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 M-w 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.

We present a stepwise inversion procedure to assess the focal depth and model earthquake source complexity of seven moderate-sized earthquakes (6.2 > M-w > 5.1) that occurred in the Afar depression and the surrounding region. The Afar depression is a region of highly extended and intruded lithosphere, and zones of incipient seafloor spreading. A time-domain inversion of full moment tensor was performed to model direct P and SH waves of teleseismic data. Waveform inversion of the selected events estimated focal depths in the range of 17-22 km, deeper than previously published results. This suggests that the brittle-ductile transition zone beneath parts of the Afar depression extends more than 22 km. The effect of near-source velocity structure on the moment tensor elements was also investigated and was found to respond little to the models considered. Synthetic tests indicate that the size of the estimated, non-physical, non-isotropic source component is rather sensitive to incorrect depth estimation. The dominant double couple part of the moment tensor solutions for most of the events indicates that their occurrence is mainly due to shearing. Parameters associated with source directivity (rupture velocity and azimuth) were also investigated. Re-evaluation of the analysed events shows predominantly normal faulting consistent with the relative plate motions in the region.

The region of interest is characterized by incomplete data sets and little information about the tectonic features. Therefore, two methodologies for estimating seismic hazard were used in order to elucidate the robustness of the results: the method of spatially smoothed seismicity introduced by Frankel (1995) and later extended by Lapajne et al. (1997) and a Monte Carlo approach presented by Ebel and Kafka (1999). In the first method, fault-rupture oriented elliptical Gaussian smoothing was performed to estimate future activity rates along the causative structures. Peak ground accelerations were computed for a grid size of 15 km × 415 km assuming the centre of the grids as epicentres, from which the seismic hazard map was produced. The attenuation relationship by Ambraseys et al. (1996) was found suitable for the region under study. PGA values for 10% probability of exceedence in 50 years (return period of 475 years) were computed for each model and a combined seismic hazard map was produced by subjectively assigning weights to each of these models. A worst-case map is also obtained by picking the highest value at each grid point from values of the four hazard maps. The Monte Carlo method is used to estimate seismic hazard, for comparison to the results from our previous approach. Results obtained from both methods are comparable except values in the first set of maps estimate greater hazard in areas of low seismicity. Both maps indicate a higher hazard along the main tectonic features of the east African and Red Sea rift systems. Within Eritrea, the highest PGA exceeded a value 25% of g, located north of Red Sea port of Massawa. In areas around the capital, Asmara, PGA values exceed 10% of g.