Attenuation of short-period body waves in Northwestern Himalayan Region, India

The attenuation of seismic wave is one of the basic physical parameters which is closely related to the seismicity and regional tectonics activity of a particular area. This is also important for seismic hazard measurement. Seismic wave attenuation for local earthquakes is determined from the analysis of direct body waves and coda waves. The dimensionless parameter, Q, is studied in the present work, which is defined as a measure of the rate of decay of the coda waves or body waves within a specific frequency band. Digital seismogram data of 75 earthquakes that occurred in Garhwal Himalaya region during 2004-2006 and recorded at 20 different stations have been analyzed to study the seismic body and coda wave attenuation. Seismic body wave attenuation characteristics are examined by estimating the frequency dependent relationship of quality factors for P -waves, Qc, and for S -waves, Q, in the frequency range 1.5-28 Hz, using 95 seismic observations with hypo central distance less than 100 km. The extended coda normalization method was applied. The average value of Q(alpha) is found to be varied from 45.10 at 1.5 Hz to 1400.0 at 28 Hz, while it varies from 109.02 at 1.5 Hz to 3987.0 at 28 Hz for Q(s). The estimated frequency dependent relation for quality factors are Q(alpha) = (29.077 +/- 8.5)f((1.16 +/- 0.01)) and Q(beta) = (67.84 +/- 13.5)f((1.18 +/- 0.02)) for P and S -waves, respectively. The rate of increase of Q(f) for P and S waves in athis region is comparable with the other regions of the Himalayas. The ratio is Q(beta)/Q(alpha) greater than unity in the entire analyzed frequency range. It indicates that scattering is an important factor contributing to the attenuation of body wave in the region. The low Q(alpha,beta) values or high, attenuation at lower frequencies and high Q(alpha,beta) values or low attenuation at higher frequency may indicate that the heterogeneity decreases with increasing depth in the study region.