NEOTECTONIC AND SEISMOTECTONIC ASSESSMENT OF THE 1961 KARA KORE EARTHQUAKE IN THE MARGINAL GRABENS OF THE AFAR RIFT (ETHIOPIA)

On the outer edge of the Afar rift, the marginal grabens are major tectonic elements that formed in the early stages of continental rifting and demonstrate present-day activity. Of societal relevance, the marginal grabens represent a significant earthquake hazard owing to proximity to population centers in the Ethiopian Highlands. In May of 1961, a large earthquake (M 6.6) occurred in the Kara Kore area, less than 300 km from the nation’s capital, Addis Ababa. This is, to date, the largest earthquake in the marginal grabens, and produced a surface rupture of 12-15 km long. The aim of this study was to assess the 1961 earthquake in terms of its rupture and fault kinematics. Available long-period seismograms were used in waveform modeling for the focal mechanism, and earthquake statistics were calculated from a modern seismicity catalog. In the field, recent faulting is observed in bedrock and alluvial fault scarps. Recent exposure along bedrock scarps is likely associated with the 1961 rupture and suggests 50-60 cm of displacement. This is consistent with the moment magnitude- displacement scaling relations. Fault plane striations demonstrate primarily normal faulting. A fault scarp in alluvium was surveyed using differential GPS. The scarp profile depicts 1.1-1.3 m of throw, which suggest it is composite displacement of the 1961 earthquake and the prior event. This could imply that the 1961 Kara Kore earthquake is a typical size event and reflects the overall fault kinematics and, hence, represents the continual increment of growth of the marginal grabens.