Observation of Atmospheric and Ionospheric Anomalies before the Nepal Earthquakes on 25th April and 12th May 2015

Identifying pre-seismic atmospheric and ionospheric anomalies holds signi�cant research importance but is fraught with challenges, particularly for earthquakes characterized by diverse magnitudes, focal depths, and focal mechanisms. This study investigates atmospheric-ionospheric disturbances associated with the Nepal earthquakes of April 25, 2015 (M=7.8) and May 12, 2015 (M=7.3), utilizing atmospheric and ionospheric parameters. Ionospheric parameters such as vertical total electron content (VTEC) and atmospheric parameters including outgoing long-wave radiation (OLR), cloud cover, and vertical temperature gradient (VTG) were collected from IGS GPS stations and INSAT 3D data provided by the Indian Meteorological Department (IMD). Notable VTEC anomalies were detected 3 and 10 days prior to the April 25, 2015 event and 2 and 6 days before the May 12, 2015 event. The study employed the inter-quartile range (IQR) and running median over one day to establish upper limit references for VTEC signatures during the 51-day period surrounding the Nepal earthquakes. The analysis revealed pronounced increases in VTEC near the earthquake epicentre, such as at stations LCK-4 and LHAZ, compared to more distant stations like IISC, HYDE, SGOC, and URUM. Prior to the earthquakes, a 54 ‒ 60% relative amplitude increase in VTEC was observed at these upper bound (UB) stations. Furthermore, examination of the global planetary index (Kp) and storm time disturbance index (Dst) over the 51-day period did not reveal any geomagnetic signatures attributable to geomagnetic storms during the seismic activity. OLR ranged from 240 to 340 watts/m2, observed four days preceding the event, while the vertical temperature gradient varied from 4.3 to 23.2°K. Daily OLR variations over the 51-day period exhibited signi�cant anomalous atmospheric responses a few days prior to the earthquakes. The shallow depth of the earthquakes facilitated enhanced energy release from the seismic zones, potentially contributing to the augmentation of anomalous VTEC patterns.


INTRODUCTION
On April 25, 2015, an earthquake of magnitude M =7.8 struck Nepal with an epicenter at Latitude 28.147°N; Longitude 84.708°E (77 km northwest of Kathmandu) and a focus depth of 8.2 km [1].This focal depth is considered shallow and more damage has occurred due to this event.The tremor lasted about fty seconds.Another M=7.3 earthquake occurred on May 12, 2015 with an epicenter 76 km eastnortheast of Kathmandu.These earthquakes may be caused by the release of stress buildup in the Himalaya due to the ongoing Indo-Eurasian plate collision.The ground motion caused by this earthquake created major disturbances in the atmosphere and ionosphere.The acoustic waves generated by the induced pressure vary with the vertical movement of the ground [2].Seismic waves with su ciently long periods of more than 10 s for higher magnitude earthquakes give the good signatures in the atmosphere and the ionosphere [3].
Davies and Bakers [4] rst noticed ionospheric anomalies during Alaskan earthquakes, since then several scientists have attempted to investigate this problem using ground [5,6,7] and satellite [8, 9,10] observations.Signatures of earthquakes in the ionosphere can be tracked from the Total Electronic Content (TEC) due to the spatial distribution of 24 satellite coverage for each IGS GPS station.
There are two schools of thought that explain the reasons for the enhancement of GPS TEC before earthquakes.One school relates it to seismogenic origin [1,19,10], and another school relates it to global geomagnetic effects [8,36].
Pulinets [21] reported anomalous changes in the total electron content (TEC) of the ionosphere using the regional variability index before the Hector Mine earthquake on October 16, 1999 with magnitude M=7.1 (Southern California, USA).The main background variations in the ionosphere are due to solar and geomagnetic activities, seismogenic and volcanic activities [13,23].
Liu et al. [23] noticed that no signi cant change in the ionosphere component was noticed under conditions of relatively stable solar and geomagnetic activity.
Calais and Minster [5], Otsuka [24], and Thomas et al. [20] correlated increased TEC variations with the effects of the Sun-Earth interaction and geomagnetic disturbances.They further pointed out that the ampli cation is not related to seismic activity.In our study, we carefully considered all potential sources of effects that may have affected TEC variations.Liu et al. [6] proposed the quartile-based process to detect abnormal signals.Astafyeva et al. [17], Calais and Minster [5], Reddy and Seemala [25] reported that disturbances of ionospheric parameters have been observed after earthquakes caused by atmospheric waves induced by ground motion.
In this article, we discuss the possible sources of ionospheric anomalies, whether related to the Nepal earthquake or to solar activity, or to geomagnetic disturbances.
We followed the standard data processing procedure by removing the different biases for the minimum TEC uncertainty values for a given epoch [21,20].
We calculated the median (m) and standard deviation (s) for GPS TEC 24 days before and 26 days after the event.Then the upper bound (UB) is m+2*sigma and the lower bound (LB) is m-2*sigma.If the observed TEC values are above the upper limit or below the lower limit, this is considered an abnormal signal.
In this study, we attempted to investigate the spatial and temporal variations of vertical total electron content (VTEC) for the 51-day period as well as daily averaged atmospheric parameters such as

