Active Rifting at an Incipient Oceanic Spreading Centre: A Study of Local Earthquake Data in Northern Afar

The end stages of continental breakup is characterised by both lithospheric thinning and magmatic intrusions. How these rifts transition from continental rifting to seafloor spreading is not well understood. It is difficult to answer such questions by studying previously rifted continental margins, as the area of transition is buried deep beneath volcanic and sedimentary sequences. Northern Afar presents a unique opportunity to resolve this problem, as it exposes subaerially the magma-rich transition from continental rifting to an oceanic spreading centre. The region therefore acts as a laboratory in which the geological signatures of continental breakup can be investigated unambiguously. For two years, between 2011 and 2013, a seismic network of 20 seismic stations was deployed in the area. I present here the hypocentral locations and local magnitudes of over 4500 earthquakes. Seismicity is focused along the western border fault and at active volcanic centres. The seismic activity is characterised by a b-value of 1.09, suggesting that extension is mainly accommodated by swarms of small magnitude earthquakes. Magma pathways beneath active volcanoes are clearly defined by seismicity spanning the entire crust. Seismic moment release is calculated and used to examine how extension is accommodated in the region. I discuss the implications that these results have on our understanding of the distribution of extension, melt storage and migration and upper mantle processes during the final stages of continental rifting.