Annals 47, 1, 2004, 01/07def 83 ANNALS OF GEOPHYSICS, VOL. 47, N. 1, February 2004 Mailing address: Prof. Minakshi Devi, Department of Physics, Gauhati University, Gopinath Bardoloi Nagar, Guwahati, 781014 Assam, India; e-mail: md555@sify.com Key words earthquake – TEC – foF2 – anomaly 1. Introduction Earthquake-induced effects are diverse in nature and their manifestations are observed right from the Earth crust to thousands of kilometres up in the atmosphere. Convention- al seismic monitoring along with knowledge of earthquake-associated phenomena like electromagnetic emissions, lithosphere-ionos- pheric coupling processes (Gokhberg et al., 1982; Oraevsky et al., 1994; Larkina et al., 1989; Hayakawa, 1999) are now widely uti- lized to disclose an association between the processes. Anomalous behavior of the ionosphere prior to an earthquake was reported as early as 1929. Since then, observations on the changes in ionos- pheric parameters like foF2, foEs, Total Electron Content (TEC) and characteristics of turbulences have been examined, to understand if there is any earthquake induced information and to devise some predictor parameters (Wolcott et al., 1969; Parrot and Mogilevsky, 1989; Blaunstein, 1999; Devi et al., 2001) within these variables. However, analyses and co-relative works for identifying earthquake precursors remain a challenging problem because of the complexi- ty of their very nature. In this paper ionospheric parameters TEC and foF2 of anomaly crest stations like Guwahati (26.2°N, 91.75°E) and Ahmedabad (23.01°N, 72.36°E) are examined, associating a number of earthquake cases. As physical and dynamic pro- cesses of the high, mid, low and equatorial lati- tude ionosphere are of different nature, we will restrict our analysis to the low latitude equatorial anomaly region only. Association of Total Electron Content (TEC) and foF2 variations with earthquake events at the anomaly crest region Minakshi Devi (1), Ananda K. Barbara (1) and Anna Depueva (2) (1) Department of Physics, Gauhati University, Assam, India (2) Institute of Terrestrial Magnetism, Ionosphere and Radiowave Propagation (IZMIRAN), Russian Academy of Science, Troitsk (Moscow Region), Russia Abstract The paper attempts to identify ionospheric parameters in association with earthquake at anomaly crest station through VHF Radio Beacon data and ground based ionosonde measurements while the Total Electron Content (TEC) parameters from RB observations are based mainly on data taken over Guwahati (26.2°N, 91.75° E), foF2 data used in the analysis were collected at Ahmadabad (23.01°N, 72.36°E). The paper describes methods and techniques adopted to examine modifications on these parameters if any, due to earthquake preparatory process- es at equatorial anomaly crest stations. The mechanism of inducement of density changes in the ionosphere is sought through the generation of strong fountain effect possibly by the development of electric field during the earthquake preparatory process. 84 Minakshi Devi, Ananda K. Barbara and Anna Depueva 2. Background Some of the strongest earthquakes of the world occurred on this region in the years 1548, 1596, 1642, 1663, 1897, 1875, though their ex- act magnitudes are not known. Some events of known intensity are those of the years 1897 (M = = 8.7), 1918 (M = 7.6), 1930 (M = 7.1), 1950 (M = 8.7) and 1988 (M = 7.3). Because of the high seismic propensity of this zone, prediction of earthquakes in this region has become a ne- cessity. Though a few short-term predictions based on statistical study have been made, these were observed to be false alarms. Fig. 1a,b. TEC variations during low solar activity period over Guwahati: a) a quiet day feature; b) a disturbed day TEC character. Note: the large enhancement in ionization density after sunset during the D-day. Fig. 2a,b. Same as in fig. 1a,b but for high solar activity periods: a) the usual quiet day diurnal variation and (b) one of the typical disturbed day TEC pattern. Note: the initial decay rate on D-day is arrested by post sunset enhancement. a b a b 85 Association of Total Electron Content (TEC) and foF2 variations with earthquake events at the anomaly crest region maximum and minimum excursions from the base are determined. We have taken any devia- tion in TEC over this excursion limits as an in- dex of anomaly. Thus, deviations in TEC from the base during and pre-earthquake days are evaluated. Attempts are then made to associate