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A comparison of free oscillations of oceanic and continental earth models

SEISMOGRAPHIC STATION DEPARTMENT OF GEOLOGY AND GEOPHYSICS UNIVERSITY OF CALIFORNIA, BERKELEY, CALIFORNIA

Abstract

It is well-known that Rayleigh and Love waves over continental and oceanic structures have different periods (T) for a given order (n). In the present study, this difference is explored over the spectrum for periods greater than 30 sec, and particularly the graver fundamental spheroidal and torsional oscillations. The oceanic model is the same as the continental model below 400 km depth, is adjusted between 400 km and 10 km to preserve the Earth's overall mass and moment of inertia, and has 5 km of crust and 5 km of ocean. For torsional oscillations, the difference in period for a given n is about 3 sec throughout the range of 2 n 120, or 0.15 per cent to 3.3 per cent. However, the two curves for the group velocity are not parallel in this range: continental group velocity increases monotonically with T, while oceanic group velocity has a broad minimum for 160 < T < 200 sec. For spheroidal oscillations, the difference in period for a given n is about 2 sec for 2 n 120, or 0.1 per cent to 2 per cent. For n = 0, the difference is only 0.1 sec or 0.008 per cent. The two group velocity curves are almost parallel with minima in the same range, 205 to 210 sec. Comparison with the precision of available measurements of the free periods shows that the presence of an oceanic crust and upper mantle is important for fitting models of the earth to any set of free oscillation observations.

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