Fault growth at a nascent slow-spreading ridge: 2005 Dabbahu rifting episode, Afar
J. V. Rowland,
E. Baker,
C. J. Ebinger,
D. Keir,
T. Kidane,
J. Biggs,
N. Hayward, T. J. Wright,
J. V. Rowland
School of Geography, Geology and Environmental Science, University of Auckland, New Zealand. E-mail: [email protected]
Search for more papers by this authorE. Baker
Department of Geology, Royal Holloway University of London, United Kingdom
Search for more papers by this authorC. J. Ebinger
Earth & Environmental Sciences, University of Rochester, USA
Search for more papers by this authorD. Keir
Department of Geology, Royal Holloway University of London, United Kingdom
Search for more papers by this authorT. Kidane
Department of Earth Science, Addis Ababa University, Ethiopia
Search for more papers by this authorJ. Biggs
Centre for the Observation and Modelling of Earthquakes and Tectonics, Department of Earth Sciences, University of Oxford, United Kingdom
Search for more papers by this authorT. J. Wright
School of Earth & Environment, University of Leeds, United Kingdom
Search for more papers by this authorJ. V. Rowland,
E. Baker,
C. J. Ebinger,
D. Keir,
T. Kidane,
J. Biggs,
N. Hayward, T. J. Wright,
J. V. Rowland
School of Geography, Geology and Environmental Science, University of Auckland, New Zealand. E-mail: [email protected]
Search for more papers by this authorE. Baker
Department of Geology, Royal Holloway University of London, United Kingdom
Search for more papers by this authorC. J. Ebinger
Earth & Environmental Sciences, University of Rochester, USA
Search for more papers by this authorD. Keir
Department of Geology, Royal Holloway University of London, United Kingdom
Search for more papers by this authorT. Kidane
Department of Earth Science, Addis Ababa University, Ethiopia
Search for more papers by this authorJ. Biggs
Centre for the Observation and Modelling of Earthquakes and Tectonics, Department of Earth Sciences, University of Oxford, United Kingdom
Search for more papers by this authorT. J. Wright
School of Earth & Environment, University of Leeds, United Kingdom
Search for more papers by this authorSUMMARY
We present a preliminary account of the near-field surface strain associated with a major magmatic rifting episode at a nascent slow spreading ridge in the Afar depression. Between 2005 September 14 and October 4, 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 dyke-induced deformation along the entire length of the segment. We document the distribution of brittle strain associated with the early part of this rifting cycle to verify the predicted pattern of deformation and constrain a conceptual model for normal fault growth in Afar, with general application to other slow spreading divergent margins. Our field investigations concentrate on the northern half of the segment, which ruptured through to the surface over a length of >30 km and a width of ∼5 km, consistent with the pattern of microseismicity recorded using a network deployed ∼1 month after the initial onset of the rifting episode on September 14. Severe ground shaking during the event was more widespread; fresh rock fall is common across the entire magmatic segment, particularly at the intersections between faults. Recent ground breaks, in the form of reactivated or newly initiated 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 generally subvertical and open along pre-existing cooling joints. Fault offset is greater than expected given the magnitude of earthquakes during the episode. The axial relief that developed consequent on fault and fissure initiation and reactivation during the 2005 Dabbahu episode is consistent with that of the entire magmatic segment. We therefore suggest that melt delivery is sufficiently frequent that favourable stress conditions for faulting are primarily achieved during dyke events.
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