THE EXCITATION OF L(G) WAVES BY EXPLOSIONS - A FINITE-DIFFERENCE INVESTIGATION

Several mechanisms have been proposed to explain the excitation of the regional Lg phase by explosions. We examine some of these mechanisms with finite-difference simulations. An energy-flux technique is used t o analyze the synthetic signals. The method is adapted from convention al array-analysis techniques and gives a clear characterization of wav e-field intensity, wave slowness, and propagation direction. The vecto r energy flux in the model is studied instead of the scalar distributi on of energy in the space-time domain. The calculations show that for an explosive source in a high P-wave velocity crust, P-wave energy can not be effectively trapped in the crust, and the nongeometrical phase S may be the primary contributor to the Lg phase. For an explosive so urce buried in a low-velocity upper crust (i.e., where the P-wave velo city is lower than the upper-mantle S-wave velocity), part of the pS w ave can be trapped in the crustal wave guide to form the Lg phase. The relative Lg excitation depends on the ratio between the P-wave veloci ty in the source region and the S-wave velocity in the uppermost mantl e. Scattering effects can transfer P-wave or surface-wave energy into Lg, but the efficiency strongly depends on both the wave frequency and the characteristic size of the scatterers. Any source process that di rectly excites S waves is more efficient for generating Lg than the af orementioned mechanisms. Spall directly generates S-wave energy, and c onsequently is an efficient source for Lg. For actual explosions, all of these factors may simultaneously contribute to Lg excitation. Deter mining their relative importance and interactions requires additional observations.