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.

Magnitude
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)

The Middle complex is minor in extent, filling some troughs in the Lower Complex. The Upper Complex appears to consist of two north-south trending belts which join together in northern Ethiopia.

Mesozoic-Cenozoic tectonics
The major tectonic elements are
1. The Ethiopian plateau
2. The Somali plateau
3. The Danakil uplift
4. The Aisha uplift
5. The Rift System

The Ethiopian plateau is an uplifted block limited on the east by the Rift System and declining on the west to the Sudan plain. The eastern margin of the plateau is elevated to between 3,000m and 4,000m while, towards the west, the plateau surface descends to between 1,200 and 1,000 metres.

The Somali plateau bounded on the west by the Rift System, is at elevations of 3,000m to 3,500m on the west, and declines gradually eastward to the Indian Ocean.

The Danakil uplift appears to be a northwesterly trending horst block with a core of Precambrian rocks and Mesozoic sedimentary rocks on the limbs. The Danakil uplift continues to the northwest as the Buri peninsula. It may continue buried under Quaternary volcanics to the Gulf of Zula.

The Aisha uplift in its northern part has a crudely anticlinal structure with Jurassic limestone in the core and Cretaceous sandstones on the limbs.

Numerous north-south faults subdivide the uplift into several horst-graben blocks. In its southern part the Aisha uplift consists of a second uplifted block bounded to the southwest by a large sinistral fault.

The Rift System consists of the Red Sea rift, Afar, the Main Ethiopian rift and Lake Stefanie and Lake Rudolf rifts. The Red Sea rift is bordered on the west by the escarpment of the Ethiopian plateau.

The margin of the Red Sea rift is interpreted as a series of stepfaulted blocks, the coastal plain forming one such step.

The Afar rift is a triangle bordered in the west and south by the escarpments of the Ethiopian and Somali plateaus, and in the northeast and east by the Danakil and Aisha uplifts.

The Danakil depression, the Erta Ale, Alayta and Tat Ali volcanic ranges, and the Gawwah, Dobi and Tendaho graben form prominent features in northern Afar. Southern Afar consists of two marginal depressions parallel to the margins of the Ethiopian and Somali plateaus, and a central relatively uplifted part.

The Main Ethiopian Rift extends for 500km from 90 30’N to 4045’N. The Rift floor varies in elevation from 1,000m to 2,000m. The Northern segment of the Main Rift is 70km to 80km wide, within the north a median trough extending from southern Afar.

The central segment of the Main Rift, 280km long and 70km to 80km wide, extends from the Yerer fault to Lake Abaya. Lakes Langeno, Shala and Abijata, to mention the major ones, fill the central depression in this segment.

The southern segment of the Main Rift consists of the Abaya-Chamo Rift, the Amaro Horst, the Galana graben and several other features.

The Lake Stefanie Rift is a narrow, north-south trending graben 100km long by 25km to 30km wide. It is bounded on both sides by steep tectonic escarpments consisting of two to three step-faulted blocks. This graben stretches to the Kenya border.

The Lake Rudolf rift is a tectonic depression which includes the Turkana basin in Kenya and the Lower Omo valley in Ethiopia. The latter widens southward between north-southtrending escarpments, and is filled with Pliocene-Quaternary lacustrine and alluvial sediments.

(Source: National Atlas of Ethiopia)

(a) the highly dissected character of the landscape over much of Ethiopia’s territory
(b) the limited extent to which flat surfaces are present in Ethiopia.

The highly dissected nature of the landscape has proved a barrier to the development of internal surface communications and has resulted in the long-term isolation of many communities in mountainous area difficult of access.

In the present day, although the major road system and the pattern of penetration and feeder roads has increased very significantly, road construction over this difficult terrain remains an expensive enterprise and maintenance costs are high.

On the other hand, differences in elevation and therefore of climate conditions have provided the scene for a wide variety of cropping patterns in agriculture.

The extreme differences in altitude coupled with high rainfall have also created the conditions for a very high potential in the production of hydro-electric power.

Extensive plains are found only in the eastern Welo and Harege Regions, and in the southern half of Sidamo and Bale Regions.

The western ‘panhandle’ of the Gambela area is the only example of a flood plain in the flat area between the Baro and Akob rivers.

Less extensive but agriculturally important area of high flat plateauland are to be found in the central highlands, particularly in northern Shewa, western Welo and parts of Gojam and Gonder Regions.

