Ind ian Journal of Rad io & Space Phys ics Vo l. 34. August 2005. pp. 22 I -232

Simultaneous ULFIVLF amplitude anomalies observed during moderate earthquakes in Indian region*

Birbal Singh

Department o f Electron ics & Communica ti on Engineering, Faculty of Engineering & Technology, R B S Co llege. Bichpuri, Agra 283 105, India

and

Vinod Kushwah, Vikram Singh & Manoj Tomar

Department of Phys ics, R B S College. Bichpuri , Agra 283 105, Indi a

and

M Hayakawa

Department of Elec tronics Eng ineering, The Uni versity of Elec tro-Communica ti ons. Chu ru , Tokyo 182 Japan

Received I Jlllle 2004; revised 18 Jalluary 2005; accepted 3 FebruOIJI 2005

A three-component search coil magnetometer to monitor ultra low frequency (ULF) magnet ic fie ld emi ssions (frequency = 0.0 I -30 Hz) and an Abso lute phase and amp litude data logger (AbsPAL) to monitor the phase and amplitude variation of very low frequen cy (VLF) 19.8 kHz fixed frequ ency NWC (A ustra lia) transmitter s ig nals have been installed at Bichpuri , Agra (Geograph Lat. 27.2°N, Long . ?ROE) in India and reg ular s imul laneous observa tions have been taken since I Aug. 2002 . The anal ys is of the nighttime VLF amplitude data has been ca rried out fo r the initial period or nine months between I Aug. 2002 and 30 Apr. 2003 and it is seen that the amplitude decreased abnorma lly by 1- 10 dB fro m normal nighttime daily averages o n six days during the whole period under consideration. The results of ULF data dnalys is show that the background magnetic fi e ld amplitudes of the three components. which are usually low. in the range of 0.03-0.30 nT are enhanced to the range of 0.26-0.96 nT. either on the sa me days or within ± 2 days of the decrease in amplitude of the VLF signal. The possible causes fo r the observed ULFIYLF amplitude anomalies are examined and it is fo und that they are caused by moderate seismi c ac ti viti es that occurred along the VLF propagation path in the reg io n. The statistica l analysis of the data using mean and standard dcv iat io n aro und the mean is carr ied out, which supports the results sa ti sfactoril y.

Keywords: ULFIYLF amplitude, Magnetic fi e ld emi ssio n, Ionosphere, Earthquakes

PACS No: 91.30 Px; 94.20 Yx

1 Introduction Several workers have reported in the past the

examples of seismo-ionospheric perturbations on the phase and amplitude of subionospheric fixed frequency transmitter signals t-3. Recently, some workers have introduced the concept of "termination times", which are defined as the times of sunrise and sunset when the phase (or amplitude) of the received signals exhibits a characteristic mInImUm, and examined the effect of several earthquakes on the propagation characteristics of such signals4

-7

. They have found that the termination times (hereafter IT)

*This paper was presented in session 57-JWG - A 1065 of the first annual meeting of the Asia Oceania Geosc ience Soc iety (AOGS) held at Singapore during 5-9 July, 2004.

showed an anomalous time shift 2-3 days before the occurrence of the earthquakes. They have interpreted the result in terms of lowering of the ionospheric reflection height by about 2 km, possibly by radon emissions or concentration of gravi ty waves during the earthquake process. The above results based on TT method were criticised on the ground that in a realistic propagation model the observed changes in IT would require the ionospheric height to be lowered much more, by 4- ~ 1 km, which can be brought out only under solar flare conditions8

.9

.

However, it has been shown that the TT method is valid for shorter propagation paths t such as that adopted by Molchanov et a/.6

.

The studies of earthquake precursors in ultra low frequency (ULF) bands if = 0.01- 10 Hz) have pro-

222 IND IAN J RADIO & S PACE PHYS. AUGUST 2005

duced more convincing results in the recent past as compared to those in higher frequency bands because of less contamination , large skin depth , and low attenuation II . IG. Further, the ULF emi ss io ns can pene­

trate the crust and pr 0 pagate through the ionosphere and magnetosphere where they may be recorded through sate llite observations I7

-19

.

