"Civilization exists by geological consent, subject to change without notice."
Will Durant (1885 - 1981) American writer, historian, and philosopher
Fig.1. Woodcut of the "terrible and great" water-flood from the Rhine river in the year 1651.
The location or site of a city is influenced by political as well as geomorphologic and geological factors. Cities are concentrated on shores of rivers or lakes which can be used as transport route and provide drinkable water. Sites where a river can be forded or waterfalls and rapids occur can become a natural meeting point for people. Harbors facing the sea are important gateways for international commerce. Peninsulas or a hilly landscape can act as natural barrier and help to defend the city against enemies. Fluvial plains are flat and provide enough space for the urban infrastructure, including zones for agriculture and forests to provide food and timber. The underground should be a stable substrate suitable for larger buildings, with a firm rock base and well-drained soil. The underground can also provide rocks for construction purpose or precious resources like salt, metals or medicated waters.
Unfortunately such favorable geological conditions are often associated with increased geological activity. River plains are often also still active flood plains, like in the case of New Orleans. Harbors, like San Francisco, developed along rugged coasts where oceanic plates move under continental plates. Islands and regions with active volcanoes and volcanic soils, like in Indonesia, are very fertile and attract people. In contrast, seismically stable regions like Siberia or the interior of Australia are strongly weathered and the soils are depleted of nutrients.
Considering the distribution of earthquakes and volcanoes and the density of population, the most dangerous areas are located around the Pacific Ocean (the notorious ring of fire). The islands and peninsulas of the Indian Ocean, the interior of Africa, and the countries in the Near East and surrounding the Mediterranean Sea are also at risk.
Fig.2. Simplified map with plate boundaries, earthquake, volcanoes and larger cities with more than one million inhabitants, modified after U.S.G.S. 2005.
Main problems of modern cities are high growth rate of the population, no adequate infrastructure and development schemes, limited space, mismanagement and corruption. The concentration of humans and infrastructure in narrow spaces increases the impact from natural hazards, and even small disasters can have great effects.
Also, modern technology and industry tend to be concentrated in single sites. At the same time globalization tends to interconnect economies worldwide - a local catastrophe can have worldwide economics repercussions.
Living in cities also poses the danger that people lose awareness of natural hazards. In slightly modified or rural areas traces of past catastrophes can be observed for a long time. Landslide scars or deposits of debris flows for example act as reminder of the instability of the underground. In urbanized areas these traces tend to be smoothed or cancelled to build new infrastructures.
The late 19th and 20th century was characterized by city planners and engineers developing technical counter-measures to prevent catastrophes, like dams, canals, and protection barriers. Most of these measures are built to prevent short term events of lower magnitude, producing the false impression that floods or massive processes in general are extraordinary large and rare events. People behind the protection wall tend to be less vigilant or reduce private mitigation measures. When an event large enough to overcome the protection barriers happens, the results are even more disastrous.
Today technical measures are combined with sociological - authorities try to teach knowledge of natural events and awareness of the possible risks to better manage the response of single individuals and society to such a danger.
BOTKIN, D.B. & KELLER, E.A. (2011): Environmental Science - Earth as a Living Planet. John Wiley & Sons: 658
CHESTER, D.K.; DEGG, M.; DUNCAN, A.M. & GUEST, J.E. (2001): The increasing exposure of cities to the effects of volcanic eruptions: a global survey. Environmental Hazards 2: 89-103