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They are at once the most predictable and unpredictable killers. We know continent-sized slabs of earth are moving beneath our feet. We know they move at a speed that is often harmless - the same rate as our fingernails grow. But sometimes, without warning, they can slip tens of metres in a second - and bring down whole cities. About a million people have died in earthquakes in the last two decades, most in a handful of huge quakes in urban areas. Yet the populations of cities at risk continue to grow. So, how can we quake-proof a city? (Photo: A general view shows excavator vehicles and rescue workers in front of a building which collapsed in the 6.4 magnitude earthquake, in the southern Taiwanese city of Tainan early on 9 February, 2016. Credit: Getty Images)