Strain accommodation by magmatism and faulting as rifting proceeds to breakup: Seismicity of the northern Ethiopian rift

[ 1] 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 x 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 similar 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 degrees E, perpendicular to Quaternary faults and aligned volcanic cones. The earthquake catalogue is complete above M-L 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 similar to 20-km-wide magmatic segments accommodates the majority of strain and controls the locus of seismicity and faulting in the upper crust.