GPS-based TEC Data
We used GPS data from International Global Navigation Satellite Systems Service (IGS) stations [27] as shown in Figure 1 to calculate the vertical total electron content (VTEC).The GPS system collects data from 24 equally spaced satellites in six orbital planes around the globe at an altitude of 20,200 km.Each satellite sent a dual frequency with the L1 (1575.42MHz) and L2 (1227.60MHz) frequencies [28,21,25].
The ionospheric assessment was determined from carrier and phase modulations recorded by dualfrequency GPS receivers.The slant TEC (STEC, 1TEC = 10 16 electrons m -2 ) were calculated from the ephemerides on the F layer (~350 km) in latitude and longitude.
We can monitor changes in the ionosphere over time using GPS in high resolution datasets.In order to spatially distribute the GPS stations, we estimated the radius of the preparation zone (km) for this earthquake using the equation [8]: R=10 (0.43*M) .(1) In equation (1), M is the magnitude of the earthquake.The Nepal earthquake preparedness zone arises (~2259 km).We have selected ten GPS stations that are within this radius.These stations are divided into 5 zones (each zone is separated within a radius of 440 km around the epicentre of Nepal earthquake) (Fig. 1).
We examined 51-day GPS time series from these 10 stations with the 30-second sampling interval.The pre-and post-seismic effect is synthesized by taking 24 days before the event and 26 days after the event in Nepal.An aftershock event near the Nepal earthquake (M=7.3) that occurred on May 12, 2015 is also considered in our study.We derived the Vertical Total Electron Content (VTEC) ionospheric parameter of the 10 GPS stations (LCK-4, LHAZ, HYDE, IISC, URUM, POL-2, KIT-3, PBRI, SGOC, and CUSV).
TEC derived from dual frequency GPS locations from [30].The effects of clock errors and tropospheric water vapour are eliminated from the relative values of STEC by calculating the differential code bias [31].
There are currently 28 GPS satellites orbiting the earth at an inclination of 55° and an altitude of 20 200 km.They broadcast information on two frequency carrier signals which are 157542 GHz (denoted f 1 ) and 12276 GHz (denoted f 2 ).The pseudo and phase information of the dual frequency signal indicates the electron content of the ionosphere.The dispersive nature of the ionosphere tends to delay the signal sent from the satellite.
The STEC slant path from a satellite to a receiver can be obtained from the difference between the pseudo ranges (P 1 and P 2 ) and the difference between the phases (L 1 and L 2 ) of the two signals [28].
where K, linked to ionospheric refraction, is 80.62 (m 3 /s 2 ).λ 1 and λ 2 are the wavelengths corresponding to f 1 and f 2 respectively.For the calculation of VTEC from STEC, the ionosphere was assumed to be a thin shell surrounding the earth.The intersection of the oblique paths from the satellite to the receiver through the ionosphere is called the ionospheric piercing point (IPP).The zenith angle χ is expressed as follows.
where α is the elevation angle of the satellite; R E is the mean radius of the Earth and h is the height of the ionospheric layer, which is assumed to be 400 km.Also, if you set the satellite and receiver polarizations to b s and b r , respectively, the vertical TEC VTEC = (STEC -(b s +b r ) • Cos (χ).( 4) IGS TEC data is maintained and monitored by the International GNSS Service (IGS).These data are accessible via the FTP (File Transfer Protocol) [32].
As a rule, the IGS website makes GPS data available in compact RINEX format [33].Using FORTRAN code [34], we converted it to normal RINEX format.The distortion of the different satellites has been corrected using the Differential Code Bias (DCB) provided by IGS [35].Receiver distortion removal was performed using a Kalman lter [31].

