Appraisal of seismic noise scenario at national seismological network of India in COVID-19 lockdown situation

We evaluated seismic background noise at national network in India using PSD, Fourier spectra, Spectrogram, and HVSR approach, before and during the nationwide lockdown declared due to COVID-19 pandemic. The analyses were performed to understand characteristics of noise wave-field in such unprecedented situation and its effect on site response at the station. SBN in long period (> 20 s), primary microseism band (10-20 s) and secondary microseisms (1-10 s) performed well and the noise levels found within the new LNM and HNM. However, in short period (< 1 s) the variation in SBN performance found significant before and during the lockdown. We observed that the SBN at each site in short period (< 1 s) is found to be about 10-12 dB noisier in the time segment prior to the lockdown. The HVSR analysis of SBN at recording sites clearly indicates that the predominant frequency for the entire region remains stable and independent of seismic noise generated before or during lockdown. A substantial variation in amplification factor, however, observed in either situation. Most of the stations across the country experienced diminished cultural noise subsequent to declaration of lockdown on 25 March 2020. Such drastic decrease in cultural noise significantly enhanced the performance of noisy stations, and the best recording stations picked the seismic phases originated from micro to small earthquakes. We suggest installation of seismometers at some depth below the surface, particularly at disturbed sites, may substantially reduce short period noise in earthquake recording.