Precursory effects in the subionospheric VLF signals for the Kobe earthquake

Physics of the Earth and Planetary Interiors 105 Ž1998. 239–248

Precursory effects in the subionospheric VLF signals for the Kobe earthquake O.A. Molchanov

a,)

, M. Hayakawa b, T. Oudoh c , E. Kawai

d

a

Institute of Physics of the Earth, Moscow, Russian Federation The UniÕersity of Electro-Communications, Chofu, Tokyo, Japan c Communication Research Laboratory, Koganei, Tokyo, Japan Communication Research Laboratory, Inubo Radio ObserÕatory, Chosi, Chiba, Japan b

d

Received 7 January 1996; received in revised form 11 July 1996; accepted 15 July 1996

Abstract The subionospheric VLF Omega signal transmitted from Tsushima, Japan Žgeographic coordinates: 34837X N, 129827X E. was continuously received at Inubo Ž35842X N, 140852X E.. This data was analyzed during an 8-month period centered on the great Hyogo-ken Nambu ŽKobe. earthquake, Mg s 7.2 on 17 January 1995, the epicenter located inside of Fresnel zone of the VLF path. To clarify the possible effect, we developed the special TT Žterminator time. method of data processing, which was useful for our short VLF path Ždistance ; 1000 km.. We discovered a statistically significant change of TT characteristics, which began a few days before the main shock and probably continued a few weeks after it as a transient oscillation with period ; 10 days. By simple modelling, it was shown that TT changes could be caused by the decrease of the VLF reflection height by ; 2 km. The possible underlying mechanisms of the effect are not defined; however, an increase of the regular electric field due to radon exhalation before the earthquake or an intensification of planetary waves by seismically influenced atmospheric turbulence might be considered. q 1998 Elsevier Science B.V. Keywords: VLF omega signal; Subionosphere; Kobe; Earthquake prediction

1. Introduction The possibility of earthquake prediction is based on the supposition on the existence of a precursor. Since ancient times, many earthquake precursors have been suggested. However, it was usually difficult to estimate the degree of coincidence in these events. )

Corresponding author. Earth Observation Research Center, National Space Development Agency of Japan, 1-9-9 Roppongi, Minato-ku, Tokyo 106, Japan.

Up to this time, no reliable precursor has been found. Recently, in relation with the great earthquake that happened near the city of Kobe in Japan on 17 January 1995 Ž5 h 46 min AM, L.T.., Mg s 7.2, whose epicenter was located at Ž34.68N, 135.08E., we checked the possibility of making use of radiophysical measurements ŽHayakawa and Fujinawa, 1994.. First, we investigated the VLF signal method, in which phase and amplitude of radio signals from VLF navigational transmitter propagating inside the earth-ionosphere waveguide are monitored. If trans-

0031-9201r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 3 1 - 9 2 0 1 Ž 9 7 . 0 0 0 9 5 - 2

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O.A. MolchanoÕ et al.r Physics of the Earth and Planetary Interiors 105 (1998) 239–248

mitter frequency and receiver distance are fixed, then the observed VLF signal parameters are mainly dependent upon the position of the reflection height h, which depends on the profile of electron density in the D-layer of the ionosphere. Therefore, the VLF signal method has become the standard for recording

short-time electron density variations in the lower ionosphere connected with solar radiation Že.g., Roentgen flares., cosmic rays ŽForbusch effect., energetic particle precipitation Že.g., Wait, 1970; Galejs, 1972., whistler-induced precipitation ŽHelliwell et al., 1973 . and lightning-induced ionization

Fig. 1. Location of transmitter ŽOmega, Japan. at Tsushima and receiving station at Inubo is shown. The line connecting these two stations is the VLF wave path, and the epicenter of the Kobe earthquake Žindicated by a cross. is about 70 km from the path. The zone of VLF signal sensitivity to perturbations in the medium during propagation ŽFresnel zone. is shown by a dashed line.

