Abstract

High-frequency strong-motion data of two recent major earthquakes of the Garhwal Himalayan region of India have been analyzed to determine the Qβ(f) for this region. Data from a local array of 24 stations, two main shocks, and several aftershocks were used to determine Qβ(f) and source parameters. An estimate of root mean square (rms) error is given. The data set in the present work includes the Uttarkashi earthquake of 20 October 1991 (Ms 7.0) and the Chamoli earthquake of 28 March 1999 (Ms 6.6) and its aftershocks. The source region of these earthquakes falls in the central seismic gap region of Himalayan frontal arc. We find the frequency-dependent Q-relationship for this region to be Qβ(f) = 112f0.97. The validity of the obtained Qβ(f) relationship is checked by comparing the source spectra at various stations with the theoretical spectra and found to be consistent. Low value of coefficient (<200) and high-frequency dependence (> 0.8) in the Qβ(f) relationship suggest that the region is seismically and tectonically active, characterized by a large number of heterogeneities. The present inversion of strong-motion data gives the corner frequencies for the Uttarkashi and the Chamoli earthquake as 0.09 and 0.20 Hz, respectively, which correspond to the stress-drop values of 31 bars and 98 bars, respectively.

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