Abstract
The synthesis of realistic oceanic Pn phases was accomplished using wave number integration. Wave trains with durations of the order of 100 s following the initial compressional wave arrival were generated for a laterally homogeneous, vertically inhomogeneous oceanic lithosphere model. The excitation of these long wave trains is primarily a result of near‐receiver water and sediment reverberations and does not involve scattering. A late arriving wave train with a group velocity of 1.6 km/s having an emergent onset and substantial duration, characteristic of oceanic abyssal T phases, was also generated even with phase velocities restricted to be greater than 5.5 km/s. The synthetic Pn wave train was compared to data collected in the southwest Pacific during the Ngendei Seismic Experiment. Many of the spectral characteristics associated with sediment and water reverberation observed in the synthetics were also observed in the data. First, it was found that the amplitude falloff rate of the Pn wave train is a strongly increasing function of frequency, in direct contrast to results predicted for forward scattered coda. This leads to a predominance of low frequencies late in the wave train which is easily interpreted in terms of the constructive interference of different modes of reverberation. Second, the presence of spectral peaks at frequencies associated with both sediment and water reverberations was observed. The predicted frequencies were computed using values of the water depth, sediment thickness, and sediment velocity obtained in an independent study. Strong alignment of the spectral peaks with the predicted frequencies illustrates the reverberating nature of the Pn wave train. The presence of substantial power in excess of 15 Hz can be attributed to the confinement of Pn to the oceanic lithosphere. The oceanic Pn phase is thus characterized as a refraction from the lower lithosphere with multiple reverberations in the oceanic water column and sediment layer comprising the subsequent coda. Complicated oceanic lithosphere models are not needed to explain the gross characteristics of Pn propagation.
Published Version
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