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Azimuthal anomalies of short-period P-wave arrivals from Nahanni aftershocks, Northwest Territories, Canada, and effects of surface topography

GEOPHYSICS DIVISION GEOLOGICAL SURVEY OF CANADA ENERGY, MINES AND RESOURCES, OTTAWA, K1A 0Y3 Canada

Abstract

Following two large earthquakes in the Northwest Territories of Canada in 1985 and one in 1988, four field surveys were undertaken to study the extensive aftershock activity. Some of the seismographs employed recorded three-component digital data, which allowed the directions of approach of short-period P-wave arrivals to be analyzed. At three stations, observed azimuthal deviations reached as much as 40° from theoretically expected azimuths as computed from the inferred hypocenters for the events. Theoretical results are presented that support the hypothesis that the observed azimuthal deviations are caused principally by local topographic slopes in the vicinity of the recording instruments. Similar theoretical results indicate that effects of local surface topography on arrival azimuths of SH and SV waves are nearly an order of magnitude less than for P waves, so that such effects will generally be unimportant in shear wave splitting studies.

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