O.A. MolchanoÕ et al.r Physics of the Earth and Planetary Interiors 105 (1998) 239–248

ŽArmstrong, 1983; Inan et al., 1988., ionosphere modification by HF transmitter ŽBarr et al., 1985. and, of course, atmospheric nuclear tests ŽWait, 1964.. Recently, a suggestion to use this method for searching earthquake precursory activity was made. Gokhberg et al. Ž1989. first reported an abnormal precursory influence of earthquakes upon subiono-

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spheric VLF propagation and suggested this as a possible method of earthquake prediction. Later, Russian ŽGufeld et al., 1994. and Japanese ŽHayakawa and Sato, 1994. colleagues accumulated more evidence on anomalies in subionospheric propagation associated with earthquakes. They analyzed deviations from nighttime monthly averages of sig-

Fig. 2. Running standard deviation of evening phase TT differences during two months from December 1, 1994 to January 31, 1995 is shown together with variations of Ý K magnetic index, solar radiation index S10.7 and rainfall index R in the Kobe area Žrelated to perturbations in the atmospheric electric field.. Abnormal behavior of each parameter is emphasized Žqualitatively. by shading. R index is counted in terms of rain probability, averaging the events on four-quarter intervals a day and two cities near Kobe. This shows that we have no candidate other than the earthquake that could be the cause of the abnormal VLF signal behavior with such sharp commencement a few days before the great Kobe earthquake.

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nal phase and found that the phase differences increased over a period from about one month to a few days ahead of the earthquake.

2. Method of data analyses and main effect We considered VLF data received at Inubo Žnear Tokyo. Žgeographic coordinates: 35842X N 140852X E. and transmitted from ‘Omega’, Japan ŽTsushima, 34837X N, 129827X E.. The relative location of the transmitter and receiving station is given in Fig. 1, together with the great circle path between them and the boundary of the first Fresnel zone Ždashed line.. This Fresnel zone is an elliptical area for which the VLF transmitter and receiver are foci ŽWait, 1964. and the value of the minor semiaxis b s w l Dr2Ž1 q

4 h 2rD 2 .1r2 y h2 x1r2 , 100 km, where, in our case, the distance D , 1043 km, wavelength l s 29.4 km and h , 80 km. The epicenter of the Kobe earthquake is indicated by a cross and is about 70 km from the VLF signal path. It is well inside of the first Fresnel zone, which means that seismogenic perturbations of atmosphere and lower ionosphere could influence the VLF signal. We examined data of signal phase and amplitude at the frequency of 10.2 kHz and for phase only at 11.3 kHz over the time period about four months either side of the earthquake. We suspected that this eight-month period would be sufficient for a statistical study because the usual time scale of reported electromagnetic precursors associated with earthquake is a few days or weeks ŽRikitake, 1976., and the maximum time scale of natural Žnonseismic. perturbations on the upper

Fig. 3. Sequential plots of the diurnal variation in VLF signal phase at Inubo Ž f s 10.2 kHz.. The whole scope of registration is 100 centicycles. The definitions of t m and te are given as the times where a minimum in phase takes place around sunrise and sunset, respectively. The value of phase at the phase minimum is defined as Nph . It is seen that abnormal behavior in the diurnal variation begins a few days before the earthquake Ž17 January is the earthquake date indicated by a star. such that we have the lengthening of daytime conditions that is found to last for a few days even after the earthquake.

O.A. MolchanoÕ et al.r Physics of the Earth and Planetary Interiors 105 (1998) 239–248

atmosphere and ionosphere, connected with magnetic storms, lightning activity, solar radiation, precipitation of energetic particles, etc. is also of the order of a few days Žsee also Fig. 2 and Section 4.. The previous works ŽGokhberg et al., 1989; Gufeld et al., 1994; Hayakawa and Sato, 1994. mentioned above have dealt with subionospheric VLF propagation over a long distance Žmore than a few thousand kilometers., but the distance involved here between Thushima and Inubo is only about 1000 km, which can be considered as short-distance propagation. Furthermore, in the case of earthquake influence, we expected to find a long-time VLF signal variation Žwith period more than one day., unlike usual shorttime variations with time scales of hours or minutes. Our initial estimations showed that it is more reliable in a statistical study to examine the deviations of terminator time t m , te instead of Žor together with. conventional deviations of phase Nph,m , Nph,e . The meaning of these parameters is shown in Fig. 3. The terminator points, where the behavior of phase Žand amplitude. has a characteristic minimum, are easily defined two times a day and the time accuracy of their determination is about 6 min. Note from the specially selected sequence of daily phase variation presented in Fig. 3 that parameter t m is decreased and te clearly increased a few days before the earthquake time. It seemed to us that this effect might have been connected with the earthquake, but that it is needed to be proven by a statistical examination.