Atmosphere Parameters
Atmospheric parameters archived from Indian National Satellite (INSAT 3D) data from Indian Meteorological Department (IMD).INSAT-3D is the rst advanced satellite launched on July 26, 2013 in India to be in the geostationary orbit and has wide coverage of marine and land region.It provides ne resolution vertical pro les of various atmospheric parameters over India and surrounding regions.The coe cients of the regression equations are determined from the results of the simulation of the radiative transfer model with different atmospheric conditions.

Global Magnetic Data
To examine global effects on the GPS time series, we archived K p and D st from the Schmidt Geomagnetic Observatory (Niemegk of the Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences).Observation of H-, D-, Z-component ground magnetometers provides information on geomagnetic disturbances.The relative amplitudes every 3 hours of the horizontal magnetic eld at 13 sub-auroral stations give the planetary index K p .The K p index and its associated indices (A p and C p ) are commonly used in ionospheric and magnetospheric studies and are widely recognized as indices measuring global geomagnetic activity.Temporal variations of the geomagnetic index parameter (such as K p and D st ) are examined during the study period to distinguish quiet days from disturbed days (Figure 2).

Background Geomagnetic Conditions for the Present Study Period
The changes on earth like volcanic and seismic activity could trigger VTEC ionospheric enhancement.
The D st index represents the low latitude disturbance of the annular magnetic eld intensity in nT and K p represents the average geomagnetic intensity on a scale of 0 to 9. The analysis of the global parameters shows Global Parameters of K p and D st Variations for the 51 days from the 1 st of April 2015 along with the Positive and Negative Anomalies derived from the Interquartile Range method, noted as ∆D st and ∆K p identi ed two major storm conditions during study period (i.e., days 6 and 47) [36] (Fig. 2).
The D st index has dropped drastically to -190 nT and -180 nT these days respectively.After two days of recovery, it appears to be a large storm capable of disrupting both the magnetosphere and the ionosphere.We noticed the positive anomaly in K p during the two-storm events and the negative anomaly in D st for the two-storm events.Lee et al. [36] and Jacobs et al. [37] indicated that K p <4 and D st >-50nT could be called geomagnetically quiet periods.In the present study, there is no appreciable solar geomagnetic activity on the ionosphere for 5 days before the events and for a period of 10 to 16 days.

