Investigation of fair weather atmospheric electric field variations from the Eastern Himalaya Syntaxis in North-East region of India

Abstract

We examine here variations of fair weather atmospheric electric field (AEF) referred to as potential gradient (PG), recorded at Namsai (27.69°N, 95.85°E) in the Eastern Himalaya Syntaxis. The work is based on data collected over 99 fair weather days spanning from November 2017 to April 2019. We aim to assess the general characteristics of PG over Namsai and further explore the possibility of deciphering seismic-associated signatures in PG. Our analysis indicates that the mean diurnal variation of fair weather PG peaks at ∼14:00 UT. Seasonal analysis shows that PG values maximized during the winter, followed by the equinox and summer. These characteristics of PG are explained in terms of variations in meteorological parameters, air pollution level, and geographical location of Namsai, i.e., the Brahmaputra river nearby and the deep forest surrounding it. We further report a significant variation in PG before the Mw 6.4 earthquake on November 17, 2017, which occurred in a blind fault near the Main Central Thrust. PG showed a bay-shaped negative anomaly lasting ∼2 h, observed ∼7 h before the event. The anomaly exceeded the 2σ limit of mean fair weather PG. The earthquake day was devoid of any significant meteorological changes. It is suggested that the PG anomaly was associated with earthquake rather than local weather changes. This study not only provides better insights into the general variability of fair weather PG over this region but also highlights the importance of AEF measurements as a potential indicator of seismic events, sensing as precursory signals several hours ahead.