Fortunately, both the VLF and ULF experiments mentioned above are available at our Bichpuri (Agra)

station (geographica l Lat. 27.2°N, Long. 78°E) and we have been conducting routine observati ons since I Aug. 2002. In the present paper, we report the results of our data analys is for the initi a l period of nine months, where we show that both the VLF and ULF signals undergo amplitude ano mali es occas iona ll y. This result is interpreted in te rms of the effect of moderate se ismi c acti viti es a long the VLF pro pa­gation path.

2 Experimental set-up We have employed AbsPAL (Absolute Phase and

A mplitude Logger) receiver obtained from LPEM Research Ltd, New Zealand to mo nitor the amplitude and phase vari ati on in fixed frequenc y transmitter signals, which has been des igned and fabricated by Dowden and Adams2o. The equipments include GPS anten na, VLF ampli f ier, service unit, DSP card , and necessary software. We use I min average amp litude data in our study .

The ULF magnetic fie ld amplitudes are mo nitored by employ ing a set of 3-componellt search coil magnetometers obtained from Lv iv Centre of Inst itute of Space Research , Ukra ine. The three sensors of the magnetometer are buried I m underground in orthogonal directions, the X-com ponent being in north -south direction. The amplified sig nals from the magnetometer are carri ed to the communicatio n unit through high quality shie lded cables . A GPS antenna is used for time synchroni zati on. T he mag netomete r works fo r the frequency band 0.01-30 Hz.

The above two experiments are running at Bichpuri , a rural area, located about ) 2 km wes t of Agra city, where loca l clcctr-ic and e lectro-magneti c disturbances are low.

3 Earthquake and magnetic storm data The earthquake data used in thi s work have been

taken from United States Geological Survey (USGS). In selecting the earthquakes for the present study, attent ion has been given to only moderate and large magnitude earthquakes (M>4), low depth , and

location s along the VLF great circle path between the NWC transmi l ling s tatio n in Australia and Agra station in Indi a.

The effec t o f magne tic storms has been examined in te rms of LKp, where Kp is 3-hour planetary index. It is well known that the values of Kp in the ranges of 0-4, 5-6, and 6-9 indicate quiet, moderate, and severe magnetic conditi o ns, respecti ve ly. In te rms of LKp, a value greater than 30 indicates severe magnetic sto rms.

4 Results and discussion We have been mo nitoring the amplitude and phase

of 19.8 kH z NWC fixed frequency transmitter signal s at Agra stat ion since 1 Aug. 2002. In Fig. I we show the great c irc le path between the transmi tter and the receiver stat io n (propagation path = 6735 km) and in Fig. 2 we show a sche matic diagram of the propa­gatio n path tlu'ough the earth-i onosphere waveguide between tran smitter and receiver and poss ible mode of intluence by an earthquake OCCUlTing in the lithosphere (Case I). A poss ible impact of the earthquake is to bring out a structura l change in the lower io nosphere, which may in turn affect the amplitude and phase of the propagating VLF signal s . We have analysed the nighttime amplitude data

'_~.r .

1350E

LONGITUDE

Fi g. I---G reat C irc le pa th between the transmitte r (N WC) in Austra li a and receiver a t Agra. in Indi a [The c ircled num bers ind icate the locations o f earthquakes (see Table I) a long the propaga ti on path .]