(Source: National Atlas of Ethiopia)

The major physical features are the results of the extensive and spectacular faulting which cracked the old crystalline block of the African continent along its eastern side producing features which stretch from Africa’s juncture with the eastern end of the Mediterranean Basin to Mozambique in the South.

The uplifting which resulted produced the high plateau – lands which are characteristic of the central regions of Ethiopia and which stand at a general elevation of between 2000 and 2500 metres above sea level.

However, the cracking also produced the clearly defined rift valley structures formed between parallel fault lines as the main blocks continued their upward movement. The floor of the Rift Valley, which stretches from the Red Sea through, (and includes the adjacent Afar Plains), south and over the border into Kenya.

It separates the main plateau area which occupies the western half of Ethiopia from the plateaulands of the south west, and, in the central regions of the country, stands at around 1000 metres below the general level of the plateaus.

Major features of the Ethiopian landscape the therefore the lines of the great escarpments overlooking the Afar plains.

These converge in the latitude of Addis Ababa to produce the Rift Valley lakes region which is a major area of inland drainage. In addition, fault structures have produced escarpments along the western borderlands of Ethiopia.

Three smaller features, the through of the lower Omo river, the highlands of the Afar horst bordering the Red Sea, and the area of the Kobar sink (Afar Depression) which is around 110 metres below sea level, are also the result of faulting.

The earth movements which affected the continental block in the eastern African region were associated with widespread volcanic activity which produced thick deposits of hard plateau basalts.

These are responsible for the flat surfaces which are characteristic of many areas in the highlands. The Bale Mountain area, for example, is the largest area of land above 10,000 metres on the continent of Africa.

However, continuing volcanic activity resulted in the building up of higher mountain areas which now dominate the plateau surface several regions, the most notable being the Simen Mountain Massif which rises to 4620 metres above sea level in Ras Dejen.

Other examples are Abuye Meda (4000 metres) which dominates the eastern escarpment area, Mt. Chilalo (4139 metres) in Arsi, Mt. Batu (4307 metres) in the Bale mountain region, Tulu Welel (3302 metres) in Western Welega Region, and Mt. Guge (4200 metres) in north eastern Gamo Gofa Region. Volcanic mountain structures are also found on the fringes and on the floor of the Rift Valley system. Examples are Mt. Wechecha (3385 metres), Mt. Ziquala (2989 metres) and Mt. Chubbe (2592 metres) in the Lakes Rift region.

The Pluvial Periods in Africa, (comparable with the Ice Ages of more northern latitudes), were responsible for the further shaping of the Ethiopian landscape, and produced the deep gorges which have dissected the high plateau lands.

The central regions of Ethiopia are most characteristic of this plateau and gorge landscape where river beds are often 2000 metres below the general level of the plateau surface, and where the hard surface basalts have resulted in abrupt edges to the gorges, overlooking a series of lower terraces which mark the underlying layers of softer sedimentaries.

The most well-known and the largest of these features is the gorge of the Abay (Blue Nile) river.

In general, the inclinations produced by uplift have resulted in the development of river systems which drain the highlands towards the north-west and the south-east, with closed river basins associated with the Rift Valley system between these two.

Ethiopia’s relief thus contributes to drainage towards both the Mediterranean Sea and Indian Ocean.

Limited area in Ethiopia lie below the 500 metre contour line. These are to be found adjacent to the Red Sea (including the area which has sunk below sea level); in the eastern and south-eastern extremities of the Ogaden area and Bale region, including the middle section of the Wabe Shebele valley and in the Gambela panhandle in western Ethiopia where the courses of the Baro and Akobo rivers enclose a section of the Sudan plains.

The Ethiopian relief includes therefore a range of altitudes stretching from below sea level to nearly 4600 metres above sea level, although, within these latitudes, this is not sufficiently high to bring the landscape within a permanent snow line as is the case in eastern Africa to the south. Within these extremes, about 50% of the land surface is above the 1500 metre contour line.

Traditional descriptions of the landscape (which are also associated with the broader classification into ecological zones) define:

(a) Woorch (the coldest highlands above 3500 metres)
(b) Dega (the cool highlands above 2500 metres)
(c) Woyna-Dega (warm lands between 1500 and 2500 metres)
(d) Kolla (the hot and relatively low lying lands below 1500 metres)
(e) Haroor (the hottest lowlands below 500 metres)

(Source: National Atlas of Ethiopia)

Areas of ancient crystalline lowlands, in northern Eritrea, northwestern Welega and southern Borena, have inselberg topography, isolated hills scattered over the well-developed plains.

