The volcanically active Main Ethiopian rift (MER) marks the transition from continental rifting in the East African rift to incipient seafloor spreading in Afar. We use new seismicity data to investigate the distribution of strain and its relationship with magmatism immediately prior to continental breakup. From October 2001 to January 2003, seismicity was recorded by up to 179 broadband instruments that covered a 250 km × 350 km area. A total of 1957 earthquakes were located within the network, a selection of which was used for accurate location with a three‐dimensional velocity model and focal mechanism determination. Border faults are inactive except for a cluster of seismicity at the structurally complex intersection of the MER and the older Red Sea rift, where the Red Sea rift flank is downwarped into the younger MER. Earthquakes are localized to ∼20‐km‐wide, right‐stepping en echelon zones of Quaternary magmatism and faulting, which are underlain by mafic intrusions that rise to 8–10 km subsurface. Seismicity in these “magmatic segments” is characterized by low‐magnitude swarms coincident with Quaternary faults, fissures, and chains of eruptive centers. All but three focal mechanisms show normal dip‐slip motion; the minimum compressive stress is N103°E, perpendicular to Quaternary faults and aligned volcanic cones. The earthquake catalogue is complete above ML 2.1, and the estimated b value is 1.13 ± 0.05. The seismogenic zone lies above the 20‐km‐wide intrusion zones; intrusion may trigger faulting in the upper crust. New and existing data indicate that during continental breakup, intrusion of magma beneath ∼20‐km‐wide magmatic segments accommodates the majority of strain and controls the locus of seismicity and faulting in the upper crust.

[1] Lower crustal earthquakes are commonly observed in continental rifts at depths where temperatures should be too high for brittle failure to occur. Here we present accurately located earthquakes in central Ethiopia, covering an incipient oceanic plate boundary in the Main Ethiopian Rift. Seismicity is evaluated using the combination of exceptionally well resolved seismic structure of the crust and upper mantle, electromagnetic properties of the crust, rock geochemistry, and geological data. The combined data sets provide evidence that lower crustal earthquakes are focused in mafic lower crust containing pockets of the largest fraction of partial melt. The pattern of seismicity and distribution of crustal melt also correlates closely with presence of partial melt in the upper mantle, suggesting lower crustal earthquakes are induced by ongoing crustal modification through magma emplacement that is driven by partial melting of the mantle. Our results show that magmatic processes control not only the distribution of shallow seismicity and volcanic activity along the axis of the rift valley but also anomalous earthquakes in the lower crust away from these zones of localized strain.

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The Miocene-Holocene East African Rift in Ethiopia is unique worldwide because it subaerially exposes the transition between continental rifting and seafl oor spreading within a young continental fl ood basalt province. As such, it is an ideal study locale for continental breakup processes and hotspot tectonism. Here, we review the results of a recent multidisciplinary, multi-institutional effort to understand geological processes in the region: The Ethiopia Afar Geoscientifi c Lithospheric Experiment (EAGLE). In 2001-2003, dense broadband seismological networks probed the structure of the upper mantle, while controlled-source wide-angle profi les illuminated both along-axis and across-rift crustal structure of the Main Ethiopian Rift. These seismic experiments, complemented by gravity and magnetotelluric surveys, provide important constraints on variations in rift structure, deformation mechanisms, and melt distribution prior to breakup. Quaternary magmatic zones at the surface within the rift are underlain by high-velocity, dense gabbroic intrusions that accommodate extension without marked crustal thinning. A magnetotelluric study illuminated partial melt in the Ethiopian crust, consistent with an overarching hypothesis of magmaassisted rifting. Mantle tomographic images reveal an ~500-km-wide low-velocity zone at ≥ ≥75 km depth in the upper mantle that extends from close to the eastern edge of the Main Ethiopian Rift westward beneath the uplifted and fl ood basaltcapped NW Ethiopian Plateau. The low-velocity zone does not interact simply with the Miocene-Holocene (rifting-related) base of lithosphere topography, but it also provides an abundant source of partially molten material that assists extension of the seismically and volcanically active Main Ethiopian Rift to the present day.

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