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 (PT) and South Tibetan Detachment Thrust, which are well-defined tectonic discontinuities in the Himalayas. The frequency-dependent attenuation of P and S waves was estimated using the extended coda normalization method. Data from 64 local earthquakes recorded at 10 broadband stations were used. The coda normalization of the spectral amplitudes of P and S waves was done at central frequencies of 1.5, 3, 6, 9, and 12 Hz. Qp increases from about 58 at 1.5 Hz to 706 at 12 Hz, and Qs increases from 105 at 1.5 Hz to 1,207 at 12 Hz. The results show that the quality factors for both P and S waves (Qp and Qs) increase as a function of frequency according to the relation Q = Qofn, where Qo is the corresponding Q value at 1 Hz frequency and “n” is the frequency relation parameter. We obtained Qp = (47 ± 2)f(1.04±0.04) and Qs = (86 ± 4)f(0.96±0.03) by fitting power law dependency model for the estimated values of the entire study region. The Q0 and n values show that the region is seismically very active and the crust is highly heterogeneous. There was no systematic variation of values of Qp and Qs at different frequencies from one tectonic unit to another. As a consequence, average values of these parameters were obtained for each frequency for the entire region, and these were used for interpretation and for comparison with worldwide data. Qp values lie within the range of values observed for some tectonically active regions of the world, whereas Qs values were the lowest among the values compared for different parts of the world. Qs/Qp values were >1 for the entire range of frequencies studied. All these factors indicate that the crust is highly heterogeneous in the study region. The high Qs/Qp values also indicate that the region is partially saturated with fluids.
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