SINGH el al. : ULFIYLF AMPLITUDE ANOMALY DURING EARTHQUAKE 223

SEISMC)-ANOMAlY IN IONIZATION LOWER IONOSPHERE BOUNDARY

CASE (1) FIXED FREQUENCY VLF TRANSMITIER

/'''},TTTT77"",,~~TrTTTT77,7~~· A~US~T_rF~rTT

~

UlF MAGNETIC FIELD SENSORS

CASE (2)

EARTH

• EARTHQUAKE

Fig. 2- The schematic diagram shows the propagation path th rough earth-ionosphere waveguide between transmitter and receiver and possible mode of influence by an earthquake occurring in the lithosphere (case I) and the propagation path of ULF signals generated from earthquake region to search coil magnetometer (case 2)

(2200h-0400h LT, L T=UT +5 .5h) for the period of nine months between August 2002 and April 2003. The purpose of choosing the nighttime data will be explained later. We have averaged the nighttime data cOITesponding to the above period using MA TLAB software and examined the variation in day-to-day data carefully. It is found that on six occasions over the whole period of analysis the amplitude decreased by 1-10 dB from the average daily variation . These identified days of amplitude decreases are 27 Aug., 17 Sep., 10 Dec. 2002 and 22 Jan. , 16 Feb. and 5 Apr. 2003.

The propagation path of ULF signals generated from earthquake region to search coil magnetometer is shown in Fig. 2 by a big arrow (Case 2). We have also carried out an analysis of the ULF magnetic field data obtained from search coil magnetometer for the period mentioned above. Unfortunately, the ULF experiment was started from 12 Sep. 2002, hence the ULF data are not available for the whole period of nine months but for eight months only. Therefore, we analyse here the data for the period between 12 Sep. 2002 and 30 Apr. 2003 The magnetic field amplitudes of the three components are usually low, between 0.03 and 0.30 nT, the X-component being the lowest, but we find that the amplitudes are enhanced to the range of 0.26-0.96 nT either on the same days or within ±2 days of those on which the VLF amplitudes are decreased. The ULF amplitudes are found to be enhanced on 18 Sep., 10 Dec. 2002 and 24 Jan., 18 Feb. and 02 Apr. in the year 2003.

Two examples of such amplitude enhancements are shown in Figs 3 and 4. In Fig. 3 we show the Bx> By, Bz components on three days between 17 Sep. and 20 Sep. 2002. The dark horizontal bands in the three

panels show the background magnetic field ampl i­tudes, whereas the vertical fringe type amplitude enhancement are due to local noise. Since, the data presented in Fig.3 correspond to the starting period of observations, they are not continuous but random, i.e. the data in the top panel is for a very small period of less than one hour between 2338 hrs and 2359 hrs L T, whereas those in the middle and bottom panels are from 2300 hrs to 0530 hrs, and for day hours between 1511 hrs and 1524 hrs respectively. It may be seen from the data in the top panel that magnetic field amplitudes of three components are very small for few minutes in the starting of the observations, but the Y - and Z-components are increased thereafter to the values of 0.56 nT and 0.61 nT, respectively .

The same amplitudes continue on the next day also as shown in the middle panel (on 18 Sep. 2002) but they appear as pearl type, because of modulation by an unknown signal at 26 Hz, possibly a dominant mode related to Schumann resonance (A.P. Nickolaenko, University of Electro Communication, Chofu, Japan, personal communication, October, 2003) or the subharmonic of power line radiation around 50 Hz. However, the amplitudes return to normal on 20 Sep. 2002 (the data on ] 9 Sep. 2002 is not available) . In Fig. 4, the data corresponding to nighttime observations on three days between 9 and 12 Dec. 2002 (the data on 11 Sep. 2002 is not available) are presented. From a glance at the three panels we find a larger enhancement in Y -component (amplitude of Y-component = 0.86 nT, amplitude of Z-component = 0.61 nT) on 10 Dec. 2002 in comparison to all other components on the rest of the two days. Similar enhancements in the ULF amplitudes and then return to normal on subsequent days are obtained in other cases also.

