Variations of intrinsic and scattering seismic attenuation with depth in western Tibet

Abstract

In this study, the depth-dependent seismic wave attenuation beneath western Tibet have been investigated using 628 local earthquakes recorded at Y2 network operated by US-China collaboration group from July 2007 to May 2011. Initially, we have calculated the attenuation of shear wave (Qs−1) using coda normalization method and coda waves (Qc−1) using single isotropic scattering model. Thereafter, we have estimated the quantitative values of scattering (Qsc−1) and intrinsic (Qi−1) attenuation using Wennerberg approach. The observations are made at five different central frequencies (1.5, 3, 6, 12, and 18 Hz) and five lapse time windows (30, 40, 50, 60, and 70 s). The results show frequency and lapse time dependence of Qc−1, Qsc−1 and Qi−1 values for western Tibet. Qc−1 and Qi−1 shows similar trend suggesting that the key mechanism is intrinsic dissipation for the decay of coda wave in the medium. Qi−1 is found to be as a dominant mechanism over Qsc−1 at all the investigated depth beneath western Tibet. Interestingly, the values of Qc−1 and Qi−1 decrease and get closer to Qsc−1 with increasing lapse time window/depth. It signifies the impact of ongoing underthrusting Indian lithosphere at deeper depth. A stronger intrinsic attenuation at shallow depth reflects the effect of the mid crustal low-velocity zone (LVZ) below Tethyan Himalaya and the southern Lhasa terrane. The obtained results in this study are also in good agreement with other reported results from seismically active regions in the world.