Using spectral amplitudes from the South Iceland Lowland (SIL) seismic network, we conduct a relative moment tensor inversion (RMTI) on aftershocks of the June 1998 Mw 5.4 event that occurred at the Hengill triple junction, southwest Iceland. Three distinct groups of spatially clustered events are observed in the region for 25 selected events that occurred during the period from 4–5 June 1998. These clusters have previously been relocated with very high accuracy using cross-correlation techniques. We use the RMTI method to determine the focal mechanisms of these events and compare our results with the SIL network mechanisms obtained using spectral amplitudes. Most focal mechanisms obtained in this study show a predominantly right-lateral strike-slip motion, similar to those obtained by the SIL network, but more consistently in agreement with the orientations of the surface faults in the area. The spectral amplitude grouping method was used to investigate discrepancies between some of the focal mechanisms obtained using RMTI and the method used in the SIL network. This resolved apparent differences in the focal mechanism solutions for two of the studied events. Cluster alignment across the presumed fault and the individual event mechanisms agree well, suggesting the occurrence of the events along a fault plane dipping steeply towards the east. Consistency in the pressure and tension axes of the focal mechanisms suggests that the region was under northeast–southwest-oriented compression during the activity. Decomposition of the moment tensors into double-couple and isotropic components and the resulting insignificant isotropic component also suggests that the styles of failure for the analyzed events was mainly due to shearing.