A widely felt Tremor (ML 3.5) of 12 April 2020 in and around NCT Delhi in the backdrop of prevailing COVID-19 pandemic lockdown: analysis and observations

Abstract

An earthquake of small magnitude (ML3.5) occurred on 12 April 2020 near the east district boundary of NCT, Delhi with maximum PGA for the event observed to be 14.13 gals. A few smaller aftershocks also occurred in the area. The estimated fault plane solution of the mainshock suggests normal faulting with some strike slip component. The focal mechanism corroborates with the NE - SW orienting lineaments mapped in the region near the epicenter. The source parameters of the event, namely, seismic moment, stress drop, corner frequency, and source radius are estimated to be 1.15 x 1014 N-m, 25.7 bars, 5.7 Hz and 300 m, respectively. The decay rate of acceleration with epicentral distance suggests a regression relation PGA = 474 D −1.347, which may be useful for understanding the ground motion in the region. A noise analyses at NDI rock and UJWA soil sites clearly suggest a significant reduction in ambient noise by ∼10 dB in the frequency band (1.0–10.0) Hz at the respective sites, during the COVID-19 lockdown situation. The reductions of the noise level improve the signal to noise ratio substantially at all the seismic stations located in the urban agglomerations, which enabled the recording of clear phases of the event and hence improved the analysis.