The Baro-Akobo basin forms a vast, flat lowland area in western Illubabor. In the lower course of the Baro river and its tributaries the water does not flow during the rains but forms extensive impassable swamps and marshes.

The lower Omo and Kibish basin are south of the Maji and Korma highlands on the northern shore of the Lake Rudolf. Much of this area is swampland where the Omo river enters this lake.

(Source: National Atlas of Ethiopia)

In Dalol-Asale district, the land falls to about 110 metres below sea level. Active volcanoes are numerous here. Some lakes in the hot Danakil depression have dried up and become salt flats.

(Source: National Atlas of Ethiopia)

The lakes sub-region, from Ziway to Chamo, is a relatively higher area than the Afar plain. East flowing rivers from the western highlands, the Meki, Bilate, and westward flowing rivers from the eastern highlands, replenish the waters of the Upper Rift Valley lakes.
 

(Source: National Atlas of Ethiopia)

The South Eastern Highlands have high mountains on their western rim, but no clear-cut boundary to the east, for they have a continuous slope running from the highest peak, Mt. Batu (4,308 metres), towards the south-east lowlands.

The areas of higher land are separated by the Wabe Shebele and the Genale rivers and their tributaries. Within the areas of higher elevation are found remnant, flat-topped tablelands known as ‘Ambas’ and jagged peaks isolated by river-eroded gorges and ravines.

The highest peaks in the country are in Gonder Region. Ras Dejen (4,620 metres) is an example. 44% of the high peaks of over 4,000 metres are found in this region.

The Afar Hills are limited in area and altitude but form a different region because they are at a higher altitude than the adjacent Afar lowlands.

This range of mountains is an isolated region stretching from the Gulf of Zula to the northern part of Djibouti Republic. The Afar Hills have steep sides and deep valleys and some volcanic peaks rise to an elevation of about 2,500 metres.

Among the significant lowland area are the central lowlands of the Rift Valley and outer lowlands surrounding the highlands.

(Source: National Atlas of Ethiopia)

The highlands include many sub-divisions or blocks separated by the deep, steep-sided valleys of the major rivers.

For example, the Tekeze separates the Tigray and Gonder massifs; the Blue Nile divides the Gojam, Shewa-Welega massifs; the Omo river separates the Kefa and Shewa-Gamo Gofa massifs. The elevations of the escarpments are lower towards the west, but are high and steep towards the Rift Valley in the east.

Gorges are found along the upper courses of the big rivers such as the Tekeze, Abay (Blue Nile), Omo, Wabe Shebele, which flow in deep canyons but widen out to broad and shallow valleys in their lower courses.

On the Addis Abeba-Debre Markos all-weather road at the Abay bridge, the bed of the Abay River is 1,500 metres below the general level of the plateau.

The general area of the central lava highlands and massifs consists of the Gonder, Welo, Gojam highlands, Shewa plateaus, some higher parts of Welega, and parts of Gamo Gofa and Ilubabor Administrative Regions.

The North Central Massifs (Gonder, Gojam and Welo) generally have a rugged surface with high elevation.

They include basins such as the Tana depression. The Shewa plateaus are dissected on almost all sides and form the watersheds of the Abay, the Awash and the Gibe drainage systems.

The South Western Plateaus (Gamo Gofa, Illubabor and Welega) receive the heaviest rainfall in Ethiopia, and are, as a result, very much dissected by the Gibe, and Baro rivers and the tributaries of the Abay river (Didessa, Dabus, Guder and Anger).

(Source: National Atlas of Ethiopia)

Over the ages, erosion, volcanic eruptions, tectonic movements and subsidence have occurred and continued through millennia to accentuate the unevenness of the surface.
Altitudes range from the highest peak at Ras Dejen, 4620 meters above sea level, down to the depression of the Kobar Sink, about 110 meters below sea level.

Most of the country consists of high plateau and mountain ranges with precipitious edges dissected by the numerous streams which are the tributaries of the major rivers such as the Abay (Blue Nile), Tekeze, Awash, Omo, Wabe Shebele, Baro-Akobo, etc.

Since the country is located within the tropics, the physical conditions and variations in altitude have resulted in a great diversity of climate, soil and vegetation.

The Great Rift Valley separates the Western Highlands and the South Eastern Highlands, and the highlands on each side give way to vast semi-arid lowland areas in the east and west and especially in the south of the country.

 (Source: National Atlas of Ethiopia)