such excursions with earthquake events. Fur- ther, extensive study on magnetic storm-in- duced TEC behaviour over this station has been made (not discussed here) to omit the magnetic storm afflicted TEC data while making this se- lection. Figure 3a,b shows two representative cases for low solar activity period. The two cases are so selected that they represent two basic fea- tures of earthquake-time density variation. Considering the type I behavior, we take the December 6, 1975 earthquake event. A clear in- crease in TEC prior to the earthquake days is noted in this case. This earthquake occurred at 11:15 h (92° East Meridian Time, EMT) with epicentre at 17.42°N (119.68°E). The figure al- so shows maximum and minimum excursion values of Q-days from its mean value, along with diurnal TEC variations of December 6. Magnitudes of daytime electron content on the two days show a clear high, relative to average quiet day TEC from the 30 day period. As an example of type II, fig. 3b represents features of the July 21, 1976 earthquake, when a significant depletion in TEC values are noted prior to and during the event. This earthquake occurred at 21:10 h (92°EMT) on that day with its epicenter at 24.78°N (98.70°E). The TEC pattern was the reverse of what was received on 6th December. In the pre-event time, a decrease of density is noted at the latitude of Guwahati and the density dips far below the minimum ex- cursion bar of quiet day TEC mean. However, such strong changes may not be clear when solar activity and background densi- ty both are high. As an example, we take the TEC values say for July 29, 1980 (fig. 4), when two consecutive earthquakes are recorded at 18:20 h (92°EMT) and 20:58 h (92°EMT) with epicenters at 29.33°N (81.26°E) and 29.60°N (81.09°E). On this occasions increase in densi- ty at Guwahati though appreciable from early morning hours, the large changes in TEC as ob- served in the low solar period are not so appar- 3. Observations 3.1. TEC and earthquake events Our attempts to understand the association between columnar electron content behaviour and earthquake events are discussed here through a few observations. This work is based on Faraday Rotation (FR) data collected through satellite Radio Beacon (RB) signal from ATS-6 (frequency 144 MHz, position 35°E) for low solar activity periods and from ETS II (frequency 136.11 MHz, position at 130°E) for high solar activity periods. Before attempting to identify earthquake-induced im- prints on TEC profiles, as it is necessary to have a knowledge on the relation of TEC with solar and geomagnetic parameters, we show in figs. 1a,b and 2a,b a few profiles observed over Guwahati at different solar and geomagnetic situations. It is seen that during high solar ac- tivity periods (1979-1981) TEC is at least 2 fold higher (fig. 2a,b) than at low solar epoch of 1975-1976 as in fig. 1a,b, indicating the need to realize the role of background density while as- sessing possible earthquake-induced features. In fig. 1a, the TEC profile of June 12, 1976 while represents quiet day (Q-day), the profile of July 1, 1976 features a disturbed day (D- day). A large increase in ionization density even after sunset hours is a significant point to be noted in the D-day, indicating processes other than normal ones are active on such days. De- pending on the solar-geomagnetic conditions, it is observed that the TEC profile shape of D-day changes (Devi et al., 2002). In fig. 2a,b, we present one Q-day event (May 25, 1980) and a D-day TEC profile (24 August 1980) of high solar periods. Here too, on the D-day the initial decay rate is arrested by injection of extra ion- ization. Keeping these features in mind, we take the following approaches while examining possible seismo-ionospheric effects though this parameter. Five most quiet day TEC values are first identified and their average figure is taken as a base of TEC free from geomagnetic distur- bances. We select a period of 30 days prefer- ably keeping earthquake day as the centre while selecting the Q-day base. Further, as Q-day TEC also show day to day fluctuations, their 86 Minakshi Devi, Ananda K. Barbara and Anna Depueva a Fig. 4. Same as in fig. 3a,b but for high solar activity earthquake event. The event shown is for 29 July, 1980. Earthquake occurrence times are shown by arrowheads. The vertical bars give excursion of standard deviation of Q-day from the base. b Fig. 3a,b. Plots showing variations