Spatial variability studies of attenuation characteristics of Q(alpha) and Q(beta) in Kumaon and Garhwal region of NW Himalaya

摘要

The present work explores the attenuation characteristics of Garhwal and Kumaon regions of northwest Himalaya, India. A comparative study of P- and S-wave quality factors (Q(alpha) and Q(beta)) is used to appreciate the different attenuation characteristics of these two regions. The strong motion data of 105 local earthquakes recorded in Garhwal and Kumaon region are considered for the analysis. The coda normalization method is implemented to compute Q(alpha) and Q(beta) at each recording site of form Q(f) = Q(o)f(n). The regional relationship obtained by using site-specific quality factor relations is Q(alpha)(f) = (55 +/- 3)f((0.90 +/- 0.06)), Q(beta)(f) = (74 +/- 5)f((0.87 +/- 0.08)) for Garhwal region and Q(alpha)(f) = (34 +/- 1)f((0.94 +/- 0.03)), Q(beta)(f) = (58 +/- 2)f((0.90 +/- 0.02)) for Kumaon region, which clearly suggest the existence of spatial variation of attenuation properties in these two regions. The Kumaon region has low Q(alpha) and Q(beta) as compared to Garhwal, which means Kumaon region has high rate of attenuation than Garhwal region, as Q is inversely proportional to the attenuation. Hence, based on obtained attenuation properties it is proposed that Garhwal region has high seismic hazard potential zone as compared to its adjacent Kumaon region. The high value of Q(beta)/Q(alpha) (> 1) obtained in the present work indicates the presence of fluid or partially saturated rocks in the earth crust for both Garhwal and Kumaon regions. The variable value of ratio (Q(beta)/Q(alpha)) obtained for Garhwal and Kumaon region specifies the different fluid saturations or fluid types existing for both the regions, and it may be the possible reason of distinct attenuation properties of these two regions.