ESTIMATION OF SCATTERING PROPERTIES OF LITHOSPHERE OF KAMCHATKA BASED ON MONTE-CARLO SIMULATION OF RECORD ENVELOPE OF A NEAR EARTHQUAKE

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 (∝ r2) 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 (lA) and from pulse duration or retardation (lT). 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 lA ≈ 150 km, lT ≈ 110 km. In the 6-25 Hz range, lA is decreasing, roughly as f{hook}-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 lT estimates. © 1990.