In Fig. 5 (Top) we show the dynamic spectra (amplitude versus time) of the enhanced Y­component corresponding to 10 Dec. 2002 shown in Fig. 4. In the middle and bottom panels we show the frequency-time spectra and power spectra of the same data, respectively. From the later two panels it is clear that in addition to the presence of a ULF signal at 1.25 Hz, there is another signal at 26 Hz, a reference to which has already been made above. However, we do not see a pearl type structure in this spectra, because of the signal at 26 Hz being weak (-10 dB) as compared to that in the case of Fig. 3, where it is much stronger (5 dB). The insignificant effect of this signal can be seen from Fig. 6, where we

224

~ ;;. ~ i .fTl ~ ..., en r ~

~

~ 8

INDIAN J RADIO & SPACE PHYS, AUGUST 2005

- I----:::~.:.

'-; -

; .

"'F---~- ---

- ~.

- I-

"'-=:-=::.... ----

AMPLITUDE, nT

-.;;. ' - ---- 1 ...,.." --

- --= r==--...:. .=-.-- _.

-""" -.-

;~ ~ - ':::-'

--.-,~,;=.--

-. !'- ' -

= ~ . . -=--.:-....-:::::: F-

- --=, -.:--- ,....

--=:F :.- _ -

- - F

-- 1-'-----~=--

~ t-:: __

--- -- -

~ :! ~ o N

Fig. 3--The X. Y. Z components of ULF emissions on 17, 18 and 20 Sep. 2002 [The data shows enhancement in amplitudes from 2340 LT, which continued on the next day (the day of the earthquake). The amplitude is normal on 20 Sep_ 2002 (the data for 19 September is not avai lable).]

'.~

8 8 8

S g 8

~ g 6

8

SINGH et lIl.: ULFIYLF AMPLITUDE ANOMALY DURING EARTHQUAKE

AMPLITUDE, nT

:. 0 - ... ... ..

II

8

~ ~ ~ ~ 8 2

U 0 . " " . . ~ . ~ " . .

Fig. 4--The same as Fig. 3 but for the days between 9 and 12 Dec. 2002 [the data for II December is not available]

225

226 INDIAN J RADIO & SPACE PHYS, AUGUST 2005

t-<:

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N I

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Lci m i30i 10. ' 202

TIME. hrs LT

L ami30. 10 12. 02

TIME X 10' s

LlllmOOi 10.12.02 5 r---~----~----r---~-----===~

o ---- .... ... ...... _-_ ..

-5 - --------- j----------:---.-.--- -:- -------- .-- ---'- 10 . _ •.. _.- . . : . .•. . ---- .-~ . -.---.- . -.j - ..... __ ._. i" --- ---- --1 ... _ .....

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a.. ~ -25 -~ ----. --- .--: . _ .... -----. - -- --.. _ .. -.. _.--_ .. . : .. . -_. -_ ..

FREQUENCY. Hz

Fig. 5----{Top) The dynamic spectra (amplitude versus time) of the enhanced Y-component, (middle) the frequency-time spectra and (bottom) the power spectra corresponding to 10 Dec. 2002

show a low pass filtered data from which the signal at 26 Hz is eliminated. The dynamic spectra in the top panel remain the same except a minor reduction in amplitude due to elimination of the signal at 26 Hz.

The occurrence of VLF and ULF amplitude anomalies within ± 2 days of each other indicate the possibility of a common causative source, which may be either solar flares (magnetic storms) or the seismic activities. In order to examine these two possibilities we have plotted the VLF and ULF amplitude variations alongwith I.Kp on 9 days including the days of anomalies and the results for five cases are shown in Figs 7-9. These cases correspond to the months in

Llilmi30i 10 .12.02 5

3

'c 2

III a :::> f--:; "-::; ..:

03.00.00 06 00 0 0

TIME. h~ LT

Lemi3(h 10.12.02 lar---r---~--~--~--~---r--~==~

5 - --- --- --- -- - -- -- - - ---- --

a -. ------ -------- -.- .---- .. _-- .. -.. '. - .. _-_ ... -.. - -

- 5 --- _.- ------ -- . - - ----- - - .... _. i- --.--. ~ ...... -- ;--. --- - j • .• - ..•

