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A study of apparent angles of emergence at Marathon Ontario from the Lake Superior data

UNIVERSITY OF WESTERN ONTARIO DEPARTMENT OF GEOPHYSICS, LONDON, Canada

Abstract

A three component array of Willmore seismometers and a hydrophone were used to record the seismic events at Marathon, Ontario during the Lake Superior crustal experiment of 1963. The first part of each record was digitized and from an analysis of the particle motion diagrams, apparent angles of emergence of the seismic rays were determined. It was found that these angles can be used to distinguish between P₂ and P_n waves. When the shot distance was less than 220 km., the first arrivals emerged with an apparent angle of 40-50°. As the shot distance was increased beyond 220 km., the rays emerged with an apparent angle of 51-70°. The apparent velocities of the (40-50) and the 51-70°) rays were 6.6 km/sec and 8.2 km/sec respectively. Further analysis of the results showed that the Moho below Marathon dipped downwards toward the south west with an angle of approximately 4°, indicating that the crust is considerably thicker on the south west side of Marathon than on the north east. A study of the ray azimuths gave some evidence of the presence of lateral inhomogeneities in the crust.