Fault Growth at a Nascent Spreading Ridge: 2005 Dabbahu Episode, Afar

We present a preliminary account of the near-field surface strain associated with a major magmatic rift event at a nascent slow spreading ridge in the Afar depression. Between 14 September and 4 October 2005, a volcanic eruption and 163 earthquakes (mb > 3.9), including seismic tremor, occurred within the ~60 km long Dabbahu magmatic segment. Results of the early response team demonstrated that ground deformation, derived from satellite radar data (InSAR), together with seismicity, is consistent with dike-induced deformation along the entire length of the segment. Our field investigations concentrate on the northern half of the segment, which ruptured through to the surface on its eastern margin over a length of > 30 km and a width of ~5 km. Severe ground shaking during the event was more widespread; fresh rock fall is common across the entire segment, particularly at the intersections between fault segments. Recent ground breaks, in the form of normal faults and fissures, opened with horizontal displacements up to 3 m and vertical displacements locally up to 5 m, but commonly ~2 m. These structures are sub-vertical and opened along pre-existing cooling joints. Reactivation of older scarps is common and demonstrates that fault growth arises as a consequence of intrusive magmatic episodes. Our observations verify the pattern of brittle strain predicted from 3-D models of satellite radar data and are consistent with the pattern of micro-seismicity recorded ~6 weeks after initial onset of the rifting episode.