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On simplified world maps,
the African plate is the Major lithospheric plate containing Earth's second largest continent.
This map of Oceanic plate ages shows sea-floor spreading ridges
adding new Oceanic plate to the the West, South, Eastern sides of the African plate.
As expected, shallow earthquakes occur around the boundaries of the African plate.
But why is there a band of earthquakes slicing through the Eastern part of the African continent?
When we go beyond a simple worldview, we discover that continental rifting
is breaking Eastern and Southern Africa into multiple plates and microplates.
Observations from both earthquakes and the Global Navigation Satellite Systems
have greatly advanced our understanding of tectonics in this region.
Let's begin by examining how Arabia is splitting away from Africa.
In the Gulf of Aden, sea-floor spreading separates the Arabian plate from the Horn of Africa.
In the Red Sea, spreading moves the Arabian plate away from Northern Africa.
The differences in direction and rate of sea-floor spreading require 5 millimeters per year of
Eastward motion of the Somalian plate away from the Nubian plate across the main Ethiopian rift.
This rift and the two spreading ridges meet at the Afar Triple Junction.
The main Ethiopian rift is the Northern part of the East African rift system that's
dominated by extension of normal faults that produce horst and graben structures.
The Great Lakes of East Africa occupy many of these down dropped valleys.
Over the past 20 years, high-precision GPS surveys
have determined motions of plates and microplates in this region.
As GPS research continues, refinements will be made,
but the following model of the East African rift system summarizes the state of the science.
Let's nail down the middle of the African continent to view
plate motions with respect to the Nubian plate.
Let's also remember that plate motions are rotations of relatively thin
shelves along Earth's spherical surface.
So, motion of the Somalian plate is a rotation about a pivot point called
the "pole of relative rotation".
While rotation rate is the same for all parts of the Somalian plate,
the linear rate of motion increases with distance from the pole of relative rotation.
In sequence, we can visualize the relative rotations of the Somalian plate
and the Victorian, Rowuma, and Lwandle microplates with respect to the Nubian plate.
In addition, there is a slowly moving San microplate in Southern Africa,
and a zone of distributed deformation in Northern Madagascar and across the Comoros.
These plates are internally ridged, but are in relative motion,
so faults and resulting earthquakes are concentrated along their boundaries.
The East Africa rift system extends from thousands of kilometers,
but the style and rate of the rifting varies along that length.
Rift zones form during extension of the lithosphere.
As the lithosphere rises and thins, magma rises towards the surface,
faults break the brittle crust, and horst and graben structures form.
Volcanoes dot the rift valleys.
In the South, the boundary between the Rowuma and San microplates is the Malawi rift
where continental rifting is in a early stage, with minor crustal thinning and a few volcanoes.
In the central region, the boundary between the Victoria microplate
and the Nubian plate is the Western rift where extension rates increase from North to South.
The Eastern rift is the boundary between the Victoria and Somalia
plates and extension rate increases from South to North.
In this region, continental rifting is at an intermediate stage,
with increased volcanic activity.
Across the boundary between the Somalian and Nubian plates at the Ethiopian rift,
extension is faster and continental rifting is at an immature stage with more volcanic eruptions.
Displaying the volcanoes of the East African rift system shows the anticipated pattern of
volcanic activity increasing from South to North along with the stage of continental rifting.
With extension as the driving force of continental rifting,
earthquakes are primarily caused by Northern faulting along the rift margins.
Animating earthquakes from 1970 to present shows a
concentration along major faults of the East African rift system,
including the Western and Eastern branches on opposite sides of the Victoria microplate.
While most of these earthquakes are caused by normal faulting,
many also have components of strike slip motion.
The largest East Africa earthquake ever recorded by seismometers
occurred near the Rukwa rift basin East of lake Tanganyika on December 13th, 1910.
Despite very strong ground shaking in the epicentral area, this magnitude 7.3
earthquake caused little damage to the homes used by indigenous peoples and no loss of life.
However, many European masonry buildings were severely damaged or destroyed.
On January 10th, 1928, a magnitude 7.0 earthquake
initiated at 8 kilometers depth beneath the Subukia valley of the Eastern branch.
Surface rupture with vertical offset reaching 2.4 meters
occurred over a distance of 38 kilometers along the escarpment forming the East edge of the rift.
Because of this sparse population in the epicentral area and the lightweight construction
of homes used by indigenous peoples of the time, damage was minimal and no lives were lost.
But modern Kenya is very different from British East Africa during the 1900's.
Metropolitan Nairobi now has a population approaching 10 million
and a major earthquake along the Eastern rift nearby could have a disastrous impact.
In 2016, a moderate magnitude 5.9 earthquake
struck near the Western shore of lake Victoria, 200 kilometers East of the Western branch.
Ground shaking was light in surrounding areas, but was very strong to violent near the epicenter.
At least 17 people were killed and more than 250 injured by this earthquake
that destroyed over 800 houses, leaving thousands homeless.
The boundary between the Nubian and Eurasian plates is another region of frequent earthquakes.
The plate boundary from the Azores to West of Gibraltar is dominated by transformed motion.
In the Eastern Mediterranean, the Oceanic lithosphere of the Nubian
plate subducts beneath Southern Italy , Greece, and Turkey.
Let's zoom in to examine the plate boundary in Southern Iberia and North West Africa.
GPS observations show the Eurasian plate colliding with the Nubian plate
at rates increasing from West to East.
Resulting deformation across the Atlas mountains is a combination of transform motion
and compression, creating a zone of transpression.
Resulting thrust faults are roughly parallel to the Atlas mountains,
with strike slip faults cutting across that trend.
These faults have produced the most destructive and deadly recent earthquakes in Africa.
On October 10th, 1980, the largest North Africa earthquake ever recorded by seismometers,
and probably the largest in the Atlas mountains since 1790,
struck Northern Algeria about 100 kilometers from Algiers.
This magnitude 7.2 earthquake produced violent ground shaking near the epicenter
at El-Asnam and was felt in Spain over 500 kilometers away.
Thrust faulting initiated at 10 kilometers depth and ruptured to the surface
with maximum fault displacement of 8 meters.
This fault produced surface rupture over a distance of 30 kilometers,
with vertical ground displacement up to 6 meters.
Secondary normal fault ruptures produced fault scarves in the overriding fault line.
On February 29th, 1960, the magnitude 5.8 earthquake at Agadir,
Morocco took more than 12,000 lives, a third of the population at the time,
making it the deadliest African earthquake since the 1754 Egypt earthquake.
How could this moderate magnitude earthquake produce
such severe ground shaking and destroy the majority of the buildings in Agadir?
The epicenter was immediately off shore and the focus was only about 3.5 kilometers deep,
so Agadir was only a few kilometers away from the origin of seismic energy release,
and young sedimentary layers in the coastal valley amplified the seismic waves.
Despite a previous town at this location being destroyed by an earthquake in 1731,
few buildings were constructed to seismic codes
because people thought that the area did not have earthquake risk.
As shown in this Earthquake hazard map, the Atlas mountains,
Mediterranean coastal area, and the East African rift system are the regions of highest hazard.
Progressive concentration of population in urban areas
within these regions have increased the potential impact of large future earthquakes.
Promoting earthquake resilience through building courts, emergency management,
and public education, are critical to sustaining economic development in Africa.
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