A Study of the Effects of the Yellowknife Crustal Structure upon the P Coda of Teleseismic Events

The short-period P codas of seven earthquakes and four underground nuclear events recorded in the Yellowknife region of the Canadian Shield are analysed both in the time and in the frequency domains. In the time domain, the application of a ‘P-detection’ filter to the earthquake events facilitates the identification of several phases (pP and sP) in the first 25 s of the P coda. The application of this filter to two nuclear events (originating at the Nevada Test Site) assists in the separation and in the identification of the crustal reverberations at the respective sources. In the frequency domain studies, the application of the spectral ratio test to six earthquake events resulted in poor agreement between the theoretical and the experimental spectral ratio curves; closer agreement was obtained for the nuclear events. Since the earthquake events did not possess the appropriate type of waveform for the spectral ratio test, it is not possible, at this stage, to pass judgment as to whether or not the crustal layering at Yellowknife fulfills the requirements of Haskell's matrix theory. Signal-generated-noise studies are based on the observation of P-generated SH and SV waves. Anomalous P-SH conversion is much less in this region than in the sedimentary basin of central Alberta. However, there are indications of appreciable anomalous P-SV conversion; the source is likely in the lower part of the crust and possibly in the upper part of the mantle at Yellowknife.