A Quantitative Evaluation of Seismic Signals at Teleseismic Distances--III Computed P and Rayleigh Wave Seismograms

Abstract

Body (P) and Rayleigh wave seismograms are computed for explosions and earthquakes using the expressions developed by Hudson; many of the features of observed seismograms are noted. For the earthquake source models used average Rayleigh wave magnitudes relative to P wave magnitudes are larger than for the explosion source models. This is true even for point sources; increasing the dimensions of the earthquake source only accentuates this difference in the relative excitation of Rayleigh waves. However the relative amplitudes are strongly dependent on the orientation of the observer relative to the fault plane of the earthquake owing to the radiation pattern of the P and Rayleigh waves. Where the observer is at an anti-node of the P radiation the relative amplitudes of the Rayleigh waves are not significantly greater than for explosions. A detailed study of observed P and Rayleigh amplitudes is now needed to see if the same effects of the P radiation pattern can also explain the observed differences in the relative excitation of P and Rayleigh waves by earthquakes and explosions.

An Examination of Some New and Classical Short Period Discriminants

“…A shallow (1-2 km), small 45° dip-slip earthquake would send out identical P-waves up and down, resulting in P-pP cancellation. These are the same events which can show compvessional first motions at all teleseismic distances; and which are difficult to discriminate by Mjtrn^, Douglas et al (1973), Blandford (1974). Another possibility is that we may find ourselves on a node of the P radiation pattern, and that this pattern will be less smoothed out by inhomogeneities at long periods than at short periods.…”

Theoretical Effect of Yield and Burst Height of Atmospheric Explosions on Rayleigh Wave Amplitudes

“…Rayleigh waves and body waves due to an atmosphere explosion modeled by an equivalent moving normal force applied to a multilayered half space have been formulated by Hudson (1969) and computed for an equivalent stationary point source at the free surface by Douglas et al (1972).…”

Complex P-Wave Seismograms from Simple Earthquake Sources

“…Douglas et al (1972) however looked at only a small number of models. In this paper we investigate a wider range of models and examine specifically the effect on the complexity of the P signal of (a) increasing the thickness of a surface layer with low P and S velocities in the source region;…”