AG_56.03.13_CHOUDHURY_correctedOK:Layout 6 ANNALS OF GEOPHYSICS, 56, 3, 2013, R0331; doi:10.4401/ag-6235 R0331 A statistical study on precursory effects of earthquakes observed through the atmospheric vertical electric field in northeast India Abhijit Choudhury, Anirban Guha*, Barin Kumar De, Rakesh Roy Tripura University, Department of Physics, Suryamaninagar, Agartala, India ABSTRACT The study of anomalous variations in the near-surface atmospheric ver- tical electric field (VEF) that have the form of bay-like depressions in strength have been used as precursors of earthquakes in various studies. We present here the first statistical report from an earthquake-prone zone in northeast India from July 2009 to July 2012. The 10 days that were me- teorologically fair and with earthquake occurrences were selected for the present analysis. The average VEF bay durations and depths were ca. 50 min to 70 min, with the corresponding magnitudes of 500 Vm-1 to 800 Vm-1. Anomalous variation in VEF before 7 to 12 hour of the impending earthquake has been observed. There was a 31% probability that a VEF bay would show as an earthquake precursor. The positive correlation co- efficient was 0.72 between the VEF bay depth and the ratio of earthquake magnitude to depth, while the negative correlation coefficient of 0.82 was calculated between VEF bay duration and the ratio of earthquake mag- nitude to depth. There was moderate correlation for distance of the earth- quake epicenter to the observation point with both VEF bay depth and VEF bay duration. The correlation of the time difference of VEF varia- tions and earthquakes with VEF bay depth was good, whereas the corre- lation of the time difference of VEF variations and earthquakes with VEF bay duration was too low to be considered. 1. Introduction The atmospheric vertical electric field (VEF) is gen- erated by thunderstorm discharges that can create a po- tential difference between the ground and the ionosphere in regions with fair weather [Price 2002, Guha et al. 2010]. The VEF is normally directed down- wards and its intensity close to the ground surface is of the order of 100 Vm-1 to 200 Vm-1 [Bennett and Harri- son 2007, Mikhailov et al. 2002]. Electric field measure- ments at the ground surface have been performed for a long time, to gather cloud charge distributions, and to study the number, intensity and polarity of thunder- storm discharges [Pawar and Kamra 2002]. It is rela- tively difficult to forecast the severity of thunderstorms due to nonlinearity in the cloud dynamics and the small temporal and spatial extensions [Litta and Mohanty 2008]. However, as the atmospheric VEF is a contribu- tor to the global electric circuit (GEC), it is one of the important parameters for further improvement of the GEC model [Fullekrug 2004]. Moreover, the study of the GEC and atmospheric electricity provide information on the solar–terrestrial weather relationships, as well as on the global temperature and climate change, which link studies of the VEF in fair-weather regions [Bering 1995, Adlerman and Williams 1996, Rycroft et al. 2000]. The fair-weather electricity is defined as the electric field induced by the space charge from an excess of pos- itive ions in the atmosphere, and excluding other phe- nomena such as local cloud electrification and dust storms [Stow 1969]. During these fair-weather days, there have been several scientific reports of uncharacteristic varia- tions in the atmospheric VEF with respect to fair weather VEF variation prior to earthquakes in various seismo-ac- tive regions of the world, as has been shown in reports from countries like Russia, Japan and China [Kondo 1968, Pierce 1976, Rulenko 2000, Hao et al. 2000, Smirnov 2001, Mikhailov et al. 2006, Zhang-Hui et al. 2011]. Interestingly, all of these reports have indicated negative variations in the VEF. These VEF variations usually occur at in- tervals of a few hours to a few days before an earthquake main shock, and they are seen as either bay-like field in- tensity decreases or oscillatory trains that last a few hours [Mikhailov et al. 2004]. These anomalous variations are usually classified as short-term earthquake precursors [Pulinets and Ouzounov 