The PWaves From the Amchitka Island Explosions

Abstract

Summary. There is evidence that the equivalent seismic sources of the Amchitka Island explosions — Longshot, Milrow and Cannikin — depart significantly from the simple model of a point compressional‐source in a layered elastic‐medium. Consequently modelling the observed seismograms using standard source‐models may not be the most efficient method of determining source properties. Here an alternative to modelling is used to obtain information on the seismic sources due to the explosions. Broad‐band (BB) estimates of the P signals are obtained from the short‐period (SP) seismograms, corrected for attenuation, and interpreted in terms of P, pP and radiation from secondary sources. the main conclusions are: (i) BB estimates of the radiated displacement from the explosions can be obtained with only a small reduction in the signal‐to‐noise ratio seen on SP seismograms; (ii) observations of differences in pulse amplitudes and spectra are not necessarily due to differences in anelastic attenuation; (iii) P and pP at a given station may differ in shape so that notches in the signal spectrum may not be related to source depth; (iv) there is evidence of arrivals that others have identified as due to slap‐down but which could be interpreted as an overshoot to pP; (v) direct interpretation of the estimated ground displacement is a better procedure for determining the seismic source properties of explosions than modelling SP seismograms using idealised models as a starting point. Copyright © 1987, Wiley Blackwell. All rights reserved