Attenuation of high frequency body waves in the crust of western Tibet

Abstract

The attenuation characteristics of western Tibet have been analyzed by estimating the seismic attenuation parameters (Qp−1 and Qs−1) at five different central frequencies (1.5, 3, 6, 12 and 18 Hz). The extended coda normalization method has been used on 469 three component local earthquakes which are recorded at Y2 network of broadband stations deployed in western Tibet during years 2007–2011. We have estimated the frequency dependent Qp−1 and Qs−1 values at each station. The results indicate strong body wave attenuation in the medium below western Tibet. The highest attenuation is observed at WT19 and WT13 stations located near to the Karakoram fault (KKF) and Indus-Yarlung suture (IYS). Across the KKF, we have subdivided the entire study area into two regions to explore the lateral variations of crustal attenuation properties. The first part covers the northeastern of KKF referred as Region 1 while the second part covers the southwestern of KKF referred as Region 2. The frequency dependent relations are found as follows: Qp−1 = (19.4 ± 4.4).10−3f−(0.9±0.2), Qs−1 = (13.3 ± 2.1).10−3f−(0.8±0.1) for Region 1 and Qp−1 = (18.4 ± 1.8).10−3f−(0.8±0.1), Qs−1 = (13.2 ± 2.2).10−3f−(0.8±0.1) for Region 2. Both the regions show strong frequency dependent nature of Q−1 values with no significant lateral variations. It may suggest the presence of similar tectonic complexities and heterogeneities beneath both the regions. We also find that all the estimated values of Qp−1/ Qs−1 ratios are greater than unity at all the frequency range which are likely to be associated with both the intrinsic and scattering phenomenon in the crust of western Tibet.