2011]. The mechanism of this anomalous electric field generation was described by Pu- linets and Boyarchuk [2004]. The nature of these effects is possibly related to variations in the stress–strain phenom- ena in the upper lithosphere that are a result of the prepa- ration of the earthquake source [Mikhailov et al. 2002]. Several possible mechanisms of lithosphere–ionos- phere interactions have been suggested in the literature. Article history Received October 16, 2012; accepted May 20, 2013. Subject classification: Vertical electric field, Earthquake, Negative bay anomaly, Lithosphere–atmosphere–ionosphere coupling. Surface air ionization caused by radon emanation into the atmosphere before earthquakes has also been in- vestigated as one of the major sources of electric field variation [Pulinets et al. 1997, Liperovsky et al. 2008, Harrison et al. 2010]. Moreover, it is evident from the study of Pulinets et al. [1998] that there is a dependence of ionospheric electron density due to seismic varia- tions on local time as well. Studies of low-latitude seis- mic regions have shown this to be explainable through zonal electric field generation due to equatorial anom- alies (Pulinets 2012), hence making the study of VEFs more important from our location, which is at a lati- tude of 23 °N. Thus, even simply from the enumera- tion of the sources of the atmospheric electric field variations, it is clear that the problem of separating out earthquake effects is still very complex. In this regard, the present study was designed to provide further in- sight into forecasting earthquakes with the help of these VEF anomalies. At the low latitude station of the Department of Physics, Tripura University, India (latitude, 23.75 °N; longitude, 91.25 °E; 42 m a.s.l.), we have been record- ing the data from atmospheric VEFs around the clock, from July 2009 to date. This report discusses the pre- liminary results of the morphological structure of the variations of the VEF with the magnitude of the nega- tive anomaly and with the anomaly duration, and the dependence of these parameters on earthquake mag- nitude and on the distance between the observation point and the earthquake epicenter. Only negative anomalies were considered; i.e., negative deviations of the VEF from the positive diurnal behavior, in the pe- riod from 24 h before an earthquake occurrence. 2. Experimental set-up To measure the atmospheric VEF, a calibrated Boltek EFM–100 atmospheric electric field monitor (EFM) was installed on the roof of the Department of Physics, Tripura University (India), at 15 m above ground level. The EFM has six sensor plates, each with dimensions of 17 cm, which are arranged symmetrically around the rotating axis. Six electronically controlled, mechanically grounded, conductive ‘choppers’ are used to alternately shield and expose the six sensor plates to the atmospheric direct current electric field. Alternate exposure and shielding of the sensor electrodes gener- ates a to-and-fro motion of charge between the ground and the sensor plates through a high resistance resistor. This moving charge promotes an alternating current, which appears as an alternating current voltage across the sensor resistor. The magnitude of this alternating current and the corresponding alternating current volt- age is proportional to the strength of the direct current atmospheric electric field. This voltage is amplified and fed into an analog-to-digital converter. The data is stored in a computer with a sampling rate of 1 datum/s. To maintain time synchronization, the internal clock of the computer is synchronized with a GPS receiver. The maximum electric field that can be measured by such an EFM is ±20 kVm-1. However, after the correct cali- bration and scaling under fair weather conditions, a scal- ing factor of 0.3 was determined, which restricted the maximum field variation to ca. ±6 kVm-1 at a height of 15 m from the ground, where the EFM was installed. The EFM detected electric field variations due to cloud activity within a radius of 30 km. 3. Analysis of data and observational results The diurnal variation of the VEF is continuously af- fected by the presence of clouds, thunderstorms (both local