Aftershock Activity and Frequency-dependent Low Coda Qc in the Epicentral Region of the 1999 Chamoli Earthquake of Mw 6.4

On 28 March, 1999 (19:05:10.09, UT) a significant earthquake of Mw 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 20ukm. 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 15ukm and then propagated updip along a NE-dipping thrust plane. Further, the epicentral distribution of aftershocks defines a marked concentration near a zone where main central thrust (MCT) takes a significant 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 five 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 61,500ukm2 with a radius of 140ukm, is studied by estimating the coda Qc 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 Qo values at different stations suggest on average a low value of the order of (30u-u0.8), indicating an attenuating crust beneath the entire region. The frequency-dependent relation indicates a relatively low Qc at lower frequencies (1–3uHz) that can be attributed to the loss of energy due to scattering on heterogeneities and/or the presence of faults and cracks. The large Qc 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 20ukm distance and to 7% at 100ukm. Hence, even 1g acceleration at the source may not cause significant damage beyond 100ukm in this region.