Detection , Analysis , and Interpretation of Teleseismic Signals 1 . Compressional Phases from the Salmon Event

The travel times and amplitude spectrums of firstand later-arrival P phases from the Salmon event are computed on the basis of polarization filter outputs. The interpretation of the P wave radiation field is made in terms of crust and mantle structure using the firstand later-arrival P phases and their amplitude spectrums. The observed seismic field corresponds with that expected from a symmetric, purely compressire source. The essential features of the observed travel times and amplitudes are explained in terms of regional mantle structures. These structures provide first-order fits to the observed data and are characterized by lowvelocity zones which terminate with rapid and continuous increases in velocity near depths of 130 km. The velocity structures also show a strong velocity gradient near 330 km. The regional models differ most strongly in the relative extent and magnitude of the velocity decrease in the low-velocity zone.