Variation of intrinsic and scattering attenuation with depth in NW Himalayas

We analysed local earthquake waveforms recorded on a broad-band seismic network in NW Himalayas to compute the variation of intrinsic and scattering attenuation parameters with depth from coda waves. First we estimated lapse time and frequency dependence of coda Q (Q(c)). Intrinsic (Q(i)(-1)) and scattering (Q(s)(-1)) attenuations were separated using Q(c) and direct S-wave Q (Q(d)). It is observed that for smaller lapse time window estimated Q(c)(-1) is close to Q(i)(-1) and both of them are much larger than Q(s)(-1) suggesting that coda decay is predominantly caused by intrinsic attenuation at shallower depth in the NW Himalayas. As lapse time increases, the Q(c)(-1), Q(i)(-1) values decrease and come closer to Q(i)(-1) values. As Q values for larger lapse time windows are affected by deeper zones, this indicates that with increasing depth relative effect of intrinsic attenuation with respect to that of scattering attenuation on coda attenuation decreases in the area under study. Both coda and intrinsic attenuation decreases with increasing depth. However, between 111 +/- 6.3 and 129.5 +/- 7.5 km intrinsic attenuation remains stable indicating possible presence of partial melts and/or fluid filled fractures. On the other hand scattering attenuation increases with increasing depth. This may be due to the fact that continental crustal material of the Indian plate underthrusts the Himalayas. The scattering attenuation characteristics of the crust are very different from those of the upper mantle in the NW Himalayas. It is also observed that at higher frequency energy loss is primarily due to intrinsic dissipation, whereas at lower frequency scattering also plays an important role in attenuating seismic wave energy in the study area.