Body and surface waves for a shallow shock that occurred on November 25, 1965, in the middle of the Nazca plate indicate a double‐couple dip‐slip source with a horizontal pressure axis in the east‐west direction. This suggests that the Nazca plate is being compressed in the direction of plate motion. A focal depth of 9 km below the ocean bottom was precisely determined from a combination of body‐ and surface‐wave data. For a dip‐slip source, the azimuthal radiation pattern for Rayleigh waves is strongly frequency dependent, and this characteristic can be used for an accurate focal depth determination. The maximum effect of the continental margin on surface‐wave amplitude was estimated assuming conservation of energy without reflections or changes in mode. Love waves are potentially more affected than Rayleigh waves by the continental margin. The effects on Rayleigh‐ and Love‐wave amplitudes due to the continental margin were measured between Galapagos and Quito.

A least squares method is proposed to invert Rayleigh and Love wave observations in source mechanism studies. When complex spectra is available the inversion scheme is linear and it is carried out in a single step. When only amplitude spectra is available the inversion scheme may be linearized and it converges to a unique solution independent of the orientation of the initial trial source. Source depth is found by minimizing errors in repeated application of the method at different trial depths. Application to real data requires a careful weighting of observations when the path structure is not well‐known. The source representation in term of seismic moment tensor components allows for a simple analysis of the surface wave data resolving power. Complex spectra of Rayleigh waves render a unique solution, but complex spectra of Love waves admit infinite solutions. If a double‐couple source is assumed, the Love wave data may have either one or three equally satisfactory solutions. When only absolute values of surface wave spectra are available, any solution may be rotated 180° in azimuth and/or all the source tensor components may be changed in sign.

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The method propos$d by Mendiguren to determine the source parameters from free oscillation data is applied to the 1970 July 31 deep Colombian earthquake. The results indicate a source propagating horizontally for about 150 km along the lithosphere and cutting across its width. The slab behaves as a guide for source propagation. The horizontal propagation velocity is determined as 3.8 km/s. The intensity of the source grew proportionately to the second power of the propagation distance. This rate of source intensity growth may be interpreted either by a fan-shaped fault model or by a cone-shaped volume source. The average slip and stress drop are estimated as 360 cm and 300 bar for the fault model. For the volume source model the transformational shear strain and stress are estimated as 11 x lo-' and 160 bar. There is no evidence of a double couple radiation preceding the P origin time. It is shown that the isotropic and deviatoric components of the moment tensor cannot be uniquely resolved when only observations of a single mode are available. It is observed that, from a statistical basis, the available oSn data for Colombian shock can be equally well explained by a pure deviatoric source model or by a source model including an isotropic component. Numerical experiments indicate that the inclusion of higher mode data does not change this situation. But, on the other hand, numerical experiments show that the available data and the scheme used for the inversion would not result in a solution including an artificial implosive component if the actual source were pure deviatoric. If the departure from a pure deviatoric source is produced by noise, it hasto be non-random, as it could be produced by lateral heterogeneities not included in the inversion scheme.