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Attenuation of High-Frequency Seismic Waves in Kachchh Region, Gujarat, India

Institute of Seismological Research, Gandhinagar, Gujarat, India babita_s{at}rediffmail.com

Dinesh Kumar and S. S. Teotia

Department of Geophysics, Kurukshetra University, Kurukshetra 136 119, India

B. K. Rastogi

Institute of Seismological Research, Gandhinagar, Gujarat, India

The attenuation properties of the crust in the Kachchh region, Gujarat, India, have been investigated. For this purpose, 49 local earthquakes having focal depths in the 3–38 km range have been used. The quality factors Q (the quality factor using P waves) and Q_β (the quality factor using S waves) have been estimated using the coda normalization method. The quality factor of coda waves (Q_c) has been estimated using the single backscattering model. Multiple scattering models have been used to estimate Q_i (the intrinsic attenuation parameter) and Q_s (scattering attenuation parameter) for the region. The values of Q, Q_β, Q_c, Q_i, and Q_s show a dependence on frequency in the range of 1.5–24 Hz for the Kachchh region. The average frequency-dependent relationships (Q=Q₀fⁿ) estimated for the region are Q=(77±2)f^(0.87±0.03), Q_β=(100±4)f^(0.86±0.04), and Q_c=(148±3)f^(1.01±0.02). The estimate of Q_c is found to be higher than Q_β in this region. This analysis supports the Zeng et al. (1991) model that predicts that the effects of intrinsic and scattering attenuation combine in a manner that Q_c should be more than Q_β. The estimated values of Q_s and Q_i vary from 529 and 183 at 1.5 Hz to 3053 and 2668 at 24 Hz, respectively. We find that the Q_c estimates lie between the estimates of Q_i and Q_s but are closer to Q_i at lower frequencies. This is in agreement with the theoretical as well as laboratory measurements. A comparison between Q_i and Q_s shows that intrinsic absorption is predominant over scattering.