and global), precipitation, geomagnetic storms, and aerosol concentrations, among other factors [Rycroft et al. 2000, Harrison and Aplin 2002, Bennett and Harrison 2007]. Thus, it is necessary to distinguish between any seismic effects on the VEF measurements and various local or global perturbations. Our geographical loca- tion is at the juncture point of three tectonic plates: the Eurasian, Indian and Australian plates (Figure 1). There- fore, it is a strategically significant place to perform a statistical study on precursory variations in VEF meas- urements before major local earthquakes. Three years of VEF data from July 2009 to July 2012 were selected for the present analysis. First of all, the fair-weather days were selected from among the whole range of data. We defined fair-weather conditions as a maximum wind CHOUDHURY ET AL. 2 Figure 1. Map of the geographical positions of all of the 10 earth- quakes for which the precursor variations were observed (triangles) along with location of the VEF instrument (open circle). The re- maining 22 of the 32 earthquakes, where precursors were not ob- served, are also shown (rectangles). Dashed lines demonstrate the limit of Eurasian, Indian and Australian plates. 3 speed of 4 m/s, less than 3 octa cloud cover, and no cu- mulonimbus clouds visible in the sky from the observa- tion site [Latha 2003], and also with no precipitation. The confirmation of fair-weather conditions was also verified according to the data from an automatic weather station (set-up at our receiver position), and from lightning im- aging satellites and meteorological and oceanographic satellites for this region. However, due to the adverse ge- ographical location of our receiver, which is located at the foot hills of the Himalayas and also in one of the re- gions with the highest rainfall in the world (Cherra- punji, with an average yearly rainfall of ca. 12,000 mm, ATMOSPHERIC PG AS EARTHQUAKE PRECURSOR EQ Dates Magnitude (M) Depth (D) M/D Dt * (hr:min:sec) VEF Depth (kV/m) VEF Duration (hr:min:sec) Dist. from Epicenter (km) Preparation Zone (km) 26.07.2009 5.2 10 0.52 08:20:59 0.714 01:01:34 1460 172 12.12.2009 5.0 10 0.50 05:15:26 0.548 00:53:32 1924 141 13.12.2009 5.1 10 0.51 11:15:47 0.633 00:51:36 140 156 01.05.2010 4.6 12 0.38 10:06:40 0.834 00:53:33 1110 95 12.06.2010 7.8 10 0.78 14:05:08 1.385 00:40:42 1763 2260 10.09.2010 4.8 15 0.32 11:22:56 0.804 01:10:22 133 116 12.12.2010 4.8 15 0.32 06:05:27 0.457 01:04:50 269 116 12.02.2011 4.0 10 0.40 07:42:30 0.515 00:54:40 70 53 24.04.2012 5.5 10 0.55 12:16:08 0.684 00:59:36 1580 232 11.05.2012 5.4 20 0.27 12:39:22 0.562 01:17:35 370 210 Date EQ Time (IST) Lat Long Depth (D) Magnitude (M) M/D Dist. from Epicenter (km) Preparation Zone (km) 2012-03-27 03:24:54.38 33.284 95.605 11.4 4.1 0.36 1167 58 2012-03-04 22:41:45.44 30.167 101.767 10.0 4.3 0.43 1217 71 2012-02-20 19:48:03.82 35.840 79.835 10.0 5.0 0.50 1757 141 2012-01-01 10:49:34.93 10.632 91.737 10.0 4.6 0.46 1431 95 2011-12-14 11:09:25.15 39.463 94.384 10.0 4.7 0.47 1798 105 2011-11-30 01:12:34.43 7.802 93.858 17.0 5.6 0.33 1766 256 2011-11-13 11:59:11.66 36.045 81.066 10.0 4.1 0.41 1703 58 2011-09-24 09:38:58.84 36.377 82.526 10.0 4.3 0.43 1650 71 2011-08-01 01:10:53.48 33.739 87.574 13.6 5.1 0.37 1192 156 2011-01-23 21:16:19.83 23.034 101.689 10.0 4.2 0.42 1066 64 2010-11-28 10:03:47.10 7.588 91.875 15.8 4.2 0.26 1770 64 2010-11-06 07:42:43.64 36.636 87.506 10.0 5.2 0.52 1503 172 2010-09-08 07:28:10.63 33.315 96.365 10.0 4.2 0.42 1199 64 2010-07-25 14:27:03.95 16.345 94.611 10.0 4.6 0.46 869 95 2010-06-22 04:44:10.84 29.872 80.428 16.3 5.2 0.32 1285 172 2010-04-27 01:52:27.72 30.432 101.424 10.0 5.1 0.51 1267 156 2010-03-10 20:02:21.77 36.426 86.966 10.0 5.1 0.51 1493 156 2010-03-03 20:05: 3.33 32.821 105.211 13.1 4.3 0.33 1713 71 2009-12-18 10:49:30.08 32.641 92.841 10.0 4.6 0.46 1027 95 2009-12-01 16:53:28.43 27.272 91.455 10.0 4.2 0.42 419 64 2009-11-20 04:24:19.36 37.157 101.978 10.0 4.2 0.42 1829 64 2009-08-09 09:32:07.30 35.578 81.647 10.0 5.1 0.51 1630 156 Table 1. Details of the 10 earthquake days where the precursors were observed. Dt *, time difference between occurrence of bay-like varia- tion in the VEF and the