- 10 _.-- - -- - -- -- •.. . - . . - -.. . -.! - ------~ ------. - j . --••• -• ~ _ •• _ •••

- 15 ---- - - - ---_. __ . - .- -- -. ...... _-.-- -- ---. -- - ----

-20 - .. ---- - --- . - .. . ---- ------ - .. ---- -- - . ---- .--- . --- --- .. - ---- -

FREQUENCY. Hz

Fig. 6--The same as Fig. 5 but the data is filtered using a low pass filter (0-15 Hz)

which anomalies have been observed. In the first two cases of VLFfULF amplitude variation there were some data gaps for 1 or 2 days because of problems in initial stage of operation and power failures. These data gaps have been filled by taking two days running mean of the data. Arrows in the top panels of each figure indicates the days of earthquakes.

The criteria of selecting these earthquakes have already been mentioned in Section 3 and details of the earthquakes are available in Table 1. As seen from Fig. 7 - Case 1, the earthquake occurred on 18 Sep. 2002 and the amplitudes of Y -, Z-components of the ULF emissions are enhanced about 18 hrs before from 2340 hrs on 17 Sep. 2002, whereas VLF amplitudes decreased one day before. There are increases in the X-, Y - and Z-components of ULF amplitudes on 13 Sep., 2002 also, but they are due to an earthquake that occurred in Port Blair (Lat. l3.04a N, Long. 93.07°E, M = 67, depth = 21 km). The Port Blair case has not been included here because we do not have long data set prior to this date for comparison, as the experiment was started on 12 Sep. 2002. In Case 2 a similar result is obtained, but the VLF amplitude is decreased on the same day rather than one day before as in the previous case. In Fig. 8-Case 3, the earth-

SINGH el al.: ULFIVLF AMPLITUDE ANOMALY DUR1NG EARTHQUAKE

Table I- The deta il s of earthquake considered [The fi ve cases of earthquakes correspond to the increasing order of the months in which the earthquakes occurred.]

227

Case Date Time Location Magnitude Depth Distance from Agra (UT)

2

3

4

5

(km)

18.09.02 120136.3 1 Lat. 13. 11, Long. 93.17 5.5 33

10.12.02 024446.68 Lat. 05.25, Long . 94.46 4.6 67

23.01.03 035840.23 Lat. 07.59, Long. 93.72 4.2 19

16.02.03 222749.86 Lat. -05.73, Long. 102.02 4.4 17

03.04.03 234824.43 Lat. 20.67, Long. 103.22 4.6 10

C ... 1 oo~--------------------~------------------------~

III

" uI 55 a ~ 50 ::; ~ 45

" ... 40 ...I

~=55 - -------------------. -- .----. -.:.-----

> 35+---_,---~---r_--~-----~----_.------~------r_---~

0.7 ~---------~----------:---__;:====11l 'c 0.6 -- x-signa ~ 0.5 -0- y-signal

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~ ~~~1--~·------~------'-~--~--~--r_--·--~----~--~--·--~---.--~--~--.--_r,----~ 12 13 14 15 16 17 18 19 20

DAYS (SEPTEMBER 2002)

56 ~--------------------~C~a~5~8~2----------------------------__, ~ 54 W 52 g sa 'J 48

~ 40

~ 44

=t 42

. ------' •

40~---_,------~------r_----~------~----_.------~------r_~--~

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~ 0.8 -0- y-signal w o 50.6

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:::> ~c------~----~~ ____ _..------~ O~--~--~--~--~--~--~--~--_r_

30 r---------------~------------------------------------------1

~ ~~ 11 ___ .. _-_-_-_-_-_-_ .. _.------------___ ,,-_____________ :.:~~~~~:~~~===:~~=====:.=~====~._....J 5 6 7 8 9 10 11 12 13

