Abstract

We compile a data set of 585 weak, broadband, and strong ground-motion recordings from 184 earthquakes covering hypocentral distances ranging from 9 to 436 km in northeast India and the northwest Himalayas. Source-normalized vertical-component Fourier amplitude S-wave spectra are used to study path-attenuation characteristics, namely, geometrical spreading and quality factor, in the two regions. A trilinear model of geometrical spreading of R-1.0 for hypocentral distance R<125 km, R0.0 for R between 125 km and 140 km, and R-1.0 for R>140 km approximates the decay of low-frequency (1.0 Hz) amplitudes in northeastern India better than the conventional theoretical bilinear geometrical spreading of R-1.0 for R<100 km and R-0.5 otherwise. In the northwestern Himalayas, the geometrical spreading is found to be R-1.0, which is the same as that of a bilinear model at hypocentral distances R<100 km. In northeastern India, normalized Fourier spectra corrected for geometrical spreading yields shear-wave quality factor as a function of frequency f, given by forumla in the frequency range of 0.9–20 Hz.

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