earthquake. Table 2. Details of the 22 earthquake days where no precursors were observed despite the fair-weather conditions. is within 350 km of the observational site), ideally clear meteorological days were rare. Next, we looked for local earthquakes that occurred in the vicinity of about 2000 km from the receiver position, but only during these meteorologically fair days. A total of 32 such earth- quakes were reported over the 3 years of the observa- tional period. Among these, there were only 10 cases of VEF variations in the form of bay-like depressions in the VEF magnitudes that were observed as precursors to these earthquakes (as listed in Table 1). Figure 1 shows the geographical positions of these 10 earth- quakes for which the precursor variations were ob- served (Figure 1, triangles), along with the location of the VEF receiver (Figure 1, open circle). The remaining 22 of the 32 earthquakes, where precursors were not observed, are indicated with gray rectangles in Figure 1, and the full details are provided in Table 2. In the pres- ent analysis, the earthquakes with earthquake magni- tude to depth ratios >0.25 were only considered. The reason behind the selecting of this ratio was to intro- duce both the earthquake magnitude as well as depth into the analysis. Also, if two successive earthquakes occurred within an interval of 24 h, they were consid- ered as a single event. Only the negative anomalies in VEF that preceded an earthquake by no more than 24 h were considered in the statistical analysis. Figure 2 shows the typical diurnal variation in the VEF for a meteorologically fair weather day (Figure 2a), a day with local lightning and precipitation (Figure 2b), and a day with fair weather and an anomalous VEF variation that preceded an earthquake (Figure 2c). In Figure 2c, the bay-like variation is highlighted by the circle, and the time of the earthquake is indicated by an upward arrow. We also checked the selected 10 days for any solar and geomagnetic storm events. Nine of CHOUDHURY ET AL. 4 Figure 2. Typical diurnal variations in the VEF for a meteorologi- cally fair-weather day (a), a day with local lightning and precipitation (b), and a day with fair weather and an anomalous VEF variation that preceded an earthquake (c). Arrow, time of the main strike of the earthquake. Figure 3. Diurnal variations in the VEF along with the earthquake for the 10 cases where a precursor was observed. Arrow to right/left, anomalous VEF variations; arrow up, time of strike of the earthquake plus ratio of earthquake magnitude to depth. 5 these 10 days were very quiet days, while one, namely April 24, 2012, was associated with a minor geomag- netic storm of Kp = 5. Figure 3 shows all of the 10 days of the bay-like VEF variations, along with the earth- quake strike times. The anomalous bay-like variations in the VEF are shown by sidewise arrows, while the times of the associated earthquakes and the corre- sponding ratios of the magnitude to the depth are shown by upward arrows (Figure 3). To determine if there was any dependency of earthquake class on the VEF variations, we selected the ratio of the magnitude to the depth of the epicenter of each earthquake as a parameter to define all 10 of the earthquakes. There was a positive correlation coeffi- cient of 0.72, with the associated 95% confidence in- terval from 0.16 to 0.93 (www.psyctc.org/stats/R/CI_ correln1.html), for the plot between the ratio of the magnitude to the depth and the VEF bay depth, as shown in Figure 4. Figure 5 shows a further plot be- tween the ratio of the magnitude to the depth and the VEF bay duration, which shows a negative correlation coefficient of -0.82, with a 95% confidence interval from -0.96 to -0.39. We then determined if there was any dependence between the distance of the earth- quake epicenter to the observation point and the VEF bay depth, and also the VEF bay duration (Figure 6). The computed correlation coefficients were 0.45 and - 0.47, with 95% confidence intervals from -0.25 to 0.84 and from -0.85 to 0.23, respectively. This indicates that there might be moderate dependence of the bay depth and the bay duration on the distance of the epicenters. Figure 7 shows the time