DAYS IDECEMBER 2002, Flg.7

Fig. 7--Variation of VLF and ULF amplitudes and LKp on nine days for the two cases of earthquakes

(km)

2253

3039

2777

4519

2652

228 INDIAN J RADIO & SPACE PHYS, AUGUST 2005

c ••• 3

Q M=4.2

J 64 "-:. < 63

i:l ~ 62 ~------------------------------------------------------------~

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20 21 22 23 OAYS IJANUARY 2003)

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24 25

65 r-----------------------------------------------------------

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DAYS (FEBRUARY 2003)

Fig. 8--The same as Fig. 7 but for Case 3 and Case 4

CaaaS

64 [-. - ---------m~ n ;:. 62 u M"'46

o 61 -_..----------0 ...... E60 ~ 59 ~58

~~ 55 ~--------~--------~------~

•

~-------T-·------- , I

40C" - ". ~ ~ - .--------: ---------

·-·---··-·--~--~--·----~i

3 4 DAYS (APRIL 2003)

Fig. <}--The same as Fig. 8 but for Case 5

I

SINGH el al.: ULFIYLF AMPLITUDE ANOMALY DURING EARTHQUAKE 229

quake occurred on 23 Jan . 2003 and the ULF amplitudes are enhanced one day after, whereas the VLF amplitude is decreased one day before. In Case 4, the VLF amplitude is decreased on the same day of the earthquake, but the ULF amplitudes are enhanced two days after. In Fig. 9-Case 5, the ULF amplitudes are enhanced one day before the earthquake, whereas VLF amplitude is decreased two days after.

In order to substantiate our result that the VLF amplitude decreases are caused by the earthquakes, we have carried out statistical analysis of the data the results of which are presented in Fig. 10. In fact, we have determined the mean (m) and standard deviation (cr) for the data corresponding to three groups of months divided over the whole period of 9 months of

observations. These three groups of months are (August, September, October), (November, December, January), and (February, March, April). The months are divided in three groups due to the reason that there is large seasonal variation in the amplitude of the VLF signal. This may be verified from Figs 7-9. It can be seen that while the range of amplitudes varies between 40 and 55 dB in the three months of Fig. 7, the same varies between 58 and 65 dB in the three months of Fig. 8 and Fig. 9. Therefore, the statistical analysis is done for the three groups of months separately . The mean and standard deviation are shown by three horizontal lines corresponding to m and m ± cr in each panel. We have also shown the variation of 'i.,Kp during the three groups of months,

AugU6t 2002-0clober 2002 65

11 21 31 41 51 61 71 81 91

DAYS

November 2002-January 2003

~ 5 65 ~ ""'!ii~cr ."1 .. ~4.2· · 1 ~1L55 -" • m

.. ~ ::,,'" """"""""" ,~,,~, ":, ;,,y:;, "=,,:,,"" ,,,,,,,,,,,,,,:,., m·o

" 21 31 41 51 61 71 81 91

DAYS

February 200J-Apr1l2003

10r--------------------------------~~-----------

~ ~ 65 j -. ~ H 0 L ~ 4.6 . +

IL 1=::~,::s'\;tC>.~7~=~~7~<~;S?S~~7~\~~~~sS""~t=>'\~~e~O~\~I\~:~:= ~~ 60 - if· \ I.J~ ~

55~--------------------------__________________ ~

11 21 31 41 51 61 11 81 DAYS

230 INDIAN J RADIO & SPACE PHYS, AUGUST 2005

separately below each panel corresponding to the amplitude variation. The amplitude data are shown by curves in each panel. The breakages in the data curves indicate non-availability of the data. The same earthquakes of Table I are shown here again by downward arrows.