in TEC (taken over Guwahati) from the Q-day base, during low solar ac- tivity period. Earthquakes occurred on (a) December 6, 1975 and (b) July 21, 1976. Earthquake commencement times are shown by arrowheads in the figure. The vertical bars give the excursion of standard deviation of Q-day TEC from the base. 87 Association of Total Electron Content (TEC) and foF2 variations with earthquake events at the anomaly crest region ent probably because of high background den- sity conditions. In this case, the epicenter lies at 5° away from the sub ionospheric point of the satellite which is at 24.12°N, 95.71°E. Results of this nature are presented else- where (Devi et al., 2001). The analysis there- fore indicates that both positive and negative ionospheric effects can be seen prior to an earthquake, and an association between ionos- pheric effects and its epicenter position cannot be ruled out. 3.2. foF2 as precursor of earthquake The changes in critical frequency of F-layer received from ground-based ionosonde at Ahmedabad are next examined to disclose vari- ations in foF2 if any, during the earthquake preparatory process. For this purpose, hourly foF2 data for 15/10 days prior to an event are considered. Further, we examine the magnitude and development of bite out, as a result of a fountain effect another input along with tempo- ral changes in foF 2 prior to an earthquake. A few representative cases in Fo f 2 features and generation of anomaly before earthquake events are described below. Two events presented in fig. 5 are for April 6 (epicenter at 26.19°N, 96.87°E) and April 27 (epicenter at 13.07°N, 119.54°E), 1994. The dark shades indicate strong density zones, prior to an earthquake and a signature of bite out is seen as two black areas around noon. It is also to be not- ed that the background density during this month is fairly large and ionization density needs to be sufficiently strong to exhibit the signature of bite out effect. Arrowheads in the figure indicate time (78°EMT) of occurrence of the earthquake. The absence of data is also marked. Figure 6 presents a representative event of a summer case where the earthquake occurred on July 30, 1996 at 23:08 h (78°EMT) with its epi- center at 14.51°N (119.95°E). Here, strong den- sity regions and anomaly (i.e. bite out) were not seen many days earlier, but a high density zone is formed two days before the event and anom- aly effect is detected 6 h before earthquake. Interestingly, all these cases (shown as repre- sentatives of different seasons) indicate genera- tion of strong density along with bite out, which must have sources other than the normal ionizing Fig. 5. foF2 variation pattern over the anomaly crest station of Ahmedabad before and during earthquakes of April 6 and 27, 1994. The arrowheads give the time of commencement of earthquakes. The density magnitude is defined by gray shades. Black zone is for frequencies > 11.00 MHz. Early morning data are not shown in the figure. High density zones and bite outs are present before and during the events. 88 Minakshi Devi, Ananda K. Barbara and Anna Depueva process. However, these features may not be well reflected in all cases especially when we have a succession of tremors at frequent intervals as shown in fig. 7 for May 1997, where there were three earthquakes of magnitudes of 5.5-6.0, on 5th, 8th and 21st day. The 5th May quake was ob- served at 11:15 h (78°MT), with epicenter at 14.91°N (119.89°E). High density zones are seen prior to the quake day but due to non availability of data at 13, 15 and 16 h on day 4, it is difficult Fig. 6. One more case showing foF2 variations, prior to and during the 30.7.1996 earthquake. In this case the darkest region is for frequencies > 10.50 MHz. Presence of bite out is to be noted before the earthquake event. Fig. 7. Same as in figs. 5 and 6 but for three consecutive summer earthquake events for May 5, 8 and 22 of 1997. The arrowheads indicate the occurrence time of each earthquake event. Here the black zone is for fre- quencies > 10.00 MHz. White circles indicate absence of data. Though bite out is not prominent, high density zones up to day 23 are significant. 