difference (Dt) between the oc- ATMOSPHERIC PG AS EARTHQUAKE PRECURSOR Figure 7. Correlation plot between the time duration of the ob- served precursor of the earthquake and the VEF bay depth (top) and VEF bay duration (bottom). Figure 6. Correlation plot between the distance of the earthquake epicenter to the receiver and the VEF bay depth (top) and VEF bay duration (bottom). Figure 4. Correlation plot between the ratio of magnitude to depth and the VEF bay depth. Figure 5. Correlation plot between the ratio of magnitude by depth and the VEF bay duration. currence of the bay-like variations in the VEF and the earthquakes, according to the bay depth and bay dura- tion. For the VEF bay depth, there was a positive cor- relation of 0.65, with a 95% confidence interval from 0.03 to 0.91, although the VEF bay duration plot was a scattered graph which showed very low correlation (Figure 7). A box plot (Figure 8) was plotted between the 10 cases where the precursors were observed (Group A) and the 22 cases (i.e., of the total of 32 fair- weather days with earthquakes) without precursors (Group B), to compare these two groups. Here, the dis- tribution was well scattered for Group A, with a mean ratio of 0.5 and a maximum ratio of 0.78 for the earth- quake magnitude to depth ratio. For Group B the dis- tribution was relatively suppressed for the earthquake magnitude to depth ratio, with a mean of 0.43 and a maximum of 0.5, from which it is hard to quantify a specific range for earthquake precursors. From the investigations of the ten earthquakes, we found the mean VEF bay-depth variation to be ca. 500 Vm-1 to 800 Vm-1, with a maximum variation of around 1385 Vm-1. Here, the mean VEF bay duration was within the range of 50 min to 70 min, with the maximum vari- ation as 77 minutes. The most probable VEF precursors were seen 7 h to 12 h before an impending earthquake, whereas the maximum variation was ca. 14 h and 5 min (Figure 9). One of the most interesting things about all 10 of these earthquakes was that all of them were shal- low; i.e., within 10 km to 20 km in depth only. From a total of 32 earthquakes reported during the time period of our data analysis, only 10 showed VEF variations, in- dicating a 31% probability of VEF earthquake precur- sors from our data, considering within 24 h before an impending earthquake, and only during the meteoro- logically clear days. 4. Discussion The atmospheric VEF near the Earth surface ex- ists under all weather conditions. Under fair-weather conditions, these VEFs arise through the GEC, due to thunderstorms occurring in meteorologically disturbed regions around the globe. In the fair weather regions, the VEF is around 100 Vm-1, and it shows an identifi- able global diurnal variation, known as the Carnegie variation. This is named after the sailing vessel on which the original marine measurements were per- formed and named [Parkinson and Torreson 1931, Guha et al. 2010]. Generally for atmospheric electricity, fair-weather electric fields are considered as negative [Pawar and Kamra 2002] in a spherical polar coordinate system, although in our analysis here we took the fair- weather electric fields to be positive, as per convention of fair-weather atmospheric electricity. Among all of the scientific reports on variations in atmospheric electric field before earthquakes, Pierce [1976] made the first observations of a 30% decrease in the electric field from its ambient value. Thereafter, sev- eral reports across the scientific literature were pre- sented that showed quasistatic anomalous electric field variations before earthquakes [Kondo 1968, Rulenko et al. 1992, Hao et al. 2000, Smirnov 2005]. The exact vari- ation is something like a negative bay-like anomaly in the VEF, with a sharp leading front and subsequent smooth recovery to the background level over a period of time, which has been observed as a few hours before several earthquakes reported from Kamchatka (Russia), Japan, Taiwan and China [Mikhailov et al. 2003, Mikhailov et al. 2006, Smirnov 2001, Kamogawa et al. 2004, Zhang-Hui et al. 2011]. Smirnov [2008] carried out a statistical study at Kamchatka from 1997 to 2002 over 103 cases of bay-like CHOUDHURY ET AL. 6 Figure 8. Box plot comparing the 10 days with precursors observed (group A) and the 22 days without precursors observed (group B) with the ratio of earthquake magnitude to depth. Figure 9. Histograms showing time difference of VEF variation (top) and earthquake and VEF bay duration (bottom). 