Spatio-temporal Variation of Ambient Noise in the Sikkim Himalaya

Ambient noise characteristics are studied at the newly constructed 27 broadband seismic stations in Sikkim Himalaya, installed to study the seismogenesis and subsurface structure of the Indian plate. Power spectral densities obtained for the ambient noise recorded in the vertical component of the seismometer lies within the defined global noise limits. The horizontal components tend to record noise higher than the global noise limits due to its higher susceptibility to induced tilts. Comparison of the installation methods reveals that the instruments installed above the ground records higher long-period noise than those installed below the ground. A change in the geography and population density across the region induces spatial and temporal variations in short-period and microseism noise. Day-time records higher cultural noise than night-time, while microseism noise dominates during monsoon. A study of the effect of the nationwide lockdown imposed due to the COVID-19 pandemic revealed a significant drop in the short-period noise recorded at stations located in regions with higher anthropogenic activity. Our study summarizes the overall ambient noise patterns, validating the stability and performance of the seismic stations across the Sikkim Himalayas.