Measuring and modelling deformation during the Dabbahu (Afar) rifting episode

In September 2005, a 60-km-long dyke intruded the Dabbahu segment of the Nubia-Arabia plate boundary (Afar, Ethiopia), marking the beginning of an ongoing rifting episode. Continued activity has been monitored using Satellite Radar Interferometry, GPS instruments and seismometers deployed around the rift in response to the initial intrusion. These data show that a sequence of 12 new dyke intrusions have reintruded the central and southern section of the Dabbahu segment. Modelling of InSAR data indicates the dykes were between 0.5 and 3 m wide, up to 10 km long and confined to the upper 10 km of crust. Seismicity data imply that the dykes were probably fed from a source near the centre of the segment. The new dykes are concentrated in areas where the 2005 dyke did not produce significant opening, implying that residual tensile tectonic stresses are higher in this location and are focusing the later intrusions. Geodetic data, which quantify the location and extension occurring in each of the events, allows the identification of regions where tensile stress has been increased. Here I demonstrate for the first time the high probability of a link involving stress transfer and dyke intrusions. Since the September 2005 intrusion, background displacement rates are significantly larger than the average secular divergence between Nubia and Arabia. Some of this deformation can be explained using viscoelastic models, which suggest an elastic crustal thickness of 13 km and upper mantle viscosity of 10 ^18.5 Pas. The presence of multiple magmatic sources around the rift zone, however, cause large residuals between the data and model suggesting that viscoelastic relaxation alone cannot account for the observed deformation. It is likely that, with a continued magma supply, dykes will continue to be intruded until the tectonic stress is fully relieved with more eruptions as the rifting episode is concluded.