7 depressions in the strength of the near Earth atmos- pheric electric field (Ez) and found the most probable bay duration to be 40 min to 60 min. The most proba- ble negative variations in Ez are around 106 Vm -1 to 300 Vm-1. No dependence of these values on the earth- quake class or the distance to the epicenter was found. The probability of earthquake forecasting over a 24 h period before an earthquake based on the Ez anomaly has been shown to be around 36%. Another statistical study was reported from Lisbon (Portugal) for the pe- riod from 1955 to 1991, which examined the influence of seismic activity on the VEF, to define an exploratory method that involved the selection of events for which the distance from the atmospheric electrical field sensor to the earthquake epicenter was smaller than the prepa- ration radius of the event [Silva et al. 2012]. In our sta- tistical investigation, for northeast India, we found that the mean VEF bay-depth variations for earthquake pre- cursors to be ca. 500 Vm-1 to 800 Vm-1, whereas the av- erage VEF bay duration was about 50 min to 70 minutes. The most probable VEF precursors were observed 7 h to 12 h prior to earthquakes. Processes like cosmic rays, solar radiation, light- ning discharge, natural ground radioactivity and aerosol concentration all contribute to the air ionization and consequent latent heat release due to water vapor con- densation on the newly formed ions, and hence the re- sulting changes in the VEF [Pudovkin and Raspopov 1992, Svensmark and Friis-Christensen 1997, Rulenko 2000, Freund et al. 2009]. Thus it is necessary to be sure enough to identify the precursor effects on the VEF from among all of these effects. Accordingly, at our lo- cation, the fair-weather VEF graph looks something like that shown in the top panel of Figure 2, which shows a quiet background level of about 120 Vm-1 to 170 Vm-1 at local morning and noon hours, with a fur- ther increase at the evening up to 250 Vm-1 to 300 Vm- 1, the dynamic range of which is consistent with other land observations [Mikhailov et al. 2002, 2003, Smirnov 2005]. When a cloud passes over our receiver and with no precipitation, the VEF reading is about 4000 Vm-1 to 4500 Vm-1, while with lightning and precipitation the VEF shows relatively chaotic positive and negative de- viations from the mean diurnal behavior of about 6000 Vm-1. In contrast, the anomalous variations of VEF for these earthquake precursors during fair-weather days is about 500 Vm-1 to 800 Vm-1. Thus the observed VEF variation is well distinguishable from local cloud pass- ing over the sensor, and from the local lightning and thunderstorm activity. As a result, in the total duration of our analysis, we found a total of 32 cases of earth- quakes during fair-weather days and among these only 10 showed VEF variations in the form of bay-like de- pressions in the VEF magnitude, which were seen as a precursory for an earthquake. A positive dependency was found between earth- quake class and VEF bay depth, which is shown in Figure 4. This signifies that with larger earthquake magnitudes (since the depths were almost equal in our observations; see Table 1) the VEF bay depth will be more. Whereas a negative correlation was observed between the earth- quake class and the VEF bay duration, which highlights that as the earthquake magnitude decreases, the dura- tion of the VEF bay variation increases, and vice versa. Our observations here contrast with those of Smirnov [2008], who monitored the Kamchatka seismicity, where there was no dependence of bay duration and bay strength on earthquake class or distance from the epi- center. However, we also observed a low dependency be- tween VEF bay duration and depth with the distance of the earthquake epicenter from the observation point. The probable cause might be due to some difference be- tween the seismicity of Kamchatka and northeast India, which is located