Frequency dependent attenuation characteristics of coda waves in the Northwestern Himalayan (India) region

Digital seismogram data of 82 earthquakes from the Northwestern Himalayan (India) region recorded at different stations during 2004-2006 were analyzed to study the seismic coda wave attenuation characteristics in this region. We used 132 seismic observations from local earthquakes with a hypocentral distance <240 km and a magnitude range of 1.2-4.9 to study the coda Q(c) using the single isotropic scattering model. These earthquakes were recorded at 20 temporary seismic stations installed in the Northwestern Himalayas (India) by the Wadia institute of Himalayan Geology, Dehradun. The Q(c) values were estimated at 10 central frequencies: 1.5, 3, 5, 7, 9, 12, 16, 20, 24, and 28 Hz using starting lapse -times of 10, 20, 30, 40, 50, and 60 s and coda window-lengths of 10, 20, 30, 40, and 50 s. The Q(c) fits the frequency dependent power-law, Q(c) = Q(0)f(n) For a 10 s lapse time with a 10-s coda window length Q(c) = 47.42f(1.012) and for a 50 s lapse time with a 50 s coda window length, Q(c) = 204.1f(0.934.) Q(0) (Q(c) at 1 Hz) varied from similar to 47 for a 10 s lapse time and a 10 s window length, to similar to 204 for a 50 s lapse time and a 50 s window length. An average frequency dependent power law fit for the study region may be given as Q(c) = 116.716f(0.9943). The exponent of the frequency dependence lawn ranged from 1.08 to 0.9, which correlates well with values obtained in other seismically and tectonically active and heterogeneous regions of the world. In our study region, Q(c) increases both with respect to lapse time and frequency, i.e., the attenuation decreases as the quality factor is inversely proportional to attenuation. The low Q(c) values or high attenuation at lower frequencies and high Q(c) values or low attenuation at higher frequencies suggest that the heterogeneity decreases with increasing depth in our study region. (C) 2015 Elsevier Ltd. All rights reserved.