Mechanisms of seismic attenuation beneath Bhutan Himalaya

Abstract

We have analyzed 614 high quality local earthquake (1.5 $\le$$M_l$ (local magnitude) $\le$ 5.5) data recorded by temporary GANSSER network of 44 broadband stations to investigate the attenuation mechanism of Bhutan Himalaya. Initially, the single isotropic scattering model is applied to study the coda wave attenuation ($Q_c^{-1}$). Subsequently, we have used the Multiple Lapse Time Window Analysis (MLTWA) to estimate the relative contribution of scattering ($Q_{\mathrm{sc}}^{-1}$) and intrinsic ($Q_i^{-1}$) attenuation to the total attenuation ($Q_t^{-1}$) under the assumption of multiple isotropic scattering with uniform half space medium. The analysis has been carried out for five different central frequencies within the range of 1.5 to 18 Hz. All the estimated values of $Q$ exhibit high frequency dependent nature. Interestingly, scattering attenuation is found to be the dominant factor attenuating the seismic waves in the crust of Bhutan Himalaya which is different from the rest of the Himalayas except Garwhal–Kumaun Himalaya and the adjacent Sikkim Himalaya. This strongly suggests that the relative role of both scattering and intrinsic attenuation varies across the Himalaya and is likely to be associated with the structural variabilities among different segments. The role of Main Himalayan Thrust ($\mathrm{MHT}$) in changing the differential stress regime across the region could be the major cause of the intra-crustal deformation which resulted in the predominance of scattering attenuation in the crust of Bhutan Himalaya.