at the juncture of three tectonic plates: namely, the Indian, Eurasian and Australian (Figure 1). Among the various models proposed in the litera- ture, radon emanation is viewed as a significant cause of such VEF variations [Smirnov 2008]. Radon is a product of uranium decay series that has a short half-life of 3.8 days, and due to this, radon shows poor intrinsic mobil- ity [Singh et al. 2010]. As a consequence of radon ema- nation, long-living ion complexes of opposite signs are formed in the near ground layer of the atmosphere. Ions of different signs have different mobilities; generally the mobility of the negative ions is 1.3-1.4-fold more than for positive ones. Under the actions of the natural atmos- pheric electric field, the positive ions would tend to move to the surface of the Earth, where they would recom- bine, but because of their low mobility, after some time, a spatial layer of positive ions will be formed at the sur- face, whereas the negative ions will move vertically up- wards. In such a way, at the near ground, an ‘electrode layer’ can be formed, along with the local electric field, which diminishes the natural atmospheric electric field, which is known as the ‘electrode effect’ [Hoppel 1967, Pulinets and Boyarchuk 2004]. It is thus expected that the changes in surface radon emission would modify the sur- face atmospheric electricity conditions and potentially lower the ionospheric properties, via the weak fair- weather conduction current that carries the negative charge upwards throughout the troposphere and strato- sphere [Harrison et al. 2010]. Moreover, more recently, Freund et al. [2009] showed that the charge generated in stressed rock can travel over distances of several kilome- ters, to reach the surface and to enhance the air ioniza- tion, which consequently induces atmospheric electrical ATMOSPHERIC PG AS EARTHQUAKE PRECURSOR perturbations. However, air ionization by increased radon release before earthquakes might be only a small part of the total ionization balance, and the identifica- tion of the atmospheric processes that are initiated by earthquake preparation processes is part of short-term earthquake prediction [Pulinets and Ouzounov 2011]. The release of radon into the atmosphere is also thought to be closely related to the state of deformation processes in the surface layers of the Earth during the earthquake preparation phase [Rulenko et al. 1992, Ru- lenko 2000]. Among the few such models proposed, Morgounov and Maltsev [2003] suggested a model that generates a quasistatic electric field in the atmosphere by considering the formation of polarization charges on the walls of rock fractures. Whereas Alekseev and Aksenov [2003] pointed out that electrical conductivity between the air and the Earth depends on the conductivity of the fluids that fill the rock pores and fractures, which ulti- mately have effects on the atmospheric electric field. More importantly, at the lower latitude regions, under relatively geophysical conditions, the equatorial anom- aly or Appleton anomaly formation occurs as a regular daytime phenomena, and might lead to generation of an anomalous zonal electric field [Pulinets 2012], thus separating our location from the higher latitude regions. This might be a reason as to why in spite having some fair-weather days, we did not observed any precursors from the China region, which is above 30° North; i.e. outside the equatorial anomaly region. Sobolev [1993] stated that these VEF variations are short-term predictors and cannot be used over the long term. So generally in this type of VEF, the variation does not follow the empirical formulae of 100.43M for earthquake preparation zones. This has also been ob- served in our records as well. From Table 1, it can be seen that 8 of the 10 earthquakes showed precursor ef- fects even outside the earthquake preparation zone. Morgounov [2004] provided an explanation of this by means of nonuniformity of the stress and strain processes in the Earth crust before impending earth- quakes. Another point to be noted from Figure 1 is that the positions of almost all of the 31 of the 32 earth- quakes under study, both with and without precursors, occurred more or less