Q-VALUE ESTIMATION OF KINNAUR REGION

Abstract

The attenuation characteristics of the Kinnaur area of the North West Himalayas were studied using local earthquakes that occurred during 2008-2009. Most of the analyzed events are from the vicinity of the Panjal Thrust (P'1') and South Tibetan Detachment Thrust, which are well-defined tectonic discontinuities in the Himalayas. We analysed local earthquake waveforms from 64 earthquakes recorded on a broad-band seismic network. The aim was to compute the variation of intrinsic and scattering attenuation parameters with lapse time window lengths, epicentral distance ranges and frequency from coda waves. Intrinsic (Q[") - and scattering (Q'1) attenuations were separated using Q. which were obtained using Aki and Chouet (1975) single scattering method and direct S-wave Q (Q) which were taken from the results of Kumar Ct al. (2014). Q values were obtained at central frequencies 1.5, 3, 6, 9, 18 and 24 Hz for lapse time window lengths of 20, 30, 40, 50, 60, 70, 80 and 90 s. Q1 and Q values were obtained using the Wennerberg's (1993) approach with Q and Qd as inputs. It was observed that intrinsic and coda wave attenuation show a decreasing trend with lapse time which in turn can be interpreted as depth, while, scattering attenuation show an - increasing trend and cross-over the values of Qi' and Q.'. This shows that intrinsic * attenuation dominates in the small lapse times (shallow depth) while for large lapse times (deeper depth) scattering attenuation dominates in the contribution towards coda wave attenuation. The geological implication of this result is that the underthrusting of the Indian plate by the Asian plate is has brought upon an increase in heterogeneities in the lapse times ranging between 40 - 60 s which in turn has resulted in an increase in the scattering attenuation with depth, against the normal trend observed in which scattering attenuation decreases with depth