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Abstract

The May 2000 earthquake cluster, around 10° N and 41° E in southern Afar, has been studied using high quality data from 12 temporary and permanent broadband seismic stations deployed in the area. 140 earthquakes have been located using P- and S-wave arrival times, a well-constrained velocity model, and a double-difference location algorithm. Source mechanisms and moment magnitudes for the four largest events (M > 4) have been obtained from moment tensor inversion. There is no clear alignment of the epicentres along a fault zone; however, the events are clustered slightly southeast of Mount Amoissa along WNW—ESE extension of the Ayelu—Amoissa (Abida/Dabita) lineament. Focal mechanisms show fault motion along WNW—ESE to east—west striking normal faults, with extension oblique to the orientation of the Main Ethiopian Rift. The non-double-couple components of the source mechanisms range from 18–25%, suggesting that the seismic activity is of tectonic origin and not volcanic. Source depths are ≤7 km, in good agreement with estimates of the elastic thickness of the Afar lithosphere. We suggest that the Gewane earthquake swarm represents remnant strain accommodation along a previous line of weakness in southern Afar related to the separation of Arabia from Africa because the focal mechanisms show north—south extension similar to many of the events in central Afar at the triple junction where Arabia is presently rifting away from Africa.

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