No abstract

Earth oscillations which followed the Chilean earthquake of 1960, May 22, have been clearly recorded by a sensitive tiltmeter situated in the Grotta Gigante near Trieste. An analysis of 85 hours of the N-S record has resolved the frequency spectrum in the frequency range 0-02 cycles per minute to 0-20 cycles per minute into a series of discrete peaks, most of which are highly significant at the 95 per cent confidence level.Comparison with other recent observations of free oscillations, as well as with theoretical values, leads to the identification of a complete sequence of 23 fundamental torsional eigenfrequencies (2 < 1 < 24); on removal of the main semidiurnal tides there is an indication that the four lowest fundamental spheroidal modes were also recorded.The tiltmeter and the method of analysis are described. For the 1 5 torsional modes with 10 < I < 24, application of a statistical t-test to squared deviations between the observed periods and theoretical periods for Earth models with (u) J. B. shear velocities and Bullen B densities, and (b) Gutenberg shear velocities and Bullen A densities, indicates that the latter model gives significantly better agreement with the Trieste data. The reported measurements of eigenfrequencies allow an important comparison with earlier measurements made at stations on the American continent.

No abstract

The tensor describing the tidal gravitational field of the Earth or of a planet is expressed in terms of the curvature parameters of the field.The tensor is then related to the orthonormal reference triads intrinsically defined within the gravity (or gravitational) field of the planet itself, i.e. the only triads recognizable locally with conceivable experiments: the canonical triad, for an observer connected with the Earth, and the tidal triad, for an observer in inertial navigation.Each system of orthonormal triads defines a triple system of mutually orthogonal congruences, to which an anholonomic system of coordinates is attached.Ricci's coefficients of rotation connecting the triads of the frames and the objects of anholonomity of the coordinate's systems are computed together with other geometrical quantities.

No abstract