ABSTRACT
Recently, amid the pandemic of COVID-2019, the north-east Delhi region experienced two small earthquakes in a short span of 1 month;the first occurred on 12th April 2020 (Mw 3.5) and the other on 10th May 2020 (Mw 3.4). These events were followed by 4 aftershocks of magnitude Mw ≤ 3.0. We carried out morphotectonic (high stream length-gradient index) and static Coulomb stress failure analyses to delineate the hidden causative fault(s) in the region. In the study, ASTER DEM data of 30 m resolution and Survey of India (SoI) toposheets on 1:50,000 scales were used for morphotectonic analysis. The analysis depicted a very high stream length-gradient (SL) and fall in elevation in the epicentral area, suggesting the area to be tectonically active with a NE-SW trending fault line. In addition, the nature of static Coulomb failure stress contours for both the main events, Mw 3.5 and Mw 3.4, suggests an NNE-SSW trending high Coulomb stress regime. Such a high coulomb stress regime is obvious at the location where a high SL index and fall in elevation were marked, which clearly indicates the presence of NNE-SSW trending a causative fault, named ‘Khanpur-Japti fault’.
ABSTRACT
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. © 2020 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.