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The data related to lithospheric electromagnetic emissions such as ultra low frequency (ULF, f = 0.01 Hz–10 Hz) and very low frequency (VLF, f = 3 kHz–30 kHz) emissions are analysed in relation to three major earthquakes (M = 6.8–8.5) occurred in Indian subcontinent during the years, 2011–2013. The equipments used to collect the data are thee component search coil magnetometer installed at Agra station (geographic lat. 27.2° N, long. 78° E) and indigenously developed amplitude measurements using a borehole and terrestrial antenna at Mathura station (geographic lat. 27.5° N, long. 77.68° E) about 70 km North of Agra station. The results show that ULF anomalies occur in the form of amplitude bursts 9 to 16 days prior to main shocks of the earthquakes whereas VLF amplitude anomalies occur coseismic and 3 days before recorded by the terrestrial antenna (the anomalies are weak and do not occur clearly in borehole antenna). These results suggest that ULF emissions (f < 0.1Hz) can travel to the observing station both via crustal and atmospheric regions, whereas VLF emissions can travel via atmospheric region only. The ULF data are compared with those obtained at two other stations of Shillong and Kachchh and malfunctioning of the sensors is ruled out. The effect of magnetic storms and lightning are not found to influence the data also. The precursory periods of 9–16 days are of the same order as 7–15 days reported by Singh et al. (2018) from the study of ionospheric perturbations caused by the same earthquakes. Hence, this study provides an interesting information about lithosphere–ionosphere coupling. [ABSTRACT FROM AUTHOR]

We have analyzed ULF magnetic field emissions data obtained from identical 3- component search coil magnetometers installed at Indian stations of Agra (Geograph. lat. 27.2°N, long. 78°E), Shillong (25.92oN, 91.88oE), and Kachchh (23.78oN, 71oE) for the months of April 2012 and September 2013 in which three major earthquakes occurred, one of magnitude M = 8.5 in Indonesian region and two of magnitudes M = 7.4 and 6.8 in Pakistan. The results of the analysis show the occurrence of ULF bursts 09-16 days prior to the occurrence of these earthquakes at Agra and Shillong stations. Such bursts are not observed at Kachchh station. Analysis of ULF data obtained from a flux gate magnetometer at a remote station Kakioka in Japan (Geograph. lat. 36.2oN, long. 140.2oE) does not show such burst also. The bursts contain electromagnetic pulses of periods 10–15s (f = 0.06–0.1 Hz). The origins of these bursts in lithospheric sources (earthquakes) are confirmed by polarization parameter analysis of data. These ULF data are not influenced by lightning and magnetic storms. A long period analysis of the ULF data for the same months between the years 2010 and 2017 shows that occurrence of such bursts are characterized by either large magnitude shallow depth earthquakes (M > 6) or multiple shallow depth earthquakes of low magnitudes (M < 6). The precursory periods of the recorded ULF bursts match very well with those reported by Singh et al. (2018) who analyzed very low frequency (VLF) and ionospheric total electron content (TEC) data recorded at Agra station. • ULF magnetic field emissions are investigated in relation to major EQs. • ULF data have analyzed over multiple stations. • Lightning activities have also been examined. • Effect of Magnetic storms have also seen. [ABSTRACT FROM AUTHOR]

Measurements of amplitude of vertical components of very low frequency (VLF) electric field emissions have been in progress at Chaumuhan, Mathura (27.5°N, 77.68°E), Uttar Pradesh, India, at the frequency of 3.012 kHz using a borehole antenna since 24 March 2011. In the present study, we have statistically analysed the VLF data from 1 January to 31 July 2015 using the mean (m) and standard deviation (s) around the mean (m ± 2s) criterion with respect to two major earthquakes (M = 7.8, 7.3) and their aftershocks that occurred on 25 April and 12 May 2015 in Khudi and Kodari regions of Nepal respectively. Results of the analysis show a spiky enhancement above the m + 2s line in the daily variation of the normalized VLF data, 7-14 days before the onset of main shocks of these earthquakes. These anomalous enhancements in the normalized values of amplitude are examined in light of various other possible causative sources such as magnetic storms, local lightning, solar flares and solar eclipse, rainfall, wind speed, temperature and pressure of the atmosphere. It was found that the VLF amplitude anomalies are not related to these sources. Further, we have developed a theoretical model for generation and propagation mechanisms of VLF emissions associated with the said earthquakes and their aftershocks by assuming an ensemble of elementary radiators randomly oriented and distributed in space and time in the preparation zones of these earthquakes. The computed electric field from the model is in good agreement with the observations. [ABSTRACT FROM AUTHOR]