Figure IO shows decreases in the amplitudes below the In-a lines in almost all cases except two, which are the cases of 22 Jan . and 16 Feb. 2003. The case of 22 January is special, in the sense that the decrease does occur in this case but above the In + a line. This is due to the reason that the range of amplitude in this month is the highest of all (64-65 dB), whereas the range of amplitudes in the two months included in this group (November and December) is the lowest of all (40-55 dB) and hence most part of the curves showing amplitude variations in January are above the standard deviation line. In the other case of 16 February, the decrease itself is just 1 dB and hence it lies within the standard deviation . The two cases of 41 SI day in the top panel and 54th day of the bottom panel , in which the amplitudes are much above the In + a lines (in the latter case below In - a line also) are possibly due to storm effect, because the 2.Kp is between 35 and 40 a few days before the enhancement in amplitudes. Hence, our finding that the occasional decreases in the amplitude of the signal are due to moderate earthquakes is supported by the results in this figure in general.

[n Fig. 11 we show the statistical analysis for ULF data for Y-coPlponent, which is done on the same lines as that shown in Fig. 10. All the Y-component data for 8 months are presented in the top panel of Fig. 11. The 2.Kp index variation during the whole period of analysis is repeated in the bottom panel. From this figure, it is clear that the amplitude enhancements are mostly corresponding to the days on which the earthquakes have occurred, except in the first case of 18 Sep. 2002, where the amplitude anomaly is not clear, because of the amplitudes of all the three components being relatively low in thi s month as compared to the others and also the data were for limited period of 15 days only in this month . From a comparison between top and bottom panels of this figure, it is seen that the amplitude enhancements are not caused by magnetic storms.

An interesting result that may be seen from these figures is that, in almost all cases (except the first) the Y -components of the ULF amplitude are larger than the X- and Z-components. It may be noted that the Y­component is aligned with east-west direction . One possibility for large enhancements in Y-components is that all the earthquakes occurred in East direction from Agra. This is in contrast to the enhancement in Z-components, which are caused by earthquakes in Paki stan and Afghanistan (north-west to Agra). Since in Figs 3 and 4, the Z-components are enhanced also alongwith Y -components, this is due to occurrence of

Stasticai Analysis for ULF for tho month of September 2002- April 2003 1.6 ,--------------- -. _~.4.4---- ..

1.4 · i 5.5 I 4.6 ,4.2 l4.6 ~ ~ 2 • + ... 1. ~ 1 · m+a V ~ 0.8~·----;-------------r-----~~~~~~-4r-+-~~~

i2 0.6 m J 1'1 ~ 0.4 f--~-++-H_t___iH___IVI_+___i_ffi_f!H;+_tt_ll__f.!_-I____v:-I-_+__lffi_V'_'_V...JI..:.ly

Q.

m-O.~ I 10 O~~~~~~~~~=_~~~~ __ ~~~~~~~

50 ,------------------------------------------------

40 .

30 :>' -w 20 ·

10 .

09101 10101 11101 12101 01101 02101 03101 04101

DAYS

Fig. II-The same as Fig. 10 but for ULF data corresponding to V-component (top) The 2. K" variation during the period of analysis (bottom)

SINGH et al.: ULFfYLF AMPLITUDE ANOMALY DURING EARTHQUAKE 231

earthquakes in both directions. The 'f.Kp variation at the bottom of each case shows the value to be < 30 on all the days under consideration, which means that the ULF-VLF anomalies are not caused by solar flares. Hence, the only poss ibility that seems to be likely is that the anomalies are caused by the earthquakes that occurred along the propagation path. The nighttime amplitude decreases caused by moderate seismic activities have already been recorded in Omega transmitter signal if = 10-13 kHz) '.

