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Part 3: Rifting in the Afar volcanic province: Modelling and kinematics |
1 Geophysical Observatory, Addis Ababa University, PO Box 1176 Addis Ababa, Ethiopia atalay{at}geobs.aau.edu.et
2 Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania, USA
3 Center for Earthquake Research and Information, University of Memphis, Memphis, Tennessee, USA
4 École et Observatoire des Sciences de la Terre, 5 rue René Descartes, F-67084 Strasbourg Cedex, France
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.
