An investigation of regional variations of coda wave attenuation in western Tibet

Regional variations of coda wave attenuation (Q(c)(-1)) have been analyzed for western Tibet using single isotropic scattering model. A total of 628 local earthquakes, having a source-receiver distance less than 200 km recorded at the Y2 network of broadband stations during 2007-2011, are used for this analysis. All the events are crustal having maximum hypocentral depth up to 40 km. The estimation of Q(c)(-1) is made at five different frequencies centered at 1.5, 3, 6, 12, and 18 Hz through lapse time windows from 30 to 70 s with an increment of 10 s starting at 1.2 times of secondary wave arrival time. We find a strong frequency-dependent nature of Q(c)(-1) in this region, which may demonstrate that the medium is highly heterogeneous and tectonically very active. The regional variations of Q(c)(-1) across the whole region have been performed, and we find a marked contrast across the Karakoram fault (KKF) at different frequencies and lapse time windows, which are consistent with the geotectonic diversity of the region. Q(0c)(-1)(Q(c)(-1) at 1 Hz) and frequency-dependent parameter (n(c)) for each station are also estimated using the power law relation Q(c)(-1) = Q(0c)(-1)f(-nc). There is a tendency to increase n(c) with increasing observed Q(0c)(-1) values, and vice versa. Variations of Q(0c)(-1) at different lapse time windows and its corresponding apparent depths are also examined. We have observed a certain rise in Q(0c)(-1) values at a 60-s lapse time window which may be due to the change in velocities, temperature, rock composition variations, and strong heterogeneity in the lithospheric mantle. The obtained values in this study are also found to be well compared with other results reported globally.