Comparison of dike intrusions in an incipient seafloor-spreading segment in Afar, Ethiopia: seismicity perspectives

Belachew, M. and Ebinger, C.J. and Cote, D. and Keir, D. and Rowland, J. and Hammond, James O.S. and Ayele, A. (2011) Comparison of dike intrusions in an incipient seafloor-spreading segment in Afar, Ethiopia: seismicity perspectives. Journal of Geophysical Research: Space Physics 116 (B6), ISSN 0148-0227.

Abstract

Oceanic crust is accreted through the emplacement of dikes at spreading ridges, butthe role of dike intrusion in plate boundary deformation during continental rupture remainspoorly understood. Between 2005 and 2009 the ∼70 km long Dabbahu‐Manda Hararorift segment in Ethiopia has experienced 14 large volume dike intrusions, 9 of which wererecorded on temporary seismic arrays. A detailed comparison of the seismic characteristicsof the seismically monitored dikes is presented with implications for dike intrusionprocesses and magmatic plumbing systems. All of the migrating swarms of earthquakesstarted from a <5 km radius zone at the middle of the Dabbahu‐Manda Hararo segment,and traveled northward and southward along the rift axis. Small magnitude earthquakesassociated with the margins of the propagating dike tips are followed by the largestmagnitude, primarily low‐frequency earthquakes. The seismic moment distributions show>80% of energy is released during the propagation phase, with minimal seismic energyrelease after the dike propagation ceases. We interpret that faulting and graben formationabove the dikes occurs hours after the passage of the dike tip, coincident with the onset oflow‐frequency earthquakes. Dike lengths show no systematic reduction in length withtime, suggesting that topographic loading and stress barriers influence dike length, as wellas changes in tectonic stress. The propagation velocities of all the dikes follow a decayingexponential. Northward propagating dikes had faster average velocities than those thatpropagated southward, suggesting preconditioning by the 2005 megadike, or ongoingheating from a subcrustal magma source north of the midsegment.

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