Abstract

A search technique is applied to determine focal mechanisms from the surface-wave spectral amplitudes of recent earthquakes in the eastern U. S. and Canada. The technique provides the five source parameters of dip, slip, strike, depth, and seismic moment through a combination of criteria requiring best correlation coefficients, least residuals between theoretical and observed spectral-amplitudes and equality between independent seismic moment estimates from Love and Rayleigh wave data.

This technique is applied to eight earthquakes of mb ≈ 5 that occurred in the North American continent in recent years. The focal mechanism results, constrained by P-wave first motions, indicate that near horizontal pressure axes are in the ENE-WSW for the 1982 Miramichi, New Brunswick (Canada) mainshock and one large aftershock (another large aftershock has ESE-WNW P-axis), the 1982 Gaza, New Hampshire mainshock, the 1982 Arkansas mainshock, the 1983 Goodnow, New York mainshock, and the 1986 Perry, Ohio mainshock. On the other hand a near horizontal tension axis in the direction NNE-SSW is found for the 1984 Wyoming mainshock in the western part of North American continent. The results obtained are consistent with the regional stress patterns and generally agree with the solutions of other investigators who used other aspects of the seismic wavefield.

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