Using a long wave propagation capability code (LWPC), Clivered et al. 9 have modeled the nighttime propagation condition along 24 kHz NAA signal path and have also found regions where the amplitude is significantly reduced . There are some other poss ibilities also, in which the nighttime amplitude of the VLF signals may be reduced. These are energetic particle precipitation in the lower ionosphere, and lightn ing activity . The amplitude decreases due to energetic particle precipitation (energy > I Me V) is not poss ible, because Agra is located at low latitude (L = 1.1 ) and its dipole field line is located in the F­region ionosphere below the bottom of the Van Allen inner radiation belt (L = 1.2), where electron losses are not due to precipitation but they are dominated by atmospheric scattering resulting in the small life-time and density of the energetic electron2 1.22.

Further, out of the five cases of decreases in amplitude, the first case corresponds to the month of rainy season in northern India (where Agra is located), whereas the next four cases correspond to the months in which lightning activities occur in the conjugate area in the opposite hemi sphere, which lies in the Indian Ocean . ThiS raises a possibility that the electric field generated from the lightening activities may penetrate the ionosphere and the E-Iayer is pulled down under E x B drift, due to which the amplitude of VLF signal is reduced. However, this possibility is ruled out due to the reason that large electric field (- 105 V 1m) generated during lightning near the ground are attenuated so much in the atmosphere that they are about 1 mV/m only in lower ionosphere. The top of the Agra field line at the equator is located at the height of 635 km above the F-region peak. Hence, the electric field of such a small magnitude may possibly affect the F-region locally, but not the E­region ionosphere.

Our result of nighttime decreases in VLF amplitude possibly under the influence of seismic activity may be questioned on the ground that there may be many

propagation modes during nighnime and the amplitude decreases may be due to interference between these modes. Hence, to avoid this si tuation one is required to study the daytime data instead of nighttime data. In order to answer thi s., we mention here that the nighttime data have been considered extensively by Gokhberg et aL.' who have shown that the amplitudes of Omega transmitter signals if = I 0-13 kHz) were influenced by moderate earthquake (M>4) in 250 out of 350 cases examined. These earthquakes occurred at epicenters as far as 700 km away from the great circle path and the disturbance could occur 1-5 days before the earthquakes. In thi s paper and some of our earlier work23 we have attempted to extend the work of Gokhberg et al.'

It may be mentioned here that we have also attempted to analyse the data using "Termination Time"(TT) method of analysis , which employs most ly daytime data from early morning to late evening. Although, we have found termination Ii me correspon­ding to evening hours, the same corresponding to morning hours could not be identified clearly. A poss ible reason for this may be because our station is located in the equatorial anomaly region, where the ioni zation in the lower ionosphere changes consi­derably in the morning hours during sumise. Further, the daytime data may be influenced by the sudden ionospheric disturbances produced by solar X-rays also. Keeping these points in view, the TT method of analysis employing daytime data has not been pursued.

The enhancement in the ULF amplitudes due to signals propagated from earthquake sources may be ques tioned on the ground that the observing station is located far away (2000-4500 km) from the sources of the earthquakes and the signal may be considerab ly attenuated in reaching the observing station. Thi s may be answered in two ways; firstly, the attenuation of ULF signals has been calculated by employing appropriate conductivity models and it has been shown that such signals may be propagated from an underground source into the atmosphere, ionosphere and magnetosphere without much attenuation 17, and secondly , Tsarev and Sasaki 24 have shown from their model calculation that such signals may be propa­gated to long distances (> I 000 km) in the middle layer crust working as waveguide.

The VLF amplitude decreases are due to additional propagation path in the ionized region of the lower ionosphere, which is, as stated before, produced by the effect of the earthquakes. The amplitude decreases

232 INDIAN J RADIO & SPACE PHYS, AUGUST 2005

in such cases are significant in comparison to those suffered during multiple retlections in the earth ionosphere waveguide.

Acknowledgement The authors are thankful to the Department of

Science & Technology, Government of India, New Dt'lhi for financial support in the form of a major research project. Thanks are also due to our research colleague 0 P Singh for useful discussions. One of the authors CBS) is thankful to the Council of Scienti fic and Industrial Research CCSIR), New Delhi for support under Emeritus Scientist Scheme.

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