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3. Statistical results The monthly-averaged variations of terminator times ² t m :, ² te : are presented in Fig. 4 together with local times of sunrise t mo and sunset teo near the end of the VLF path Žat Tokyo.. It is not surprising that the variation in terminator times correlates with sunrisersunset times, but with ‘sunrise’ in VLF signal behavior occurring a little later, D t m s ² t m : y t mo ) 0, and VLF ‘sunset’ happening earlier, D te s ² te : y te0 - 0. It is worthwhile to note that < D te < D t m both for phase and amplitude variations, and it is very difficult to find any earthquake signals in these characteristics. However, variations are clearly observed in the TT Žterminator time. differences of phase and amplitude d te s te y ² te :, which are presented in Fig. 5. To estimate the statistical importance of these deviations, we calculated the seasonal dispersion of data s s ²Ž te y ² te :. 2 :1r2 , averaging over the whole period of observation, and plot the 2 s level. Both TT differences exceed the 2 s level a few days before the day of the main shock of the earthquake Žshown by a vertical dotted line., and suggested that the relation between the TT spike and the earthquake occurrence was not coincidental. It seems rather probable that the disturbance continues after the main shock and appears here as a transient with a sharp commencement occurring before the earthquake, especially in the amplitude TT plot ŽFig. 5b.. If we suppose that third maximum has been

Fig. 4. The temporal variation of monthly averaged values Žrunning means. of t m and te for phase Žsolid line. and amplitude Žthin line. of the VLF signal during the observation period and for f s 10.2 kHz. The corresponding values of te for f s 11.3 kHz Žphase. are given by the dashed line. The star-lines indicate the times of sunrise and sunset observed at Tokyo. The vertical line marks the time of the Kobe earthquake. We have no data for the period from 5 October to 23 October Žthe same for other figures..

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missed for some reason, then the oscillation period is about 10 days. Results for morning TT phase differences are not so clear, but are usually in anti-phase

with evening ones and show the same 10-day period, as shown in Fig. 6a,b. To clarify this 10-day dependence in our TT data, we found the distribution of

Fig. 5. Temporal evolution of terminator time ŽTT. differences of te Žin hours. from the monthly average values for phase Žpanel a, solid line. and amplitude Žpanel b, solid line.. Experimental uncertainty of the measurements is 0.06 h. The "2 s level Žtwice the standard deviation. is also plotted for the sake of comparison Žthin lines.. We notice a remarkable deviation in the evening terminator time just a few days before the earthquake, well above 2 s . This event probably continues after the shock as transient oscillations with period ; 10 days, especially noticeable on the amplitude plot.

O.A. MolchanoÕ et al.r Physics of the Earth and Planetary Interiors 105 (1998) 239–248

the evening TT variations Ž te . against period T Žin days., averaged over the whole time of analysis Ž8 months.. This normalized spectrum SŽt . is presented in Fig. 6c. Spectral maxima near t s 9–10 days and t s 17–19 days are rather evident and one can also note a hint of a maximum at 5–6 days. The evolution of amplitude of the 10-day TT variations SŽ10. during 4 months around the earthquake date is presented in Fig. 6d. In an attempt to prove that the relationship between the VLF event observed here and the Kobe earthquake is not coincidental, we analyzed the behaviour of well-known influences upon VLF transmitter signals. The results are presented in Fig. 2, where the variations of magnetic indices connected with precipitated ionization of the ionosphere, variation of solar radiation related with photoionization, and variation of rainfall index connected with lightning electric field perturbations are shown. It is

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evident that all of these are not producing the abnormal VLF phenomena observed here. 4. Discussion It is not very easy to provide a perfect explanation of the observed effect. We have carried out a simple computation of subionospheric VLF propagation based upon the formulation in Wait Ž1970.. The observed VLF electric field Ez is as follows: Ez s WE 0

Ž 1.

0

where E is the field in free space and W is an attenuation function connected with the properties of the medium and described as a sum of modes: `

WsB

Ý Cn Sn2 H0Ž2. Ž kSn D . .

Ž 2.

ns0

Here, D is distance, k is the wave number in free

Fig. 6. Ža. Comparative variation of phase TT differences at the evening and morning. Žb. The detailed illustration of the change in phase terminator times Ževening and morning. for January 1995 only. An enhanced deviation in te is observed a few days before the earthquake, with the change in t m found to be in an opposite phase with that in te . Žc. Normalized distribution of spectrum of te variations on period T, star-line for phase variations, solid line for amplitude. Žd. Temporal evolution of SŽ10. amplitude around the date of the Kobe earthquake. Labels the same as in Žc.. This plot was obtained by averaging over "1 month around the time of each point.

