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Earthquake distribution in Ethiopia

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The Afar Depression, the Main Ethiopian Rift and the Escarpments constitute the main seismic belts in Ethiopia. These regions belong to East African Rift system and their seismicity can be explained in terms of plate tectonics.

According to the theory of plate tectonics the earth’s surface may be divided into a number of rigid spherical caps whose boundaries are the seismic belts of the world. It is the relative motion between these caps, or plates, which causes earthquakes.

There are three major types of plates boundaries at which the relative motion occur. The first type is a boundary created when two plates are moving apart from one another.

This is typically found at a mid-oceanic ridge where new plates are being formed. The East African Rift system belongs to this category. Such boundaries are called accreting or diverting boundaries.

The second type of boundary is created where two plates are moving towards one another. This is called converting boundary.

Oceanic trenches like the Marianas and certain mountain ranges, such as the Himalayas are located along such boundaries. The third type of plate boundary occurs along transform faults where the relative plate motion is parallel to the boundary.

In all three types of boundaries strain accumulates slowly over a period of years. The abrupt release of this accumulated strain generation disturbance in the earth’s crust and this disturbance is known as an earthquake. Such earthquakes are generally associated with faulting volcanic activity.

Historical records over the last 600 years and recent instrumental observations show that there have been earthquakes in Ethiopia mainly in the Afar Depression, the escarpments, and the main Ethiopian rift.

Explanation of earthquake parameters employed in producing the earthquake distribution map is given subsequently.

Epicentre and focal depth
The initial disturbance which sends out the main waves is confined to a limited region of the earth’s interior whose linear dimensions do not ordinarily exceed the order of a few kilometres.

The centre of this confined region is called the focus (sometimes the hypocentre) of the particular earthquake.

The point of the earth’s outer surface vertically above the focus is known as the epicentre while the distance from the epicentre to the focus is called the focal depth.

In general focal depths are grouped in three categories, namely, shallow (0km-60km), intermediate (61km-300km), and deep (301 km-700 km). Almost all of the earthquakes that occur in Ethiopia are of shallow focal depth.

Earthquake size as determined by instruments is measured on a logarithmic scale called the Richter Magnitude Scale. Earthquake magnitude is related to the energy released during the occurrence of an earthquake. Empirically this is given by:
Log E = 5.8 + 2.4m
where m is the body wave magnitude and E is the energy in ergs.

Earthquakes with magnitude greater than five can cause destruction over a restricted area while earthquakes with, say, magnitude 4.5 can cause slight damage near the epicentre.

As can be inferred from the map there have been numerous earthquakes in Ethiopia whose magnitudes are greater than, or equal to, 5 (red).

These earthquakes have either been felt or caused damage. For example, the 1961 E.C. (1969) earthquake at Serdo, Central Afar, had a magnitude of 6.5 and completely destroyed the town killing 24 people and injuring 167 others.

Earthquakes whose magnitudes are less than 5 (green) are also shown on the map. In some area such earthquakes also caused damages; example, Awara Melka area, 1973 E.C. (1981). Earth quakes whose magnitudes are not determined (blue) are mainly from historical records.

(Source: National Atlas of Ethiopia)