Focal depths in the range 8 to 24 km below the seafloor were determined for seven of the larger shocks associated with the Great Sumba earthquake of August 19, 1977. Depth control was achieved by modeling digitized long‐period P waves recorded by the World‐Wide Standard Seismic Network. If the base of the rupture at the time of the mainshock were no greater than the depth of the deepest aftershock, the stress drop would be nearly 500 bars. Amplitude and first motion data from the P waves were inverted for point source moment tensors. The inversion of Rayleigh and Love wave spectra computed from digital seismograms recorded by the Seismic Research Observatories (SRO) and the Abbreviated Seismic Research Observatories (ASRO) broadened the source study to include earthquakes with moments as small as 1024 dyn cm: too small for either traditional first motion solutions or P wave modeling. The inversion procedure was applied to data in the passband from 30 to 120 s. The propagation phase for the Love and the Rayleigh waves was calibrated with signals from two earthquakes for which the source phase was inferred from the P wave solutions. Agreement with the corresponding P wave solutions is nearly the same whether or not the smaller eigenvalues of the inner product matrix are retained in the inversions. The double couple components generally account for more than two thirds of the total moment. Normal faulting along an east to northeasterly strike characterizes the mechanisms in the principal zone of aftershock activity. Strike slip mechanisms consistent with horizontal compression normal to the arc prevail in the delayed aftershock zone located 200 km northwest of the mainshock. Of the earthquakes subjected to this surface wave analysis the foreshock stood out as having felt a significantly higher apparent stress, perhaps as much as one half an order of magnitude higher than the others. These results suggest that accurate source parameters for moderate to small magnitude earthquakes can be determined from surface waves recorded by the SRO and ASRO. In particular, the mechanisms of small magnitude foreshocks could be obtained in this way.

Amplitude data from direct and near‐source reflected phases are inverted to obtain point‐source moment tensors. The inversion scheme is computationally efficient, and the results can be interpreted without the uniqueness problems that plague many geophysical inversion schemes. This follows from the linear relationship between the moment tensor components and the recorded waveforms. The L1 norm is used as an optimum solution criterion, thereby allowing first motions to be included in the data set. A mixed data set is warranted when only a small number of amplitude measurements are available. Displacement amplitudes at the recording stations are estimated by seismogram modeling in the case of the shallow earthquake and by the application of an optimum lag inverse filter in the case of the deep earthquake. The inverse filter is designed to remove the combined effects of the recording system and signal distortion owing to anelasticity. Long‐period P waves from an intraplate earthquake located between the Caribbean arc and the mid‐Atlantic ridge at a depth of 25 km reveal a source with a moment time function in the far field that has rise and fall times of 2±1 s. By implication the duration of faulting was short in comparison with shallow earthquakes of similar size at active plate margins. Approximately 89% of the total moment of 0.8×1025 dyn cm pertains to a change in deviatoric stress, which is represented almost totally by a double couple. A 20% increase in the double couple component was achieved by a systematic steepening by 5°–8° of takeoff angles for ray paths to teleseismic distances computed from the Herrin travel times. A submoho source depth is assumed, consistent with generally accepted models of oceanic lithosphere. The double couple component from the moment tensor is similar to the first motion solution but is dominated by a strike‐slip rather than a dip‐slip radiation pattern. Amplitudes and first motion polarities from a deep earthquake beneath the Bonin arc yield a moment tensor that is 72% double couple and 19% compensated linear vector dipole. A similar steepening of the ray paths in this case consistent with a 7% reduction in the compressional velocity at the source depth results in a double couple component of more than 90%. Lateral heterogeneity in the source region precludes a simple interpretation of this apparent velocity reduction. Our results demonstrate that the inversion of body wave amplitude data for the unconstrained moment tensor can yield essentially pure double couples.