Attenuation of coda waves in the Garhwal Lesser Himalaya, India

Qc estimates for the Uttarkashi and the Chamoli regions of the Garhwal Lesser Himalaya have been obtained by analyzing the coda waves of 159 local earthquakes recorded during 2008 and 2009 employing a 12-station seismological network. Earthquakes around the Uttarkashi region are located in the epicentral distance range of 5.0 to 93.9 km, focal depth range of 1.63 to 42.13 km, and coda magnitude range of 0.2 to 2.9, whereas earthquakes around Chamoli region are located in the epicentral distance range of 19.8–109.2 km, focal depth range of 1.36 to 40.72 km, and coda magnitude range of 1.0 to 3.0. The coda waves of 30 s duration, recorded on 982 seismograms, have been analyzed in seven frequencies range centered at 1.5, 3.0, 6.0, 9.0, 12.0, 18.0, and 24.0 Hz for four to five lapse time windows (LTW) using the single backscattering model given by Aki and Chouet (J Geophys Res 80:3322–3342, 1975). Mean value of Qc estimates vary from 76 at 1.5 Hz to 2201 at 24.0 Hz for LTW range of 10–40 s and from 216 at 1.5 Hz to 3243 at 24.0 Hz for LTW range of 50–80 s (for the Uttarkashi region) and from 147 at 1.5 Hz to 2273 at 24.0 Hz for LTW range of 20–50 s and from 188 at 1.5 Hz to 2826 at 24.0 Hz for LTW range of 50–80 s (for Chamoli region). The Qc values thus obtained showed a clear dependence on frequency and LTW and frequency dependence Qc relationships, Qc = Q0fη, for LTWs that have been obtained as Qc = 57f1.20 (10–40 s), Qc = 97f1.07 (20–50 s), Qc = 116f1.03 (30–60 s), Qc = 130f1.03 (40–70 s), and Qc = 162f0.95 (50–80 s) for Uttarkashi region and Qc = 107f0.95 (20–50 s), Qc = 115f0.96 (30–60 s), Qc = 128f0.95 (40–70 s), and Qc = 145f0.95 (50–80 s) for Chamoli region.