The Earth’s mantle is made up of Tectonic plates that are forever moving as the Earth cools down. This happens over billions of years, and one of the most notable ones is around the Pacific rim where Earthquakes, (seismic activity) happens quite often
The Rift Valley in Ethiopia lies along the edge of the African Tectonic Plate, and continues down through parts of Somalia and into Kenya.
Just recently a rift opened up in Kenya, which seismologists seem to think is the pre-cursor of a major tectonic plate movement, splitting apart a part of Africa, which could include Ethiopia.