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 is of research importance but also meets difficulties, especially for earthquakes with varying magnitudes, focal depths and focal mechanisms. In this paper, atmospheric‒ionospheric disturbances associated with earthquakes in Nepal (April 25, 2015, M = 7.8 and May 12, 2015, M = 7.3) are investigated using atmospheric and ionospheric parameters. Ionospheric (vertical total electron content (VTEC)) and atmospheric (outgoing long wave radiation (OLR), cloud mask, vertical temperature gradient (VTG)) parameters are archived from IGS GPS stations and INSAT 3D data from Indian Meteorological Department (IMD) of Ministry of Earth Sciences, Government of India. The abnormal VTEC signal was noticed 3 days and 10 days prior to April 25, 2015 event and 2 days and 6 days prior to the May 12, 2015 event. Inter-quartile range (IQR) and associated running median over one day were determined as the upper limit reference to a signature of VTEC for the 51-days period of the Nepal earthquake, it can be clearly observed that the total electron content (TEC) has increased from the limits of UB (upper bound) at the stations closest to the earthquake epicentre such as LCK-4, LHAZ than the far stations such as IISC, HYDE, SGOC and URUM. Prior to these earthquakes, UB observed a 54‒60% increase in relative amplitude of VTEC. The overall geomagnetic storm condition was thoroughly examined using the global planetary index (K_p) and storm time disturbance index (D_st) over the 51-days period. The IQR range method was used to analyse its abnormal positive and negative signals. We found no geomagnetic signatures caused by geomagnetic storms during the seismic regime The OLR varied from 240 to 340 watts/m² observed 4 days before the event. The vertical temperature gradient varied from 4.3 to 23.2 K. Daily variations of 51 days for the OLR showed good anomalous atmospheric responses a few days before the event. The shallow depth of the earthquake gives the best coupling, releasing a large amount of energy from the seismic zones, and could be a causal factor in the enhancement of anomalous VTEC patterns.