INTRODUCTION

East Africa is tectonically coming apart. The ancient African continent, hundreds of millions of years old, is being broken up by faults accompanied by volcanoes and lava flows. Millions of years from now, if you think about the region's future as a geologist might, it will probably be a group of large islands that are actually small continents in the Indian Ocean, like the island of Madagascar. In fact, the northeastern part of Africa has already broken away from the rest of the continent to form the Arabian Peninsula. Arabia and Africa are now separated by the Red Sea, a newly formed ocean.

This chapter discusses two cities in the Rift Valley, Nairobi and Addis Ababa (Figure 27.1), both of which have a major earthquake hazard (Gouin, 1979); however, the hazard differs between the two cities.

NAIROBI

West and south of the Red Sea, the African continent appears to be still together, but it is cut by huge faults that form the African Rift Valleys. The eastern Rift Valley, called the Gregory Rift in Kenya, is bounded by great faults, and the center of the valley is downdropped below a high plateau, similar to the much larger continental plateau marking the tectonically more stable part of Africa to the west. Kenya's largest city, Nairobi, is on the plateau at an elevation of 5400 feet (1645 m), about the same elevation as Nairobi's sister city of Denver, Colorado. The high elevation means that even though Nairobi is on the Equator, it has a comfortable climate, accounting for its recent growth as a major commercial center, with a population greater than 3.1 million. By 2025, its population is expected to grow to five million.

Nairobi is a relatively new city, founded in 1899 as a rail depot between the cities of Mombasa, on the Kenyan coast, and Kampala in Uganda. In 1906, Nairobi's population was only 11,500. A year later, this small town was made the capital of British East Africa, in large part because its climate was agreeable to Kenya's English colonial rulers. Nairobi grew to its present population in the last few decades as a result of the large number of wealthy Kenyans and Europeans who established businesses there and built skyscrapers, and the migration to the city of poor Kenyans looking for work and to improve their lot in life.