O.A. MolchanoÕ et al.r Physics of the Earth and Planetary Interiors 105 (1998) 239–248

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space depending only on the frequency f, B and the product of excitation and height dependent factor Cn are constants, supposing a fixed D and f, H0Ž2. is the Hankel function of second type, and finally: Sn s a n y i

e n D˜ 2 kh

ra n

Ž 3.

where a n s w1 y Žp nrkh. 2 x1r2 and h is height of the reflection point, e n s 1 for n G 1. Lastly, D˜ is a function that is connected with dissipation of VLF energy in the conductive ground and ionospheric

medium. The observational values AŽ t . and w Ž t . are determined by the following relation: Ez s Real  Aeyi w 4

Ž 4.

where A is the amplitude and w is the phase of the VLF signal. A comparison of Eqs. Ž1. and Ž4. leads to equations for A and w . We have supposed the following: Ža. five modes of propagation, from n s 1 Ždominant mode. to n s 5; Žb. dissipation factor D˜ s 0.06, which corresponds to attenuation of the first dominant mode 3.0 dBr1000 km at long dis-

Fig. 7. Computed results of the expected diurnal variations for phase Žthick line. and amplitude Žthin line with small squares.. Times of open circles correspond to te in Fig. 2 and d te is the change of te due to upper atmosphere perturbation. Ža. Diurnal variation of phase and amplitude for normal behavior of the ionospheric height hŽ t .. Žb. Diurnal variation of phase and amplitude for perturbated behavior of ionospheric height h˜ Ž t .. Žc. Supposed hŽ t . Žsolid line. and h˜ Ž t . Žthin line with crosses.. h tp is the value of h near the terminator point.

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tance, f s 10.2 kHz; Žc. the height of VLF wave reflection in the upper atmosphere is 85 km at night and 75 km at day; and Žd. duration of the terminator time change is 2 h. Based on these suppositions, our theoretical results compare favorably with the regular diurnal variation in phase and amplitude, observed experimentally, as shown in Fig. 7a. To obtain the observed changes in the terminator times during the seismically perturbed period, we need only to assume the total decrease in the reflection height by D h ; 2 km as shown in Fig. 7b,c. Of course, our modelling is a rather crude one, not including, e.g., lateral limitation of the perturbation zone in the ionosphere, diurnal changes of dissipative modes Ž n ) 5. at near distance and so on, comparing with more sophisticated calculations Že.g., Poulsen et al., 1993; Baba and Hayakawa, 1995.. Indeed, results of our simple modelling can be considered only as estimations. At present, it is difficult to suggest a plausible physical or chemical mechanism of reflection height decrease and corresponding modification of density and conductivity profile. The simplest idea for these changes might be connected with intensified radioactive radon exhalation before an earthquake, and the resultant increase of the electric field at the upper atmosphere similar to that considered in a theoretical scheme developed by Pierce Ž1976.. Indeed, there have been many papers on intensified radioactive gas observations before earthquakes Že.g., King, 1989; Yamauchi, 1992., and there is a report of increased radon ion density Žby about 10 times. before the Kobe earthquake. Recently, Fuks and Shubova Ž1994. reported a clear change in VLF transmitter signals found just after accidents at large nuclear stations. The change of atmospheric electrical parameters after radioactive fallout from a nuclear plant was reported by Israelsson and Knudsen Ž1986., and by Retalis and Pitta Ž1989.. This could be considered as a simulator of radioactive gas influences upon the atmosphere and lower ionosphere. The other reason could be an intensification of planetary atmospheric waves with a period of about 10 days as it can be seen from Figs. 5 and 6. There are a few reports Že.g., Pancheva et al., 1991; Yi and Chen, 1993; Wu et al., 1994. that these waves could penetrate from the lower atmosphere to the lower ionosphere and have characteristic periods in the

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range of 2, 5, 10 or 16 days. We have shown that our VLF TT variations have about the same period Žsee Fig. 6c. with a dominant 10-day spectral component that is enhanced over the time interval from about 1 month before the earthquake to the time of the main shock ŽFig. 6d.. This enhancement is not obviously related with seismicity and might be a seasonal effect. However, the increase of a transient-like quasi-periodic oscillation created by some shock-type influence a few days before the earthquake seems a reasonable explanation for the TT behavior depicted in Fig. 5a,b. How atmospheric turbulence can lead to quasi-periodic oscillation and especially the shockwave transportation to the lower ionosphere is not clear. Whatever this process might be, we believe that subionospheric VLF propagation can be considered as a rather promising candidate for a short-term earthquake precursor.

Acknowledgements We thank L. Turivnenko for technical assistance and Craig J. Rodger for helpful discussions about the text. The authors greatly appreciate the useful advice of our referees.

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