No abstract

Ð On 28 March, 1999 (19:05:10.09, UT) a signi®cant earthquake of M w 6.4 occurred in the Garhwal Himalaya (30.555°N, 79.424°E). One hundred and ten well-recorded aftershocks show a WNW-ESE trending northeasterly dipping seismic zone extending from a depth of 2 to 20 km. As the main shock hypocenter occurred at the northern end of this seismic zone and aftershocks extended updip, it is inferred that the main-shock rupture nucleated on the detachment plane at a depth of 15 km and then propagated updip along a NE-dipping thrust plane. Further, the epicentral distribution of aftershocks de®nes a marked concentration near a zone where main central thrust (MCT) takes a signi®cant turn towards the north, which might be acting as an asperity in response to the NNE compression due to the underthrusting of Himalayan orogenic process prevalent in the entire region. Presence of high seismicity including ®ve earthquakes of magnitude exceeding 6 and twelve earthquakes of magnitude exceeding 5 in the 20th century is presumed to have caused a higher level of shallow crustal heterogeneity in the Garhwal Himalaya, a site lying in the central gap zone of the Himalayan frontal arc. Attenuation property of the medium around the epicentral area of the 1999 Chamoli earthquake, covering a circular area of 61,500 km 2 with a radius of 140 km, is studied by estimating the coda Q c from 48 local earthquakes of magnitudes varying from 2.5±4.8. These earthquakes were recorded at nine 24-bit REFTEK digital stations; two of which were equipped with three-component CMG40T broadband seismometers and others with three-component L4-3D short-period seismometers. The estimated Q o values at dierent stations suggest on average a low value of the order of (30 0.8), indicating an attenuating crust beneath the entire region. The frequency-dependent relation indicates a relatively low Q c at lower frequencies (1±3 Hz) that can be attributed to the loss of energy due to scattering on heterogeneities and/or the presence of faults and cracks. The large Q c at higher frequencies may be related to the propagation of backscattered body waves through deeper parts of the lithosphere where less heterogeneities are expected. An important observation is that the region north of MCT (more rigid highly metamorphosed crystalline rocks) is less attenuative in comparison to the region south of MCT (less rigid slightly metamorphosed rocks (sedimentary wedge)). The acceleration decays to 50% at 20 km distance and to 7% at 100 km. Hence, even 1g acceleration at the source may not cause signi®cant damage beyond 100 km in this region.

Seismographs record earthquakes and also record various types of noise, including anthropogenic noise. In the present study, we analyse the influence of the lockdown due to COVID-19 on the ground motion at CSIR-NGRI HYB Seismological Observatory, Hyderabad. We analyse the noise recorded a week before and after the implementation of lockdown by estimating the probability density function of seismic power spectral density and by constructing the daily spectrograms. We find that at low frequency (<1 Hz), where the noise is typically dominated by naturally occurring microseismic noise, a reduction of ~2 dB for secondary microseisms (7–3 s) and at higher frequency (1–10 Hz) a reduction of ~6 dB was observed during the lockdown period. The reduction in higher frequencies corresponding to anthropogenic noise sources led to improving the SNR (signal-to-noise ratio) by a factor of 2 which is the frequency bandwidth of the microearthquakes leading to the identification of microearthquakes with Ml around 3 from epicentral distances of 180 km.