SEPARATION OF SCATTERING ATTENUATION AND INTRINSIC ABSORPTION USING MULTIPLE LAPSE TIME WINDOW METHOD

Abstract

The study presents estimation of scattering attenuation and intrinsic absorption adopting multiple lapse time window method to estimate the two factors. This method has been given by Hoshiba (1991) and based on comparison of observed seismic wave energy integrals estimated over three different finite lapse time windows with the synthetic energy density distribution obtained using isotropic multiple scattering model. The method has been applied on the data set of eleven local earthquakes (2.0 ML 2.9) occurred in the Garhwal l-limalaya in 2013. The events fall in the vicinity of the Main Central Thrust (MCT). The 114 digital recordings of these events have been analysed in three lapse time windows of 0-15, 15-30 and 30-45 seconds. For each lapse time window, the recordings are filtered in the three frequency bands, viz. 1-2 l-lz, 2-4Hz 4-81-lz. The seismic energy is estimated from the mean squared amplitudes of band pass filtered seismogram. The estimated energy is corrected for local site amplification and source effects using coda normalization method. The energy corrected for geometrical spreading is normalised and plotted with respect to the hypocentral distance. From the ratio of the estimated energy from the synthetic energy distribution in different time windows the two parameters namely, the reciprocal of extinction length (L) and seismic albedo (Bo) are estimated. From these parameters, intrinsic absorption and scattering attenuation are estimated. The estimates of intrinsic absorption in the three frequency bands, viz. 1-2 Hz, 2-41-lz 4-81-lz are 0.008,0.003 and 0.002. Scattering attenuation comes out to be 0.050, 0.060, and 0.061. The estimates of seismic albedo in the three frequency bands are: 0.87. 0.95 and 0.97.The extinction length varies between 26 kms to 235 kms with an average of 58 kms for the frequency band 1-2 Hz. For the frequency band of 2-4 Hz these values vary from 40 to 195 kms with an average of 80 kms. For frequency band of 4-8 Hz, the extinction length varies from 45 to 197 kms with average value of 88 kms. The results show that the area is scattering dominant. This is mainly due to the heterogeneities present in the area. Records show that a number of earthquakes occur in this area. Due to this there are a number of fractures that are developed in the subsurface of the earth. This also explains the large amount of scattering attenuation which is occurring in this area.