Abstract

Arrival times, slownesses and azimuths for coherent signals arriving between P and 60 secafter PP have been measured for 12 earthquakes recorded at the Yellowknife array at distances between 90° and 115°. The slowness (dT/dΔ) and azimuth values for P indicate that corrections to dT/dΔ for local structure beneath the array are small and can be neglected. A statistical study of arrivals from 10 events at distances less than 103° did not demonstrate conclusively the existence of PdP waves, and revealed a pattern of slowness values for precursors to PP similar to that observed by Wright and Muirhead (1969) at a distance of 106.0°. Further, a more detailed examination of three events at distances of 93.1°, 105.5° and 114.5° showed the presence of precursors with slowness values of about 10 sec/deg. These results required the development of an asymmetric reflection hypothesis in which the large amplitudes of these waves are produced by cusps in the travel-time curve near 20° and lateral variations in the uppermost regions of the Earth.

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