Source parameters of aftershocks of the 22 April 1991 (MW = 7.7) Costa Rica and the 25 April 1992 (MW = 7.1) Cape Mendocino, California, earthquakes are determined using a grid search inversion of P, SH, and SV amplitude ratios recorded by sparse local networks of three-component broadband and short-period stations. The inversion procedure consists of computing synthetic seismograms for three fundamental fault orientations for all source-receiver pairs over a range of source depths; calculating the complex envelopes of the observed and synthetic seismograms to determine peak amplitudes of P, SH, and SV waves; combining the fundamental fault amplitudes for all possible values of strike, dip, and rake, at 10° increments; and determining the best fault orientation and depth as the one that yields the smallest misfit between observed and synthetic P/SH, P/SV, and SV/SH amplitude ratios. The ambiguity in the sense of motion on the nodal planes, arising due to the use of amplitude ratios, is resolved by examining P-wave polarities. The sensitivity of source parameters to uncertainties in earthquake location and crustal structure is explored. For events with good station coverage, focal mechanism determinations are stable for a wide range of assumed values of crustal structure, earthquake location, and depth. Source parameters for many of the largest events (M > 3.4) are also determined by inversion of broadband displacement waveforms using a similar grid-search technique. Comparable results were obtained using both broadband waveforms and amplitude ratios. Focal mechanism solutions for 20 aftershocks of the Costa Rica earthquake reveal a complicated faulting geometry, indicating active thrust, normal, and strike-slip faults in the back-arc of Costa Rica. The 1992 Cape Mendocino earthquake occurred at the intersection of the North American, Gorda, and Pacific plates. While the mainshock was associated with underthrusting of the Gorda plate beneath the North American plate, fault plane solutions for 70% of the 38 largest aftershocks indicate that these events result from either motion between the Gorda and Pacific plates or from internal deformation within the Gorda plate.