Coda Q Estimates in the Koyna Region, India

The coda Q, Qc, have been estimated for the Koyna region of India. The coda waves of 76 seismograms from thirteen local earthquakes, recorded digitally in the region during July–August, 1996, have been analyzed for this purpose 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 magnitude 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 length the estimated Qc values vary from 81 to 261 at 1.5 Hz and 2088 to 3234 at 24 Hz, whereas the mean values of Qc with the standard error vary from 148913.5 at 1.5 Hz to 2703938.8 at 24 Hz. Both the estimated Qc values and their mean values exhibit the clear dependence on frequency in the region and a frequency dependence average attenuation relationship, Qc 96f1.09, has been obtained for the region, covering an approximate area of 11 500 km2 with the surfacial extent of about 120 km and depth of 60 km. Lapse time dependence of Qc has also been studied for the region, with the coda waves analyzed at five lapse time windows from 20 to 60 sec duration with the difference of 10 sec. The frequency dependence average Qc relationships obtained at these window lengths Qc 66f1.16 (20 sec), Qc 96f1.09 (30 sec), Qc 131f1.04 (40 sec), Qc 148f1.04 (50 sec), Qc 182f1.02 (60 sec) show that the frequency dependence (exponent n) remains mostly stationary at all the lapse time window lengths, while the change in Q0 value is significant. Lapse time dependence of Qc in the region is also interpreted as the function of depth.