P-wave coda analysis of short-period array data and the scattering and absorptive properties of the lithosphere

Abstract

Short-period registrations of events at regional distances usually show a considerable amount of energy arriving after the initial P onset. This is caused by the scattering of the wavefield at heterogeneities along the propagation path. I have studied the P coda of Indonesian earthquakes recorded at the Warramunga array in central Australia. The data show a decrease of the coda energy relative to the energy of the direct wave with increasing source depth. Coherency measurements by means of semblance-enhanced stacks reveal that the coda of deep-focus events is mostly incoherent across the array, while the coda of shallower events contains a certain amount of partly-coherent energy. This points to the existence of heterogeneities with various scale lengths. Small-scale heterogeneities of only a few kilometres correlation length beneath the array are responsible for the generation of the incoherent part of the coda, while the partly-coherent arrivals are produced by larger-scale heterogeneities in the deeper lithosphere and by reflections from plane layering. In order to study further the small-scale fluctuations of the seismic parameters I have computed the power spectral density of the P coda in four frequency bands between 0.75 Hz and 6 Hz. An energy-flux model (Frankel & Wennerberg 1987) which assumes multiple scattering and allows for the independent determination of intrinsic Q and scattering Q has been fitted to the data. The results show an almost linear increase of intrinsic Q with frequency. Q is about 300 at 1 Hz. The frequency dependence of scattering Q is somewhat less pronounced with a value of about 340 at 1 Hz and an exponent of 0.85. A comparison of this frequency dependence with results from single-scattering theory imply a correlation length of about 5.5 km and a rms velocity fluctuation of 5 per cent for the scattering medium.