Seismicity and earthquake source mechanism study in the East African rift

Sammanfattning: A new earthquake catalogue of the Horn of Africa has been compiled for the period from 1960 to 1993. The threshold magnitude of this catalogue is found to be 4.5 (mb). Spatial and temporal variation of seismic energy release and b-value are investigated for this catalogue. The b-value is lower in the continental part of the Afro-Arabian rift system (Afar and southern Sudan) than in the southern Red Sea and Gulf of Aden rifts which is in agreement with the increasing fault segmentation from north to south. From the recent spatial distribution of earthquakes in the East African Rift, Danakil & Aisha horsts and the Tanzanian Craton are behaving as prototypes of microplates.Earthquake source parameters in some parts of the East African Rift are studied for both the pre-WWSSN (World-Wide Seismographic Station Network) eras as well as afterwards. Digitized old seismograms from Wiechert-type instruments and bulletins are used to analyze source parameters for three pre-WWSSN events. The old analog seismograms are scanned and digitized using the nXscan software. P-wave first-motion readings and waveform data from short-period, broadband and long-period instruments are employed to investigate the earthquake source mechanisms.A total of 10 earthquakes in the magnitude range from 4.8 (mb) to 6.9 (Mw) are studied for fault mechanisms using P-wave first-motion readings and waveform inversion techniques. The results show both normal and oblique slip deformations with left-lateral strike-slip components. This indicates sinistral motion of the Somalia plate with respect to the Nubia plate, in agreement with the general E-W extension of the East African Rift.

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