Crustal structure across Afar, Ethiopia, from surface wave tomography

We present the results of a study of the group velocity dispersion characteristics of fundamental-mode surface waves across Afar, Ethiopia. We analyze surface wave data for more than 200 events recorded in 2007 and 2008 by the NERC Afar Consortium experiment and the NSF Rochester seismic networks deployed in northern Ethiopia. We first apply a dispersion analysis using the frequency-time analysis to obtain the group velocity of the fundamental mode of Rayleigh waves for periods ranging from 3 to 15 sec. We then invert path-averaged dispersion curves to obtain 2D maps, of group velocity for discrete periods using a method that is the generalization to two dimensions of the Backus and Gilbert method. The resulting tomographic maps show a significant low velocity anomaly below the Dabbahu and Hararo magmatic segments, the site of recent dyke intrusions and maps out crustal anomalies within the Afar rift segments. Our results represent the first detailed surface wave study of crustal and uppermost upper mantle structure of Afar and could lead to significant progress towards a better understanding of the lithospheric structure and magma distribution in the area.