Ionosphere VTEC Variations
We rst examined the in uence of global geomagnetic effects on daily uctuations recorded in GPS data.The total vertical electronic content (TECU) is derived from GPS data and the daily mean values of VTEC at these stations are calculated (Figure 3).
The study period can be divided into two categories, geomagnetically disturbed days and quiet days.The K p values increased on days 6 and 47 (K p ~9) and D st ~-180nT.These two days are considered geomagnetically disturbed days.At 5 days before both events (i.e. the 25 th day; and the 42 nd day) and the period from 10 th to 16 th days, no signi cant enhancement in the global parameter was detected in raw data and anomalies.These days therefore became quiet days.
The in uences of the geomagnetic disturbance on the VTEC of all stations are clearly visible at days 6 th and 47 th in 10 stations IGS GPS (Fig. 3).
Galav and et al. [38] and Kumar and Singh.[39] reported of the ampli cation of TEC during geomagnetic storms at low and medium latitudes.
Rama Rao and et al. [40] reported of the temporal in uence of geomagnetic storm on the navigation system due to GPS-TEC at different latitudes in the Indian continent.The increased delay during periods of thunderstorm activity re ects the signi cant increase in TEC in EIA areas.The number of phase shifts in the TEC GPS signal can be seen during the storm weather regime.
The ten GPS locations used in this study can be divided into two categories based on their spatial distribution, namely Equatorial Ionization Anomaly (EIA) Region and Non-Equatorial Ionization Anomaly (Non-EIA) Region.The HYDE, IISC, SGOC, PBRI and CUSV GPS locations are in the region of the Equatorial Ionization Anomaly (EIA).
Due to E×B vertical drift; this region is characterized by an increased electron density at magnetic latitudes of 10°± to 20°± in the F region [43].AIE can result in a strong latitudinal gradient of the TEC gradient on the equator side and on the polar side, the latter being more intense [42].
Our results are consistent with the EIA, VTEC values ranged from 30 to 90 TECU in the EIA region and from 24 to 45 TECU in the non-EIA region.
On the 6 th day, a geomagnetically disturbed day, the increase in VTEC at many stations KIT3, POL2, URUM, LHAZ, LCK4 is observed, about 60% sharp rise compared to the quiet value (Fig. 3).
On the 47 th day, during a strong magnetic storm, ten IGS stations showed gradual increase in VTEC.The observed VTECs for 10 IGS stations were categorized in ascending order relative to the epicenter in the earthquake preparedness zone [29].
The two event regimes in the present analyzes are indicated by a dotted line in the raw data.The rst dotted line represents magnitude M=7.8 and the second dotted line represents magnitude M=7.3 (Fig. 3).
The observed VTEC showed good peaks during geomagnetic storms K p >5 and D st <-50 nT and sub geomagnetic storms K p >4 and D st around -40 nT.On the 6 th and 46 th day we noticed a very good increase in the signal of 60% at KIT3, POL-2, URUM, LHAZ and LCK-4 compared to the other stations.
Lee et al. [20] and Huang [22] also reported the enhancement of VTEC during substorm conditions consistent with global solar activity.
We haven't registered any anomalous magnetic storm conditions on days 15, 22, 26, 28, 36, and 40 at the stations LCK-4 and LHAZ.We observed spikes in the raw data in nearest stations where as we were not noticed such spikes by other remote stations.We saw an increase in observed VTECs of 30-60% over previous days for nearest IGS GPS stations.Such spikes were not noticed by GPS stations in other areas.The small variation in VTEC (20 to 30%) is generally attributed to the diurnal variability of the ionosphere [41].We could not distinguish the VTEC variation due to global disturbances based on station locations in the EIA region and the non-EIA region, and they showed a similar trend in both regions during this period.
The nearest stations LCK-4 and LHAZ showed a signi cant increase in their VTEC value from 28 to 38 from April 22, 2015 their increase is about 45-50% during quiet days, two days before the event on April 25, 2015, then decreased gradually and returns to normal after the earthquake.Since the earthquake in Nepal occurred at shallow depth, there are three possible explanations for the anomalous variations in TEC, such as: -Acoustic shock waves during topside vibrations [15]; -The electric eld generated by the voltage variation in the rocks of seismic zones [45]; -Release of radon into the lower atmosphere [11].
The release of radon gas from the micro cracks formed in the crust and the surface seems to be the third possible ampli cation of the pre-earthquake TEC [11].Therefore, the observed change in VTEC can be correlated with the seismogenic variation just before the earthquake since there is less in uence of the global geomagnetic effect on the VTEC.
After the main earthquake of April 25, 2015, the major aftershock occurred on May 12, 2015 (M=7.3,Lat: 27.809 0 N, Lon: 86.066 0 E, D: 15 km).This aftershock is also included in our current analysis to study ionospheric variations during these two events 150 km apart.During the main shock, we observed signi cant variations in VTEC at stations in Zone-1 and Zone-2 three days before the main shock occurred on the 25 th day, while we observed sharp peaks on days 36 and 40 before the May 12 th aftershock (42 nd day).Deviation from this distribution could be related to strong geomagnetic solar activity and impending seismic events.After the event, the signi cant disturbance was observed in area 1 and area 2 on April 25, 2015, and May 12, 2015 (Fig. 3).

