Estimation of scattering properties of lithosphere of Kamchatka based on Monte-Carlo simulation of record envelope of a near earthquake

Abstract

Several models which describe the intensity of single and multiply scattered waves radiated by an instant point source into a medium with uniformly dispersed scatterers and uniform absorption are reviewed. The model of multiple low-angle scattering (MLAS) is then considered and we show that in the realistic case of dominantly forward scattering one can employ the model to determine the mean free path l from pulse broadening of scattered waves. This fast (∝ r 2 ) pulse broadening leads to r −2 amplitude decay and can explain known fast-amplitude decay of short-range magnitude calibration curves and of peak acceleration attenuation laws. Since analytical theory is lacking for an important case of scattered body waves at source distances around r = l , we employ the previously developed technique of Monte-Carlo simulation of a scattered wavefield to obtain a set of theoretical formulae and master curves. These enable us to estimate mean free path l in two independent ways: from intensity ratio of direct and scattered waves ( l A ) and from pulse duration or retardation ( l T ). In both cases, the estimates must depend only weakly on errors of Q determination. We applied the developed theory to the interpretation of records of earthquakes near Kamchatka recorded by frequency selecting (‘ChISS’) stations. For Shipunsky (SPN) station in the 1.5–6.0 Hz frequency range the estimates are l A ≈ 150 km, l T ≈ 110 km. In the 6–25 Hz range, l A is decreasing, roughly as ƒ −0.65 . We could expect that improved theoretical coda shapes will resemble the observed ones, leading to accurate intrinsic Q estimates. This is not the case however, and our Q estimates depend in fact on the choice of lapse time window. This indicates that uniform medium models are insufficient for interpretation. We could demonstrate directly the depth dependence of l based on l T estimates.