Coda Q estimates in the Koyna region, India

The coda Q, Q(c), have been estimated fbr the Koyna region of India. The co da waves of 76 seismograms from thirteen local earthquakes, recorded digita lly in the region during July-August, 1996, have been analyzed for this pur pose at nine central frequencies viz., 1.5, 2.0, 3.0, 4.0, 6.0, 8.0, 12.0, 16.0 and 24.0 Hz using a single backscattering model. All events with magni tude less than 3 fall in the epicentral distances less than 60 km and have focal depths which range from 0.86 to 9.43 km. For the 30 sec coda window l ength the estimated Q(c) values vary from 81 to 261 at 1.5 Hz and 2088 to 3 234 at 24 Hz, whereas the mean values of Q(c) with the standard error vary from 148 +/- 13.5 at 1.5 Hz to 2703 +/- 38.8 at 24 Hz. Both the estimated Q (c) values and their mean values exhibit the clear dependence on frequency in the region and a frequency dependence average attenuation relationship, Q(c) = 96f(1.09), has been obtained for the region, covering an approximate area of 11 500 km(2) with the surfacial extent of about 120 km and depth o f 60 km. Lapse time dependence of Q(c) has also been studied for the region, with th e coda waves analyzed at five lapse time windows from 20 to 60 sec duration with the difference of 10 sec. The frequency dependence average Q(c) relat ionships obtained at these window lengths Q(c) = 66f(1.16) (20 sec), Q(c) = 96f(1.09) (30 sec), Q(c) = 131f(1.04) (40 sec), Q(c) = 148f(1.04) (50 sec) , Q(c) = 182f(1.02) (60 sec) show that the Frequency dependence (exponent n ) remains mostly stationary at all the lapse time window lengths, while the change in Q(0) value is significant. Lapse time dependence of Q(c) in the region is also interpreted as the function of depth.