Anomalous VTEC from UB, LB s
The deviation of the daily values of the total electronic content near an epicenter a few days before the main shocks has been observed by many researchers [7,24,19,21,16,55].
For detection of the abnormal signal in the ionospheric parameters, Liu et al. [14] proposed an analysis of protocol based on quartiles, which has been adopted to study the precursor signatures of certain earthquakes [46].
The hourly median (M), lower ( rst) quartile (LQ) and upper (3 rd ) quartile (UQ) for the 24 days before and 26 days after the Nepal event were calculated for the same Universal Time (UT) for each station during the study period.
Taking into account the normal distribution with mean (m) and standard deviation (s) for the GPS-TEC, the expected values of M and LQ and UQ are respectively denoted by m and 1.34 s [47].
The lower bound (LB)=M-1.5(M-LQ)and the upper bound (UB)=M+1.5(UQ-M)are calculated for any TEC anomalies.Here, the probability of the observed TEC in the interval (UB, LB) is about 54%.The UB, LB and TEC variations observed 24 days before and 26 days after the event (Figure 4).
The VTEC as well as the upper and lower bounds for ten IGS stations is shown in Fig. 4. The panels in Fig. 4 are arranged in ascending order of epicentral distance from respective stations.Abnormal VTECs were measured according to the standard protocol of Liu et al. [6].The blue dotted line shows the upper bound, the red dotted line shows the lower bound, and the green line shows the observed VTEC (Fig. 4).
The days of the Nepal earthquakes were mentioned with the dotted line at days 25 and 42.
If the observed VTEC crosses UB, it shows the anomalous situation in the ionosphere.The abnormal situation occurs during the disturbance from both external and internal sources.During the days of geomagnetic storms (6 and 47) and substorms (1,18,20,31,43), we noticed an increase in VTEC on the UB signal.But apart from the external in uences of geomagnetic storms and substorms, we also observed a signi cant increase in VTEC on the UB in LCK4 and LHAZ (i.e.Zone-1 and Zone-2) during the 15 th , 22 nd , 26 th , 36 th and 40 th days.
As explained in the previous section, there is no external in uence (global parameter) on these days and there is no positive or negative anomaly in both K p and D st on these days.On obtaining results, we have two sources: an external source which is a global phenomenon and should be replicated in all areas of EIA and non-EIA; the second cause of disturbance is earthquake-induced activity, which is local and expected to spread depending on the magnitude [29].
So, in our current analysis, we xed a good increase in stations in Zone-1 and Zone-2, which are near to the Nepal earthquakes M=7.8 and M=7.3, respectively.We identi ed as a precursor signal for the 25 th event on day 22 nd (three days before) and day 15 th (10 days before; more distinct) and for the 42 nd event of M=7.3 on day 36 th (6 days before) which is clearer signal and such variations have not been observed in stations of other zones.
The ionized atmosphere of the Earth consists of several layers, namely mesosphere (60-85 km), E (85-155 km), F (155-550 km), with the main contribution coming from the F-layer.This electron density varies in the equatorial and polar regions for different reasons, with the equatorial regions being affected by geomagnetic activity while the Polar Regions are affected by ionization through the coupling of energetic particles and the magnetosphere.The production of photoelectrons in the ionosphere changed directly with solar irradiance.
Positive slope of the TEC from morning to noon is noted, and with the sudden loss of photoelectrons the negative slope of the TEC is noted from noon to night.The rate of electron production on atomic oxygen and molecular nitrogen could be the causal factor for the positive and negative slopes of the TEC from morning to night.The anomalies could be observed with effects around or after the date of the earthquake [6].
The variations of TEC over the 24-hour cycle can be considered as signals of a Gaussian distribution.The deviation from the Gaussian pattern of the TEC signal has been highlighted as showing the anomalous effect related to earthquakes observed by several researchers [48].Therefore, the average value of such a signal is far from zero.
The observed TEC increased from the limits of UB, LB in LCK4, LHAZ (Fig. 4).The relative amplitude of 54-60% of the observed VTEC of UB, LB is noted in the study.Afraimovich and Astafyeva [18] studied a few cases of preseismic precursors using TEC ampli cation a few days before earthquakes.They found that in some cases the enhancement may re ect global changes in ionization caused by solar and magnetic activity.While in a few cases they were correlated with local seismic activity.
Ouzounov and et al. [49] found that the source of the enhancement of TECs during the Tohoko earthquake was related to the seismogenic origin, as they got a very weak cross-correlation with the global geomagnetic eld.Positive and negative anomalies were calculated from the UB, LB analysis above as VTEC←UB and VTEC←-LB.These positive and negative anomalies have been represented in Figure 5 in ascending order as per the distance.The nearest stations received a stronger signal than the farthest stations.

