The horizontal propagation of P waves through scattering media: Analog model studies relevant to long-range P_n propagation

We compare seismograms produced through the use of analog models of scattering and nonscattering Earth structure to access the effect of lateral heterogeneities on the horizontal propagation of P waves. Our results are applicable to propagation out to 16° and to periods as short as 5 sec. We find that layers of scatterers within the mantle near the turning points of P waves can cause these phases to have long coda similar in certain respects to the coda observed in long-range P_n. The overall appearance of this coda is similar to that produced in laterally homogeneous models that include low-velocity crustal material near their surfaces. Scatterers at shallower depths in the mantle or crust have a different effect. Those in very uppermost mantle seem to suppress the horizontal propagation of P waves while those in the crust have very little overall effect, at least at the perod range we studied. We also find that the scatterers have an amplifying effect on the horizontal propagation of the very longest periods (>20 sec), an effect that may be related to the scattering-controlled boundary waves discovered by Biot (1968).