Seismo-volcanic crisis in Afar: the 2005 Boina rupture-eruption sequence

Abstract

Continental rupture models emphasize the role of faults in extensional strain accommodation; extension by dike intrusion is commonly overlooked. A major rifting episode that began in September, 2005 in the Afar depression of Ethiopia provides an opportunity to examine strain accommodation in a zone of incipient plate rupture. Earthquakes recorded on a temporary seismic array, direct observation of fault patterns, and geodetic data document ongoing strain and continued dike intrusion along the ~60 km-long Dabbahu-Wal’is rift segment. Epicentral locations lie along a ~3 km-wide, ~50 km-long swath that curves into the SE flank of Dabbahu volcano; a second strand continues to the north toward Gab’ho volcano. Two additional dike intrusions in June and July, 2006 indicate a central feeding source for basaltic magmas (Keir et al., 2009; Hamling et al., 2009). We interpret the depth distribution of microseismicity as the dike intrusion zone; the dikes rise from ~10 km to the near-surface along the ~60 km-long length of the tectono-magmatic segment. Focal mechanisms indicate slip along NNW-striking normal faults, perpendicular to the Arabia-Nubia plate opening vector. The seismicity, InSAR, continuous GPS, and structural patterns all suggest that magma injection from lower or sub-crustal magma reservoirs continued at least 3 months after the main episode. Cote et al (submitted) measure the low frequency content of volcano-tectonic earthquakes recorded on a temporary broadband seismic array following the September 2005 dike intrusion and volcanic eruption sequence in the Afar rift, Ethiopia. Stations at all 9 sites show signals with long low frequency codas. Strong variations in signal amplitude, frequency content and coda length are seen even between stations separated by only 2 km. The patterns of seismicity after the 2005 dike intrusion provide a 3D perspective of magma feeding systems maintaining the along-axis segmentation of this incipient seafloor spreading segment.