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

Abstract Identifying pre-seismic atmospheric and ionospheric anomalies holds significant 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 significant 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.