M4.6 - Kariba, Zimbabwe

Tectonic Summary

Seismotectonics of the East African Rift System

The East African Rift System (EARS) is a 3000 km long Cenozoic age continental rift extending from the Afar Triple Junction between the horn of Africa and the Middle East, to western Mozambique. Segments of active extension occur from the Indian Ocean west to Botswana and the Democratic Republic of the Congo (DRC). It is the only rift system in the world that is active on a continent-wide scale, providing geologists with a view of how continental rifts develop over time into oceanic spreading centers like the Mid-Atlantic Ridge.

Rifting in East Africa is not all coeval: volcanism and faulting have been an ongoing phenomenon on the continent since the Eocene (~45 Ma). The rifting began in northern East Africa, and led to the separation of the Africa (Nubia) and Arabia plates in the Red Sea and Gulf of Aden, and in the Turkana area at the Kenya-Ethiopia border. A Paleogene mantle superplume beneath East Africa caused extension within the Nubia plate, as well as a first order topographic high known as the African superswell which now includes most of the eastern and southern segments of the Nubia plate. Widespread volcanism erupted onto much of the rising plateau in Ethiopia during the Eocene-Oligocene (45 – 29 Ma), with chains of volcanoes forming along the rift separating Africa and Arabia. Since the initiation of rifting in northeastern Africa, the system has propagated over 3000 km to the south and southwest, and it experiences seismicity as a direct result of the extension, as well as active magmatism.

Today extension and magmatism are localizing in distinct eastern, western, and southwestern branches, marking the edges of two or more microplates between the sub-parallel zones of extension in East Africa. The borders of the microplates (known as Victoria and Rovuma) with the Nubia plate to the west and Somalia plate to the east now represent the most seismically active zones on the continent where normal faulting earthquakes occur on a monthly basis. They are also the sites of volcano-tectonic earthquakes. Along the rift, models of sparse GPS data and earthquake slip vectors predict average spreading rates of 1-4 mm/yr, increasing from north to south in the western branch, and increasing from south to north in the east.

The three limbs of the Afar triple junction zone experience major earthquakes, as well as frequent volcanic eruptions and dike intrusions. The magnitude of earthquakes during volcano-tectonic events is usually less than M 6, but large volumes of magma accommodating plate opening may occur during these intense, smaller magnitude swarms. The largest earthquakes recorded in this area occurred in a swarm, along a section of the evolving Arabia-Nubia plate boundary in Afar in August 1989. Studies of this earthquake swarm show that the events occurred as a result of slip on conjugate normal faults bounding the narrow Dobi graben. The swarm, made up of 25 M ≥ 4.2 earthquakes, occurred over 48 hours from 11:17 UT August 20, 1989. The events were all shallower than 15 km as a result of the thin crust beneath the triple junction compared to the surrounding continental crust. Conversely, the deepest earthquake ever recorded on the African continent (~62 km) also occurred beneath the Afar triple junction on November 8th, 1978. This M 4.9 earthquake occurred beneath an active spreading segment, and was probably associated with the movement of magma at depth during the rifting episode.

The western branch is divided into three main segments. The northern and middle segments border the Nubia - Victoria microplate boundary, and the southern segment borders the Nubia - Rovuma boundary. The northern segment comprises the West Nile region (WNR), and Lakes Albert, Kivu, and Edward (LAKE); the middle segment includes Lakes Tanganyika and Rukwa (LT, LR); and the southern segment encompasses Lake Malawi and central Mozambique (LM). The two largest instrumentally recorded events in Africa occurred ~ 300 km north of Lake Albert, in an area of Mesozoic (250-65 Ma) rifting. These M 7.1 earthquakes struck four days apart in May 1990, both at ~ 15 km depth. Their tectonic relationship to the Albert rift system or Mesozoic faults remains unclear. The most recent major earthquake to strike the region occurred on February 22nd, 2006. This M 7.0 event struck at a depth of 11 km in central Mozambique around midnight local time, killing two and injuring 15. The earthquake was felt in Mozambique, Zimbabwe, Swaziland, South Africa, and Zambia, but no major structural damage was reported.