89 Association of Total Electron Content (TEC) and foF2 variations with earthquake events at the anomaly crest region to make a statement on bite out. However, on the day of the event a bite out is observed. On May 8, another tremor was detected at 08:23 h (78°EMT) with epicenter at 24.89°N (92.25°E). Strong density zones in noon and afternoon hours, prior to the earthquake day are clearly seen in the ionogram. But, we cannot say if bite out was present because of want of data during noon hours, however bite out was seen after the earth- quake. So, is the case with the event of May 22, when the tremor occurred at about 04:00 h. How- ever, we cannot make a statement on bite out on May 19, 20 and 21, as there were no continuous data. The high density regions in noon and after- noon periods as noted almost on all days from May 3 to 23rd, subside after May 23rd. The overall pattern described above seems to be maintained during major earthquakes too, one such event is shown in fig. 8. 3.3. Major earthquake On 26th January 2001, Ahmedabad region experienced a very strong earthquake of scale 8.1, and was followed by a number of tremors. It is interesting to note the high densities indi- cated by strong foF2 values prior to the event (fig. 8). Bite outs are also visible before quake days but high density contours disappeared shortly before the onset of earthquake. This de- crease in density and the position of the epicen- tre of the earthquake (23.36°N and 70.33°E), is to be noted. Due to mass destruction, no ionos- pheric data were available just after the onset of earthquake, which continued for 3 days. 4. Discussion Although ionospheric parameters behave in a complex manner, scientists have been at- tempting to associate some of their features with seismic events (Wolcott et al., 1969; Ha- yakawa, 1999; Depueva and Rotanova, 2001). One of the approaches is by analysing the de- velopment and inhibition process of anomaly by mapping the magnitude and spread of its crest and trough, before and during earthquake events using topside ionospheric data (Depueva and Ruzhin, 1993, 1995). In this exercise too, we are trying to see the anomaly effect during the earthquake preparatory process. Increase in magnitude of TEC, which is observed to be Fig. 8. A case of very strong earthquake of M = 8.1 experienced around Ahmedabad on 26 January, 2001. The large density contours followed by depletion in density prior to the event (at 09:00 h, 78°EMT) are clearly seen. Here the black region corresponds to frequencies > 11:50 MHz. Due to mass destruction no foF2 data are avail- able just from the onset of the event for 3 days. 90 Minakshi Devi, Ananda K. Barbara and Anna Depueva higher than maximum excursion limits in day to day TEC fluctuations (figs. 1a,b and 2a,b), in- dicate that an inflow of ionisation might have taken place in the earthquake preparatory re- gion from an additional source. Similarly, a de- crease in TEC suggests that an outflow of ion- ization has taken place from the afflicted site. Such changes in TEC variations for a num- ber of strong earthquake events (Devi et al., 2001) also indicate that high-density TEC con- tours are often associated with earthquakes having their epicenters near to the equator or away from the observational site. The study further indicates that TEC depletions (like the event of July 1976) are often observed when the epicenter lies very near to the observation- al site. It is also observed from foF2 variations over Ahmadabad that enhancement as well as biteout are fairly well detected phenomena prior to an earthquake. But unlike the case of TEC, decrease in density is not clearly seen even if the epicenter lies near to the receiving site. However, for the major earthquake of January 2001, when its epicenter was at Ah- medabad, a decrease in density was detected prior to the event. The depletion features as observed in TEC are prolonged in comparison to what is seen through foF2. These enhance- ments (as well as depletion) in density values along with development of anomaly if consid- ered as earthquake-induced perturbations in the atmosphere, an extra agent (besides the normal ionizing agencies) associated with the earthquake preparation processes in shaping density profiles has to be active. As E× B drift is one of the significant contributors in con- trolling density at anomaly crest stations like Ahmedabad and Guwahati (Devi et al., 2002), it is necessary to invoke electric field trig- gered by earthquake preparatory processes that generate E× B drift (Depueva and Ruzhin, 1993, 1995). In fact, emission of ELF fields prior to earthquakes was also detected near the epicenter (Larkina et al., 1983; Parrot and Magilevsky, 1989). Depletion and