NARRATOR: Most earthquakes originate in deep fractures in the crust, miles beneath the surface. These giant fractures crisscross the globe, splitting the planet's 50-mile thick crust into around a dozen huge rocky slabs.

BILL MCGUIRE (University College London): The earth is enclosed in a rigid shell, if you like. This shell is made of different plates, rigid rocky plates, which move around at about the same speed as your fingernails grow. There's this, sort of, dance of the plates going on, all the time, around the planet's surface.

NARRATOR: Geologists call this dance, "plate tectonics." The planet's internal heat moves the huge plates. Above the hottest zones, molten rock rises and solidifies, creating new crust. The new crust jostles for space, forcing crust on cooler edges to grind against other plates or push beneath them. But wherever or however plates collide, they generate earthquakes.

BILL MCGUIRE: Earthquakes happen because these huge chunks of rock that form the earth's crust don't slide against one another nice and smoothly, they stick and they lock. So you're getting this enormous accumulation of strain, and then you're getting it released, in a matter of seconds, during the earthquake.

NARRATOR: The quakes can originate tens of miles down, but the energy they generate creates deadly vibrations on the surface.

BILL MCGUIRE: The amount of energy generated in the biggest earthquakes are comparable to many, many thousands of nuclear bombs going off. There's just a huge amount of energy, absolutely vast.

NARRATOR: Haiti lies directly above a network of massive faults, where the Caribbean plate meets the North American plate. As the plates slowly creep past one another, the rock distorts and stretches like rubber, building up enormous stress.

Since 2003, Calais and his colleagues used ground movement data to calculate stress levels at the plate boundary. Calais realized a deadly quake was inevitable.

ERIC CALAIS (Purdue University): What we saw was a fault being loaded just like a rubber band.

NARRATOR: Simple math allowed him to calculate the precise elastic strain on the fault.

ERIC CALAIS: The last earthquake on that fault occurred about 250 years ago. And it's building up elastic energy at seven millimeters per year. Seven times 250 is about 1.8 meters.

NARRATOR: That's almost six feet of stored strain, equivalent to 100 Hiroshima atom bombs.

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