Comparison of dike intrusions in an incipient seafloor‐spreading segment in Afar, Ethiopia: Seismicity perspectives

摘要

Oceanic crust is accreted through the emplacement of dikes at spreading ridges, but the role of dike intrusion in plate boundary deformation during continental rupture remains poorly understood. Between 2005 and 2009 the ～70 km long Dabbahu‐Manda Hararo rift segment in Ethiopia has experienced 14 large volume dike intrusions, 9 of which were recorded on temporary seismic arrays. A detailed comparison of the seismic characteristics of the seismically monitored dikes is presented with implications for dike intrusion processes and magmatic plumbing systems. All of the migrating swarms of earthquakes started 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 earthquakes associated with the margins of the propagating dike tips are followed by the largest magnitude, primarily low‐frequency earthquakes. The seismic moment distributions show >80% of energy is released during the propagation phase, with minimal seismic energy release after the dike propagation ceases. We interpret that faulting and graben formation above the dikes occurs hours after the passage of the dike tip, coincident with the onset of low‐frequency earthquakes. Dike lengths show no systematic reduction in length with time, suggesting that topographic loading and stress barriers influence dike length, as well as changes in tectonic stress. The propagation velocities of all the dikes follow a decaying exponential. Northward propagating dikes had faster average velocities than those that propagated southward, suggesting preconditioning by the 2005 megadike, or ongoing heating from a subcrustal magma source north of the midsegment.