Comparison of dike intrusions in Afar: Seismic perspective for source and extent

The spatial and temporal characteristics of dike intrusions vary widely depending on various factors such as the state of tectonic stress, stress and deformation history and depth and pressure of the magma source region. Understanding these factors and related processes is vital in estimating the contribution of magmatism in strain accommodation during continental breakup and formation of oceanic crust. Since the onset of the magmato-tectonic activity in 2005 in Afar, dikes of different extent and characteristics, as observed by seismic and geodetic methods, intruded different parts of the 60 km-long Dabbahu - Manda Hararo rift. We will present observations of the dike intrusion processes based on seismicity associated with the 11 -13 August 07, 11 -14 November 07 and 31 March - 04 April 08 dike injections. These dikes intruded along axis regions to the N and S of the mid-segment. The syn- and post-dike intrusion hypocenters delineate a 60 km-long, 5 km-wide zone along the rift axis. In profile, hypocenter distributions outline an aseismic zone immediately south of the mid-segment. The seismicity associated with the August 2007 dike intrusion and fissural eruption showed a northward migration. The very low seismic moment (<1% of the total geodetic strain) indicates accommodation of strain aseismically. The November 2007 dike intrusion, the largest in terms of length and duration of the above dikes, was intruded as two discrete dikes. It started with tectonic earthquakes that migrated at 0.5 m/s for the first 5 hrs. This suggests rock failure happened before the injection of the fluid. The occurrence of very low-frequency earthquakes coincides with the slowed propagation at 0.15 m/s for the next 15hrs. The second discrete dike started propagating southwards with the same speed after 10hrs. Seismicity is mainly distributed shallower than 6 km. Relatively higher seismic moment (~3.3% of the total geodetic strain) is explained by propagation of the dike within the brittle crust. The seismicity associated with the March/April 2008 dike intrusion also migrated to the south starting from the same position as the November dike, but the seismicity was restricted to depths between 2km and 9km. During the same time period higher levels of activity were recorded in the rift north of the mid-segment. This 10km-long dike propagated as one discrete dike at 0.5 m/s for 18hrs. The seismicity continued at the tip of the dike for several days. The lack of seismicity before the migrating swarm of earthquakes from the dike injections can be considered as a result of sudden change in the interaction between magma pressure within the source and tectonic stresses. The variation in the 3D distribution of the earthquakes indicates different magma source depths and pressure for the dikes beneath the mid-segment.