Atmospheric Anomalies
The diurnal variations of various atmospheric parameters during the earthquake in Nepal are shown in the Figure 6.Atmospheric parameters such as outgoing long wave radiation (OLR), vertical temperature gradient (VTG) were archived from Kalpana satellite space grid data on the epicenter of the earthquake for 51 days.Increase in OLR from 240 to 340 watts/m 2 , increase in vertical temperature gradient from 4.3 to 23.20 K were observed before Nepal earthquakes.Among all parameters, OLR, AOT and vertical temperature gradients are signi cantly increased just before the quake with higher coe cients of variation (40-50%).
Only several researches have reported on the OLR variability in the seismic activity zone during the earthquake preparation period [6, 9,49].The rate of change of OLR and radiation is directly proportional to the thermodynamic process over seismically active regions [9].
An abnormal character of the OLR is constructed analogously to the anomalous thermal eld [50].Our results show that the OLR started an upward trend from April 15 th and peaked on April 24 th and 25 th just before the earthquake in Nepal.Increased tectonic activity in the seismic zone increases anomalous latent heat ux and eventually increases radiation rapidly.Such an increase in radiation has also been observed in most places of recent earthquakes [49,44], occurred in: -Japan (M=9.The observed increase in the OLR anomaly, VTG, is likely to be correlated with the signal from the earthquake preparatory zone before the earthquake in Nepal.

Spatial Variations of Atmosphere and Ionosphere Parameters
The spatial variation of TEC, OLR and VTG was investigated during the study period near the Nepal earthquake fault radius.The spatial coverage of VTEC over the study period is shown in Figure 7.
VTEC readings are between 25 and 30 TECU on April 24, 2015, just one day before the earthquake struck Nepal in the epicentral region.On the day of the earthquake, the TECU rose to 40-45.The following day, April 26, 2015, an increase in the level of TECU can also be observed.This is clearly recognizable by the signi cant changes in VTEC over the seismic zone.The one-to-one relationship in the ionosphere before a large earthquake gives the correct signature.
The spatial coverage of the OLR and the vertical temperature gradient over the study period is shown in Figure 8.The high-resolution datasets were archived from NCEP NCAAR Reanalysis-2 data [51].
The dense gridded variation of the OLR and vertical temperature was observed during the study period.
OLR values are in the range of 240-260 watts/m 2 on April 24, 2015, just one day before the Nepal earthquake hit the epicentral region.On the day of the earthquake, the OLR rose to 330-350 watts/m 2 .
The next day, April 26, 2015, also shows a high OLR value in the range of 280-290 watts/m 2 .The possibilities for enhancement in OLR are due to two reasons [52]: the rst possibility is that we can expect the increase by trapping all the outgoing long wave radiation; the other possibility is that an ampli cation of OLR by earthquakes and volcanic eruptions.
In our study, the OLR variability is in the range of 110-130 watts/m 2 , which may have increased due to the earthquake in Nepal.The increase in OLR near the earthquake site is due to the piezoelectric effect in addition to terrestrial emissivity [9].
Similarly, the vertical temperature variability is shown in Figure 9.We observed that the VTG values vary from 12 to 14 on April 24, 2015, just one day before the earthquake in Nepal in the epicentral region.On the day of the earthquake, the VTG rose in the following units to 22-24.The next day, April 26, 2015, an increase in the level of VTG in the range of 20-22 can also be observed.Therefore, thermal anomalies are also observed in the largest faults and areas where major earthquakes occur.

