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

Abstract

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 <240km and a magnitude range of 1.2–4.9 to study the coda QC 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 QC values were estimated at 10 central frequencies: 1.5, 3, 5, 7, 9, 12, 16, 20, 24, and 28Hz using starting lapse-times of 10, 20, 30, 40, 50, and 60s and coda window-lengths of 10, 20, 30, 40, and 50s. The QC fits the frequency dependent power-law, QC=Q0fn. For a 10s lapse time with a 10-s coda window length QC=47.42f1.012 and for a 50s lapse time with a 50s coda window length, QC=204.1f0.934. Q0 (QC at 1Hz) varied from ∼47 for a 10s lapse time and a 10s window length, to ∼204 for a 50s lapse time and a 50s window length. An average frequency dependent power law fit for the study region may be given as QC=116.716f0.9943. The exponent of the frequency dependence law n 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, QC 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 QC values or high attenuation at lower frequencies and high QC values or low attenuation at higher frequencies suggest that the heterogeneity decreases with increasing depth in our study region.