Rifting in the Horn of Africa: The Eritrea Seismic Project (June 2011 - October 2012)

Abstract

SEIS-UK provided a loan of 6 broadband seismic systems (5 x CMGESP, 1 x CMG3T, 1 x CMG40T) for a period of 16 months, for deployment in a network that extended ~500 km across Eritrea from Asmara in the highlands to Assab on the edge of the Danakil in southern Eritrea. The SEIS-UK deployment was coeval with a BHP-Billiton funded array of 13 seismometers in the Danakil depression, Ethiopia (GEF loan 885) and the YOCMAL array of 50 seismometers in Yemen (GEF loan 873). The Eritrean systems recorded continuously at 100/50Hz with relatively minor data losses due to equipment problems. All data are archived on the SEISUK system and will be sent to IRIS (Incorporated Research Institutions for Seismology) and made publicly available in October 2015. The use of the Eritrean seismic data, together with Ethiopian and Yemen data has allowed us to produce high-resolution images of the crust, upper and mid-mantle (to depths of 900 km) beneath the Horn of Africa. Body-wave and surface wave tomography show focussed, melt related, anomalies in the top 100km of the lithosphere with multiple upwellings transcending the transition zone with their origin likely in the lower mantle. Receiver function estimates of transition zone structure show evidence for a stable melt layer atop the 410 km mantle discontinuity, suggesting the upwellings in the transition zone have a significant hydrous component. Receiver functions have also provided the first estimates of crustal structure in Eritrea, providing further understanding on the breakup history of Afar. Fortuitously, the seismometers were deployed just days after the eruption of Nabro Volcano in Eritrea, thus they (together with stations in Ethiopia) have provided important data to characterise the nature of this, the first ever recorded eruption of the volcano. Data from these stations (together with data from other regional networks) continues to be used to measure seismic anisotropy from splitting of SKS-waves and local earthquake S- wave data, estimate locations of local seismicity and to produce Pn tomographic images of the uppermost mantle. All these results have been presented at international conferences and have been and are currently being written up for leading international journals.