CONCLUSIONS
The ionospheric (vertical total electron content (VTEC)) and atmospheric (OLR, cloud mask, vertical temperature gradient (VTG)) parameters are studied from March to June 2015 to identify the pre-seismic signatures before the earthquakes in Nepal (April 25, 2015, M=7.8 and May 12, 2015, M= 7.3).VTEC data is obtained from data from 10 IGS GPS stations and atmospheric parameters are archived from INSAT 3D data from IMD. Interquartile range (IQR) and UB/LB methods are used to identify anomalies in the Total Electron Content (TEC) dataset and correlate with earthquakes in Nepal.The main results of this study are as follows.
1. Ionospheric disturbances such as vertical total electron content (VTEC) of a permanent GPS station closest to the earthquake epicentre in Nepal are signi cantly enhanced before the main event on April 25, 2015 and aftershock of May 12, 2015; and very large anomalous signals were noted 3 days and 10 days before the April 25 event and 2 days and 6 days before the May 12 event.

2.
EIA can result in a pronounced latitudinal TEC gradient on the equator and on the polar side, the latter being more intense, as indicated by high Vertical Total Electron Content (VTEC) for IGS GPS stations HYDE, IISC, SGOC, PBRI and CUSV compared to URUM, KIT-3, POL-2, LCK-4 and LHAZ stations.The anomaly of the geomagnetically disturbed day in the ionospheric data is clearly visible in our study.5.The ionospheric (vertical total electron content (VTEC)) and atmospheric parameters are found to be effective in identifying earthquake precursors before Nepal earthquakes.

ACKOWLEDGEMENTS
The authors are thankful to the Director General Dr. Sumer Chopra, Institute of Seismological Research (ISR), Gandhinagar, Gujarat, India for his encouragement, scienti c support and permit us to publish this work.Thanks to Indian Meteorological Department (IMD) of Ministry of Earth Sciences, Government of India (New Delhi, India) for providing us various atmosphere parameters from INSAT 3D [53].Thanks for the globally covered IGS GPS network [54] for having high-frequency data for the research purpose FINANCING There is no funding from any external project for the establishment of the multi parametric geophysical observatories for earthquake precursory research in Kachchh, Gujarat, India.Our Ministry Department of Science and Technology, Government of Gujarat has provided nancial support to establish multi parametric geophysical observatories for earthquake precursory research in Kachchh to study earthquake precursors.
outgoing long wave radiation (OLR), cloud mask, vertical temperature gradient (VTG) for the period of 51 days during earthquakes in Nepal (April 25, 2015, M=7.8 and May 12, 2015, M=7.3).Data from the permanent GPS stations of the IGS are used to calculate the total vertical electron content over a period of 51 days with a sampling interval of 30 seconds.Atmospheric parameters archived from Indian Meteorological Department (IMD) INSAT 3D data [26].DATA AND METHOLOGY Data from different experiments were used to study the anomalies of the earthquakes of April 25, 2015 and May 12, 2015 in Nepal.VTEC measured with 10 GPS receivers from IGS stations LCK-4, LHAZ, HYDE, IISC, URUM, POL-2, KIT-3, PBRI, SGOC, and CUSV.The outgoing long wave radiance (OLR) and vertical temperature gradient (VTG) were derived from the Indian Meteorological Department (IMD) of Ministry of Earth Sciences, Government of India (New Delhi, India) website [26] and included in the study.

3 . 4 .
The Vertical Total Electron Content (VTEC) increased above the upper bound and noticed positive anomalies in the following GPS stations (LCK-4 and LHAZ) and the VTEC increased to 20-35 compared to the previous days.Atmospheric parameters such as Outgoing Long wave Radiation (OLR), Vertical Temperature Gradient Anomaly (VTG) increased their values before earthquakes in Nepal.

Figure 1 Location
Figure 1

Figure 4 Observed
Figure 4