Radiation pattern of crustal phases of New England earthquakes

The amplitudes of Pn-waves from New England earthquakes recorded by a regional, short-period seismic network have been studied to determine the effects of the double-couple source radiation pattern. Both observed and synthetic seismograms show that Pn amplitudes change dramatically with source-to-receiver azimuth. These changes are primarily due to the take-off angles of the seismic rays relative to the nodal planes of the focal mechanism solution. The recordings of a seismic refraction blast show no appreciable change of amplitude with azimuth. The focal mechanism solutions found from inverting the Pn amplitudes of several well-recorded earthquakes are consistent with those obtained from the first-motion readings. This study has demonstrated that it is possible to calculate focal mechanisms of earthquakes using the polarities and amplitudes of Pn-waves recorded at a small number of azimuthally distributed local stations.