near the tectonic plate bound- aries, although there was an exception in one case where the precursor was being observed, and its posi- tion was situated far to the west, near Mumbai, which is good distant from any tectonic plates (Figure 1). However the probable cause of this is still a matter of further investigation. These pre-seismic variations on the atmospheric electric field could be due to a surface ionospheric coupling mechanism [Kamogawa et al. 2004]. Kamogawa [2006] and Hayakawa [2006] sum- marized three possible coupling mechanisms. They highlighted that the gases released from the ground during plate motion before major earthquake shocks modulate the properties of the entire atmosphere. Sec- ondly, ground motions might also excite atmospheric gravity waves that propagate upwards. Lastly, electro- magnetic radiation produced by processes acting in the ground before earthquakes could also initiate ionos- pheric effects. Liperovsky et al. [2008] highlighted that different physical mechanisms acting together are nec- essary to explain the same precursor event as well. But Pulinets and Ouzounov [2011] raised questions against this, and they introduced a model known as the Litho- sphere–Atmosphere–Ionosphere Coupling (LAIC) model. This model discusses a mechanism that exists between different layers of the atmosphere, which might explain the linkage of events occurring simultaneously before an earthquake, between the ground surface, the at- mosphere and the ionosphere. So, from the above discussions, it is perhaps rea- sonable to suggest that radon emanation might not be the sole source of VEF variations before earthquakes. Although from Figure 8, for the ratio of earthquake magnitude to depth above 0.5, there is a possibility of getting a precursor; however, this is still hard to con- firm, and it might not always be a governing factor. A more complex mechanism deep inside the Earth crust could be responsible for all of these variations in the VEF, and this link might map its effects on the entire at- mosphere before an impending earthquake. Neverthe- less, the main objective of this study was only to demonstrate the VEFs from the earthquake prone zone of northeast India, and discuss the probable causes of these perturbations. As we do not have any radon-mon- itoring devices in the vicinity of the receiver to poten- tially correlate this with our observed results, we cannot at present verify this hypothesis. Furthermore, from Figure 3, it appears that 90% (i.e., 9 out of these 10 earthquakes) of the precursors appear before 14:00 IST and 70% (7 out of 10) earthquakes strike in be- tween 16:00 and 24:00 IST. This might not be mere co- incidence, as Pulinets et al. [1998] showed dependence of the local time on the seismic ionospheric variations. However, at this stage our main objective was the ratio of earthquake magnitude to the depth, so these further aspects are beyond the scope of the present study. 5. Concluding remarks The present study is the first preliminary statistical observation made from a region of the globe where no similar previous studies have been made. From the pres- ent analysis, good correlation was found between the VEF bay depth and bay duration with the ratio of the CHOUDHURY ET AL. 8 9 earthquake magnitude to the depth, while low depend- ence was found for both the distance of the earthquake epicenter to the observation point, and the time of the observed precursor of the earthquake, for both the VEF bay depth and VEF bay duration. Various sources have indicated that these changes in ionization are due to radon emanation or might be due to seismo-ionosphere coupling; we might have observed the decrease of VEF before earthquakes but no consensus has been reached. This makes the subject very interesting, but neverthe- less complex, which the need for extensive studies here. 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Geophys., 54, 818-827. *Corresponding author: Anirban Guha, Tripura University, Department of Physics, Suryamaninagar, Agar- tala, India; email: anirban1001@yahoo.com. © 2013 by the Istituto Nazionale di Geofisica e Vulcanologia. All rights reserved. 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