001, 2002] based on the simpler direction-finding system, indicated that the region of seismoatmospheric perturbation is relatively away from the direction of the future EQ epicenter, but that the seismoatmospheric regions tend to be close to the land area. This means that seismoatmospheric perturbations are likely to be located not in the sea (ocean), but in the land area, which seems to be likely to be related with the generation of those seismoatmospheric perturbations. Recently Hayakawa et al. [2007] have proposed a generation mechanism of such seismoatmospheric perturbation on the basis of the possible changes in geochemical quantities associated with EQs, leading to the tropospheric propagation anomaly. Their analysis is indicative of a possible relation between the abnormal phenomena of the reception of over-horizon VHF signals and EQ preparation stage. One can suppose the following scenario of the phenomena in a seismically active region. A variation of tectonic stress in the vicinity of faults results in the gradual groundwater squeezing out from the higher depths toward the Earth s surface. The warmed groundwater filtrates upward from the higher depth to the Earth s surface to provide a weak increase in the ground temperature similar to that detected before an EQ [Gorny et al., 1998; Tronin, 1999; Qiang et al., 1999; Tramutoli et al., 2001; Tronin et al., 2002]. Additionally, there may be an increase in the contents of water vapor, CO 2,CH 4, and other optically active gases, which, in turn, leads to the local greenhouse effect followed by the ground temperature increase [Tronin, 1999]. The lead time of about one week of abnormal over-horizon VHF signal, seems to be consistent with these satellite results on the ground temperature measurement. If the favorable meteorological conditions occur in the near-surface layer over seismically active regions, it can produce the air humidity and temperature inversion followed by the abnormal variations of the air refractive index. Such meteorological conditions can persist in the vicinity of a coastal line. This seems to be consistent with the second observational fact in section 3. Eventually it can lead to the radio ducting and over-horizon VHF wave propagation [Kushida and Kushida, 1998; Fukumoto et al., 2002; Fujiwara et al., 2004; Yonaiguchi et al., 2007b]. The direction-finding results for all of these three event studies, indicate that the direction of scattering for over-horizon VHF signals is just around the great circle path. This direction is acceptable as a combined effect of the physical consequence of seismoatmospheric perturbation over the land and the preferred direction in scattering problem. As the result, we can regard that the seismoatmospheric perturbation is extended, at least, to the path determined by the direction finding, so that the radius of seismoatmospheric perturbation is more than 100 km or so. [26] Next, we discuss natural VHF radio noises associated with EQs. While, the VHF natural noises with smaller amplitude, but above the background, are found to have their azimuths relatively close to the epicentral direction. This conclusion is in an excellent agreement with our recent finding for a recent Japanese big EQ named Niigata Chuetsu EQ [Hayakawa et al., 2006]. These VHF natural electromagnetic noises are likely to be generated relatively close to the epicenter, with a possible radius of the order of 50 km or so. This relatively small area of VHF noise generation, would enable us to locate the position of a future EQ epicenter, by means of triangulation from a few observing sites. The lead time of about one week of these less intense VHF electromagnetic noises behind the EQ is consistent with the result by Hayakawa et al. [2006]. How about the generation mechanism of those natural emissions? As compared with the generation of much lower frequency emissions such as ULF emissions, higher-frequency waves (such as VHF) are easily generated because of a much smaller wavelength. However, the generation mechanism is poorly understood. The essential one difference of this VHF noise with respect to the overhorizon VHF transmitter signals, is that these VHF natural noises tend to be relatively close to the epicenter, unlike the considerably large dimension of seismoassociated atmospheric perturbation as seen from over-horizon VHF transmitter signals. They take place nearly at the same time, or simultaneously, which might indicate that the fundamental origin of these two phenomena is in common, probably geochemical effects. Hayakawa et al. [2002] suggested the following scenario. We might expect some modification in the lower atmosphere because of seismicity. Quasi-static electric fields decrease in the regions with enhanced air conductivity because of any preseismic gas and water releases in the atmosphere. Then, the electric field becomes reduced in the lower 8of10

9 troposphere, and thereby the probability decreases of the cloud-to-ground strokes in such a contaminated area. Simultaneously, the electric field grows inside and above the thunderclouds, and hence the number of horizontal and tilted intercloud (or intracloud) discharges will grow, resulting in the higher-frequency radio noises such as in VHF. It is interesting to note in this connection that some strange phenomena such as a sheet lightning, luminescence of mountain tips, etc. were occasionally observed prior to and during strong EQs [Finkelstein and Powell, 1970; Derr, 1973; Hedervari and Noszticzius, 1985]. [27] Acknowledgments. This research is partially supported by NiCT as R&D promotion scheme funding international joint research, to which we are grateful. References Derr, J. S. (1973), Earthquake lights: A review of observations and present theories, Bull. Seismol. Soc. Am., 63, Enomoto, Y., and H. 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10 Tsutsumi, A, Y. Enomoto, and H. Hashimoto (1999), Relationship between geo-electric change signals and meteorological lightning, in Atmospheric and Ionospheric Electromagnetic Phenomena Associated With Earthquakes, edited by M. Hayakawa, pp , TERRAPUB, Tokyo. Yonaiguchi, N., Y. Ida, M. Hayakawa, and S. Masuda (2007a), Fractal analysis for VHF electromagnetic noises and the identification of preseismic signature of an earthquake, J. Atmos. Sol. Terr. Phys., 69, , doi: / j.jastp Yonaiguchi, N., Y. Ida, and M. Hayakawa (2007b), On the statistical correlation of over-horizon VHF signals with meteorological radio ducting and seismicity, J. Atmos. Sol. Terr. Phys., 69, , doi: /j.jastp T. Goto, M. Hayakawa, Y. Ida, and Y. Yasuda, Research Station on Seismo Electromagnetics, University of Electro- Communications, Chofu, Tokyo , Japan. whistler.ee.uec.ac.jp) 10 of 10