Abstract: The preliminary results of a collaborative study on the amplitude variation of 24kHz sub-ionospheric NAA VLF transmitter signals transmitted from Cutler, Maine (Lat. 44.6°N, Long. 67.2°W) and monitored simultaneously at Budapest (Lat. 47.5°N, Long. 19.17°E), Hungary and Agra (Lat. 27.2°N, Long. 78°E), India are presented. The time segments of the propagation paths are so chosen that they lie in the post-midnight hours over Budapest and sunrise hours over Agra. The results show that the amplitude at Budapest decreases after midnight hours by about 3dB whereas the same at Agra increases by 5dB during sunrise normally. The anomalous enhancements and reductions in the amplitude variation during the three month period of July–September 2002 along Cutler–Agra great circle path (GCP) are examined in the light of seismic, solar flares, and magnetic storm effects. It is found that the occasional amplitude reductions are caused by earthquakes (M >5) which occurred along the GCP, and the enhancements are caused by solar flares. The magnetic storms do not seem to influence the data except in the case when associated with large solar flares. [Copyright &y& Elsevier]

Abstract: Employing a GPS receiver we have monitored ionospheric total electron content (TEC) at Agra (Geographic Lat. 27.2°N, Long. 78°E), India since 24 June, 2006. In this paper we have analysed 15months of data for the period between 01 September, 2006 and 30 November, 2007 statistically. The results show anomalous depletions and enhancements which are examined in the light of magnetic storms and earthquakes. As magnetic conditions have remained quiet throughout the period of data except some sporadic cases, majority of the TEC anomalies are interpreted as due to earthquakes. We find that out of 43 cases of moderate and high magnitude earthquakes (M ⩾5.0) there are 23 cases in which both depletions and enhancements occurred and in the remaining 17 cases (no data available in three cases) either of enhancement or depletion occurred. In general the anomalies occurred 0–9days before the earthquakes as precursors. The correlation coefficient of 0.8 is found between the two sets of data which satisfy null hypothesis. A mechanism responsible for TEC anomalies under the influence of earthquakes is discussed also. [Copyright &y& Elsevier]

The experimental data obtained from three different techniques such as (i) subsurface VLF electric field measured with the help of a borehole antenna (ii) total electron content (TEC) of the ionosphere measured with a GPS receiver and (iii) VLF amplitude of subionospheric fixed frequency (f = 19.8 kHz) NWC (Naval Communication Station) transmitter signals monitored with SoftPAL receiver are analysed in relation to two major Nepal earthquakes occurred on 25 April (M = 7.8) and 12 May (M = 7.3), 2015. The VLF electric field data and fixed frequency VLF transmitter signal data are taken from Mathura and Agra stations in India whereas TEC data are taken from three IGS stations, one of which Lucknow (Lck3) is located in India and others Lhasa (Lhaz) and Wuhan (Wuhan) are located in China, all within 800 km from epicenters of two earthquakes except Wuhan which is located at about 2865 km. The results of the analysis indicate that while borehole and TEC data show anomalies in normal variations within 8–15 days prior to the earthquake of 25 April, the same occur within 5–7 days prior to the earthquake of 12 May. The subionospheric variations of VLF amplitude also show an anomaly 4 days before the later earthquake. Occurrence of these anomalies is examined critically in the light of local lightning and magnetic storms. However, these factors are not found to influence the data. A brief description of generation and propagation mechanisms of seismogenic emissions is also presented. [ABSTRACT FROM AUTHOR]