The Afar Depression: interpretation of the 1960-2000 earthquakes

We studied the seismic activity of the Afar Depression (AD) and adjacent regions during the period 1960-2000. We define seven distinct seismogenic regions using geological, tectonic and seismological data. Based on the frequency- magnitude relationships we obtain b-values of about 1 for the different regions. The pattern of the distribution of the location of epicentres fits with the known active fault zone in the AD and the axial volcanic ridges. The Bab el Mandab area and the Danakil-Aysha'a blocks are less active. For 125 intermediate to strong earthquakes the seismic moment and source parameters were calculated. The results of the fault plane solutions for the Afar Depression indicate mainly strike-slip and normal sense of movement originating from fault planes striking NW-SE. These results indicate a clockwise block rotation described previously as a bookshelf model in central AD. There are a few right-lateral faults east of Massawa with E-W-striking fault planes. At the southern Red Sea, north of the Danakil block, the mixed focal mechanisms, with axial plane striking NW-SE, comprise several reverse faulting, strike-slip motion and normal faulting. Right-lateral movement was also calculated for a cluster of seismic events between the Manda Hararo and Alyata volcanic ridges along NW-SE-striking faults. Along the N-S-striking faults in the escarpment, at the western Afar margins, there are two distinct clusters of epicentres. The strong earthquakes at the southern cluster exhibit normal or strike-slip motions. The intermediate to small earthquakes in the northern cluster exhibit reverse and strike-slip motions. Mainly normal faults were calculated along NE-SW-striking faults of the Ethiopian East African Rift. Estimates of the seismic efficiency suggest that the maximal values are about 50 per cent or less, implying that most of the motion is taken aseismically.