Abstract

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 to analyze the synthetic signals. The method is adapted from conventional array-analysis techniques and gives a clear characterization of wave-field intensity, wave slowness, and propagation direction. The vector energy flux in the model is studied instead of the scalar distribution 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 cannot be effectively trapped in the crust, and the nongeometrical phase S* may be the primary contributor to the Lg phase. For an explosive source buried in a low-velocity upper crust (i.e., where the P-wave velocity is lower than the upper-mantle S-wave velocity), part of the pS wave 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 velocity in the source region and the S-wave velocity in the uppermost mantle. 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 directly excites S waves is more efficient for generating Lg than the aforementioned mechanisms. Spall directly generates S-wave energy, and consequently is an efficient source for Lg. For actual explosions, all of these factors may simultaneously contribute to Lg excitation. Determining their relative importance and interactions requires additional observations.

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