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Lapse-Time Dependence of Coda Q in the Source Region of the 1999 Chamoli Earthquake

Abstract In the present study the attenuation of seismic-wave energy in and around the source area of the Chamoli Earthquake of 29 March 1999 is estimated using aftershock data. Most of the analyzed events are from the vicinity of the main central thrust (MCT), which is a well-defined tectonic discontinuity in the Himalayas. The method of a single backscattering model is employed to calculate frequency dependent values of coda Q ( Q c ). A total of 30 aftershock events are used for Q c estimation at central frequencies 1.5, 3, 6, 9, 12, 18, and 24 Hz through five lapse-time windows from 10 to 50 sec starting at double the travel time of the S wave. The observed Q c is strongly dependent on frequency, which indicates that the region is seismically and tectonically active with high heterogeneities. The variation of Q c has also been estimated at different lapse times to observe its effect with depth. The variation of Q c with frequency and lapse time shows that the lithosphere becomes more homogeneous with depth. Q c -values for higher frequencies increase very fast with depth within about the top 63 km of the lithosphere and then become more or less constant beyond this depth. This indicates that turbidity at higher frequency decays very fast with depth, and the mantle may be transparent to high-frequency waves. The variation of Q c at 1.5 Hz with lapse time matches quite well with those predicted by Gusev (1995). However, the frequency parameter n in the relation Q c = Q 0 f n , where Q 0 = Q c at 1 Hz, does not follow the expected pattern given in his model. This could be due to faster depth decay of turbidity as mentioned previously.