Capturing magma intrusion and faulting processes during continental rupture: seismicity of the Dabbahu (Afar) rift

Continental rupture models emphasize the role of faults in extensional strain accommodation; extension by dyke intrusion is commonly overlooked. A major rifting episode that began in 2005 September in the Afar depression of Ethiopia provides an opportunity to examine strain accommodation in a zone of incipient plate rupture. Earthquakes recorded on a temporary seismic array (2005 October to 2006 April), direct observation of fault patterns and geodetic data document ongoing strain and continued dyke intrusion along the similar to 60-km long Dabbahu rift segment defined in earlier remote sensing studies. Epicentral locations lie along a similar to 3 km wide, similar to 50 km long swath that curves into the SE flank of Dabbahu volcano; a second strand continues to the north toward Gab'ho volcano. Considering the similar to 8 m of opening in the September crisis, we interpret the depth distribution of microseismicity as the dyke intrusion zone; the dykes rise from similar to 10 km to the near-surface along the similar to 60-km long length of the tectono-magmatic segment. Focal mechanisms indicate slip along NNW-striking normal faults, perpendicular to the Arabia-Nubia plate opening vector. The seismicity, InSAR, continuous GPS and structural patterns all suggest that magma injection from lower or subcrustal magma reservoirs continued at least 3 months after the main episode. Persistent earthquake swarms at two sites on Dabbahu volcano coincide with areas of deformation identified in the InSAR data: (1) an elliptical, northwestward-dipping zone of seismicity and subsidence interpreted as a magma conduit, and (2) a more diffuse, 8-km radius zone of shallow seismicity (< 2 km) above a shadow zone, interpreted as a magma chamber between 2.5 and 6 km subsurface. InSAR and continuous GPS data show uplift above a shallow source in zone (2) and uplift above the largely aseismic Gab'ho volcano. The patterns of seismicity provide a 3-D perspective of magma feeding systems maintaining the along-axis segmentation of this incipient seafloor spreading segment.