enhance- ments as seen on density profiles, may be the result of earthquake-associated E× B drift when electron density may flow into or out of the observing station depending on its loca- tion, if an electric field of sufficient magni- tude is developed at the ionospheric heights. There are reports related to possible effects of earthquake generated electric fields at ionos- pheric heights (Gokhberg et al., 1984; Parrot, 1995). Considering that such electric field does develop during earthquake preparatory processes and it penetrates to E-layer heights resulting in enhanced electrojet strength, the upward E× B drift will then be amplified. We therefore examine the effects of such field, re- flected as bite outs in density profiles while dealing with foF2, as one of the inputs for identifying seismo-ionospheric effects. How- ever, the anomaly effects generally being very regular and strong during equinoxial months, we have taken summer and winter events on- ly, to avoid ambiguities in analysis and inter- pretation. In fact, the development or inhibi- tion of anomaly during earthquake preparato- ry processes have also been examined by ear- lier workers (Depueva and Rotanava, 2001). While analysing topside electron density over a dip latitude zone of ± 30°, they showed that considerable changes in the development processes of the anomaly do take place prior to an earthquake and also that the position of the epicentre is critical in controlling the mag- nitude of anomaly. However the magnitude of such electric fields is still a question. REFERENCES BLAUNSTEIN, N. (1999): Remote sensing of short term ionospheric phenomena, indicators of precursors of earthquakes by use of radiophysical methods, in Progress in Electromgnetics Research Symposium Pro- ceedings, Taipei, vol. 1, pp. 132. DEPUEVA, A. and N. ROTANOVA (2001): Low latitude ionos- pheric disturbances associated with earthquakes, Ann. Geophysics, 44 (2), 221-228. DEPUEVA, A.H. and YU.YA. RUZHIN (1993): The equatorial earthquake preparatory stage as a reason of «fountain» effect in the ionosphere, Preprint IZMIRAN, N82 (1029), p. 10. DEPUEVA, A.H. and YU.YA. RUZHIN (1995): Seismoionos- pheric «fountain» effect as analogous of active space experiment, Adv. Space Res., 15 (12), 151-154. DEVI, M., M.K. BARMAN, A.K. BARBARA and M. DEPUEVa (2001): Total electron content near anomaly crest as precursor of earthquake, Ind. J. Radio Space Phys., 30, 209-213. DEVI, M., M.K. BARMAN and A.K. BARBARA (2002): Iden- 91 Association of Total Electron Content (TEC) and foF2 variations with earthquake events at the anomaly crest region tification of quiet and disturbed days through TEC pro- file features over anomaly crest region, J. Atmos. Solar Terr. Phys., 64, 1413-1423. GOKHBERG, M.B., Y.A. MORGUNOV, T. YOSHINO and T. TOMIZAWA (1982): Experimental measurement of elec- tromagnetic emissions possibly related to earthquakes in Japan, J. Geophys. Res., 87, p. 7824. GOKHBERG, M.B, N.I. GERSHENZON, I.I. GUFEL’D, A.V. KUSTOV, V.A. LIPEROVSKIY and S.S. KHUSAMEDDIMOV (1984): Possible effects of the action of electric fields of seismic origin on the ionosphere, Geomagn. Aeron., 24, p. 183. HAYAKAWA, M. (Editor) (1999): Atmospheric and Ionos- pheric Electromagnetic Phenomena Associated with Earthquakes (Terra Scientific Publishing Co., Tokyo), pp. 996. LARKINA,V.I., A.V. NALIVAYKO, N.I. GERSHENZON, M.B. GOKHBERG, V.A. LIPEROVSKY and S.I. SHALIMOV (1983): Observations of VLF emissions related with seismic activity, on the Interkosmos-19 satellite, Geomagn. Aeron., 23, 684-687. LARKINA, V.I., V.V. MIGULIN, O.A. MOLCHANOV, I.P. KHARKOV, A.S. INCHIN and V.B. SCHVETCOVA (1989): Some statistical results of very low frequency radio wave emissions in the upper ionosphere over earth- quake zones, Phys. Earth Planet. Int., 57, 100-109. PARROT, M. (1995): Use of satellite to detect seismo-elec- tromagnetic effects, Adv. Space Res., 15 (11), 1,27- 1,35. PARROT, M. and M.M. MOGILEVSKY (1989): VLF emission associated with earthquakes and observed in the iono- sphere and the magnetosphere, Phys. Earth Planet. Int., 57 (1-2), 86-99. ORAEVSKY, V.N., YU.YA. RUZHIN and A.H. DEPUEVA (1994) Seismoionospheric precursors and atmospheric elec- tricity, Turkish J. Physics., 18, 1229-1234. WOLCOTT, J.H., D.J. SIMONS, D.D. LEE and R.A. NELSON (1969): Observations of an ionospheric perturbation arising from the Coalinga earthquake at Kurile islands on August 11, 1969, Nature, 226, p. 1239.