Volcanism and seismicity

Volcanic and seismic activity are the most evident expressions of the deformation processes acting at lithospheric plate boundaries; they give rise to the most important geological risks related to the active processes shaping the surface of our planet. Being an area of active deformation, the rift valley is characterized by numerous volcanic edifices, most of which are still active or showing evidences of very recent activity, and by widespread seismicity.

 

Volcanic activity
About 50 volcanic edifices in Ethiopia have documented activity during the Holocene. The majority of recent volcanoes are located within the Afar depression; among the active volcanoes, the Erta Ale is surely the most famous, being characterized by a basaltic lava lake on its summit caldera that has been active more or less continuously in the last 120 years. On the same volcanic alignment, the DallaFilla volcano gave rise -during 2008- to the biggest eruption ever recorded in Ethiopian territory in historical times. Characterized by current activity is the Dabbahu volcano whose activity started during September 2005 with a dyke emplacement, a strong seismic swarm and a small eruption, followed by other 14 dyke intrusions and 4 small eruptions, the last of which occurred during May 2010. In June 2011, a major eruption occurred at Nabro volcano, at Eritrea-Ethiopia border; many other volcanoes (e.g., Dallol, Ardoukôba, Manda-Inakir, Alayta, Dubbi) have documented activity during the XIXth and XXth centuries.
To the South of the Afar depression, many volcanic edifices have documented historical activity: an example is the Fantale volcano, in the northern part of the Ethiopian rift valley, which produced basaltic lavas at the beginning of the XIXth century. Analogously, basaltic lava flows have been erupted in the first half of the XIXth century from small volcanic centers close to the Kone caldera. More to the South, obsidian flows from the flanks of the Aluto volcano have been at around 2000 years b.p.; this volcano (like many others in the region) is characterized by strong fumarolic activity and analysis of satellite imagery has evidenced episodes of deformation (such as rapid uplift up to 10-15cm during 2004 and 2008) that indicate a significant activity of the magmatic system.

Recent volcanoes in Ethiopia (click to enlarge)

 

Small cone in the northern Ethiopian Rift Valley (click to enlarge)

 

The June 2011 Nabro eruption (Nasa image) (click to enlarge)

 

 

Seismic activity
The whole Ethiopia rift valley is characterized by a diffuse seismic activity, typically represented by events with magnitude around 5-5.5; analysis of the seismic activity in the last 150 years has evidenced that earthquakes with larger magnitude are less frequent although they have occurred in the past.
The majority of seismic activity is localized within or around the Afar depression: this region is characterized by events that have reached or have exceeded magnitude 6. Generally the strong earthquakes are associated to seismic swarms that may last for months, as in the case of the Kara Kore seismic crisis that produced -on the western escarpment of the depression between May and September 1961- more than 3500 quakes with magnitude equal or higher than 3.5 and an estimated maximum magnitude of 6.5. Other strong seismic sequence is the one that occurred during March-May 1969 in central Afar, where about 250 events with magnitude equal or higher than 3.0, at least 12 with magnitude higher than 5, and 4 with magnitude around 6 destroyed the small town of Serdo. A strong seismic sequence also occurred during August 1989 in the Dobi area: in this case, a first earthquake with magnitude 6.2 was followed in the successive 40 hours by 14 events with magnitude higher than 5, two of which that reached magnitude 6.1 and 6.3. In Afar, the seismic activity is often localized close to major volcanoes and associated to eruptions, as shown by the sequence (with events characterized by magnitude up to 5.5) that in 2005 accompanied the Dabbahu eruption.

 

Earthquakes in Ethiopia (click to enlarge)