Africa, Alaska and Sumatra: Earthquakes 27 June-3 July 2013


Home / Africa, Alaska and Sumatra: Earthquakes 27 June-3 July 2013

Africa Quake – Uganda’s M5.2

The East African Rift Valley – tops a deep mantle hotspot which raises the continental crust. Image credit: USGS

Although most large-scale seismic activity occurs at plate margins, the otherwise-stable continental interiors are by no means immune. The M5.2 which occurred near Kigorabaya in Uganda on 2 July is just such an example.

East Africa is tectonically fascinating. A deep mantle hotspot below Ethiopia is raising the continental crust, causing it to split. Put simply, the two northernmost breaks in the crust are associated with the creation of new ocean crust in the Red Sea and the Gulf of Aden, while that to the south is splitting the continent of Africa and has generated the East African Rift Valley.

Extensional movement along this axis causes significant movement along normal faults (downward faulting) so that medium-sized tremors, while nowhere near as frequent as in some other areas, cannot be considered unusual.

US Earthquakes: Alaska

Alaska, where the Pacific plate moves northwards and subducts under the North American plate, is regularly the location of the largest earthquakes in the US and proved to be so again this week, with a tremor of M5.7 south of the Fox Islands. The epicentre of the ‘quake was to the south of the plate boundary, indicating that it was caused by deformation within the Pacific plate rather than by movement along the interface between the two plates.


Like Sumatra, Alaska is the location for many large-magnitude earthquakes: indeed, it has the dubious distinction of being the location for the second-largest earthquake on record, an M9.2 which occurred in Prince William Sound in 1964.

Small Earthquakes, Major Subduction Zones

The week’s earthquakes illustrate the significance of tectonic setting. Alaska and Sumatra produced major earthquakes – yet for their settings they were minor: while the relatively minor M5.2 was noteworthy within an environment (the East African Rift) where earthquakes of M7 or more are rare.


USGS. Real time earthquake map. (2013). Accessed 3 July 2013

Yeats, R. Active Faults of the World. (2012). Cambridge University Press.

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