The continents are slowly drifting beneath our feet, the patterns of land and sea constantly changing. East Africa is being gradually pulled apart by the forces of plate tectonics; in several million years a new ocean will form, but at the present day a 3000km long valley and a string of volcanoes mark the rift. The established view is that stretching takes place during earthquakes as two blocks slide past each other on faults. Yet recent geodetic observations performed using ESA satellites have shown that magmatic activity may be more continuous and widespread than previously believed. Scientists are working on a new paradigm, one in which magma plays a crucial role in continental rifts.
The ISMER project uses ESAâ€™s radar satellites Envisat and ERS1/2 to measure ground deformation associated with volcanoes and faults in the East African Rift. ISMER is the first to perform a systematic program of observations along the East African Rift, to determine the temporal and spatial distribution of magma-driven processes and their implications for rift development. So far, surveys of the Kenyan and Main Ethiopian Rifts have been carried out and three seismic swarms have been studied (Dabbahu, 2005; Natron, 2007 and Karonga, 2009). Together these studies act as the basis for innovative numerical models of continental rift formation, and aim at demonstrating the use of InSAR data for hazard monitoring and geothermal exploration in developing countries.
InSAR is a satellite-based geodetic technique which compares the phase of successive radar images to produce a map of range change and thus ground displacement in the satellite line of sight. The displacement is accurate to ~ 1cm (limited by delays caused by atmospheric water vapour) and can be mapped at a spatial resolution of ~ 90m (limited by the digital elevation model). This project makes use of an extensive archive of radar images collected by the ESA satellites ERS1, ERS2 and Envisat covering nearly 20 years.
Seismic swarms have drawn attention to two dike intrusion episodes in East Africa. A dike is a vertical magma body which fails to reach the surface. The first occurred in 2005 in Afar, Ethiopia and was unusually large: 60 km long and 8 m wide. The second occurred in 2007 in Tanzania, and the surface deformation reflect a mixture of both fault motion and dike intrusion.
In less mature sections of the East African Rift, such as the Malawi Rift, there is little or no magmatic influence, and the extension takes place on normal faults. In December 2009, a sequence of earthquakes occurred in the Karonga region of Malawi. Although these earthquakes were of moderate size only, the southern East African Rift has an unusually large thickness (35-40~km) of crust within which earthquakes can occur with the potential to generate magnitude 7-8 normal-faulting earthquakes.
The geomorphology of the area is dominated by the 100-km long Livingstone Fault; the down-thrown block to the west of the fault has created a topographic low, occupied by Lake Malawi and its sediments. The deformation patterns seen in the satellite images are consistent with the rupture of a shallow, west-dipping fault in the hanging wall of the Livingstone Fault. Although magmatism and dike intrusion are important components of continental rifting even in immature sections of the East African Rift System, and the earthquakes did not follow a simple mainshock-aftershock pattern, there is no evidence for the involvement of magmatic fluids.
East Africa, like Iceland, is in a prime setting to exploit geothermal power: rifting above anomalously hot mantle. Thus volcanoes have significance both as a source of geohazard and a valuable resource. Geothermal energy appeals as an alternative as it is carbon-neutral and readily available in regions that may not have access to conventional energy sources.
Only a handful of the volcanoes in the East African Rift have any monitoring equipment and most are not considered dangerous. However, this is more due to a lack of information than a realistic appraisal of the hazards. Using ESA radar data, deformation at 8 volcanic edifices in Kenya and Ethiopia have been detected in this study, many of which are close to large populations (e.g. Addis Ababa, Nairobi). From the satellite observations alone, it is not clear whether magma migration or the flow of geothermal fluids around a hot magma chamber drives the deformation. The most likely solution is that it can be observed the dynamics of a coupled hydrothermal-magmatic system. Additional ground-based studies are now planned, including magneto-tellurics, micro-seismicity and dynamic gravity to study the best case example, the Aluto volcano in Ethiopia.