Love- and Rayleigh-wave signals recorded at stations of the Global Digital Seismograph Network were used to retrieve focal-mechanism information for moderate-magnitude (mb ≧ 5.0) aftershocks that followed the large (MS = 7.7) Colombia earthquake of 12 December 1979. Source mechanisms compatible with the observed Love/Rayleigh spectral amplitude ratios were identified following a comparison against theoretical values calculated for a suite of source models. P-wave first motions observed at worldwide stations were then used to remove ambiguities in the sense of slip and strike orientation for most of the events examined. The focal depths of the events were fixed at values determined from depth-phase arrival times. In the surface-wave comparison, a reference earthquake with known focal mechanism and depth was used to calibrate the procedure and to minimize the path and size effects.

Aftershocks whose epicenters lie within the asperity region from which most of the moment release originated during the large Colombia earthquake denote thrust faulting similar to that of the main shock, consistent with occurrence along the interplate zone. The few near-trench events in the vicinity of the asperity may have resulted from secondary deformation away from the major plane of rupture rather than from a propagation of interplate rupture seaward to the trench axis. These near-trench aftershocks, which are seaward of the region of maximum energy release, do not provide strong evidence for seismogenic failure of the plate interface. At least one of the events is a normal-faulting earthquake that occurred either above the interplate boundary or in the subducting Nazca plate.

Source mechanisms computed for aftershocks located northeast of the asperity but within the length of the rupture reported for the main shock are also consistent with interplate slip. The aftershock activity observed north of this distance is associated with a separate, and perhaps offset, segment of the Nazca-South America plate boundary.

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