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Short-period P-coda characteristics in the eastern Canadian Shield

SEISMOLOGY DIVISION OBSERVATORIES BRANCH, OTTAWA, Canada

Abstract

The short-period P-coda of eight earthquake events (recorded on magnetic tape) and one underground nuclear event (standard Canadian network seismogram) are analyzed in both the time and frequency domains in order to determine the transmission characteristics of the crust in the eastern section of the Canadian Shield. Surface particle motion diagrams indicate that there is appreciable anomalous P-S conversion in this region, starting at the base of the Labrador trough and extending to the upper mantle. The spectral ratio test, when applied to short-period records, is found to be capable of resolving thin crustal layers only. Coherency studies of teleseismic events recorded over a lateral distance, the same order of magnitude as the thickness of the crust in this region, show that the coherency (0.5 average) is much lower for the horizontal component than for the vertical component and also does not appear to vary appreciably with epicentral distance, whereas the vertical component does. Since for many of the records the signal-to-noise ratio of the vertical and horizontal components is comparable, these results indicate the possibility of the horizontal transfer function being more unstable over this region than the vertical transfer function.

From these observations it is concluded that (1) in the region of the eastern Canadian Shield where there is a surface layer of Proterozoic sediments about one km thick overlying basement rocks, the short-period body-wave transmission characteristics are more similar to those of the western Canada sedimentary basin in central Alberta than to those of the western part of the Canadian Shield near Yellowknife, Northwest Territories and (2) in order to resolve both thin and thick crustal layers by means of the spectral ratio test, both short- and long-period records of the P coda are required.

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