The Devastating Haiti Earthquake: Questions and Answers

The earthquake that devastated Haiti Tuesday was the strongest temblor to hit the island nation in more than 200 years. The magnitude 7.0 quake caused tremendous damage that officials have yet to fully characterize, and the death toll may run into the thousands.

What caused the Haiti earthquake, and why was it so devastating? Here are answers to these and other questions:

What caused the earthquake?

The shaking started on Tuesday, Jan. 12, at 4:53 p.m. EST (21:53 UTC) in the Haiti region, just 10 miles (15 km) southwest of Port-au-Prince.

The Haiti earthquake occurred at a fault that runs right through Haiti and is situated along the boundary between the Caribbean and North American plates, which are rocky slabs that cover the planet and fit together like a giant jigsaw puzzle. These two plates constantly creep past one another, about 0.8 inches (20 mm) a year, with the Caribbean plate moving eastward with respect to the North American slab.

"Twenty millimeters a year of slippage is very small, and that’s not what people felt," said Carrieann Bedwell, geophysicist with the U.S. Geological Survey (USGS) and the National Earthquake Information Center (NEIC).

Rather, they felt the release of energy from the Enriquillo-Plaintain Garden fault system. "The two sides of the fault line moved past each other in an east-west direction and that's what caused the energy release and the Haiti earthquakes," Bedwell said.

The high magnitude of this quake took scientists by surprise, as this system of faults hasn't triggered a major temblor in recent decades. The fault has, however, been linked to some historical big ones in 1860, 1770, 1761, 1751, 1684, 1673 and 1618, though none of these has been confirmed in the field as associated with this fault, according to the USGS.

What does a magnitude 7.0 mean?

Magnitude measures the energy released at the source of the earthquake. Since magnitudes are given on a logarithmic scale, a 7.0-magnitude earthquake would correspond to 10 times greater ground motion than a 6.0-magnitude temblor. The total amount of energy released by the earthquake, however, goes up by a factor of 32 for every unit increase in magnitude.

Geoscientists also look at an earthquake’s intensity, which measures the strength of shaking produced by the earthquake at a certain location and is determined from the effects that shaking has on people, structures and the environment.

How rare was the Haiti earthquake?

The Caribbean isn't exactly a hot zone for earthquakes, but they're not unheard of in the region.

Yesterday's earthquake was one of the largest ever to hit the area — the last time an earthquake this strong struck Haiti was in the 18th century.

Haiti takes up about half of the island of Hispaniola, while the Dominican Republic lies on the other side. In 1946, an 8.0-magnitude earthquake shook Samana, in the Dominican Republic, causing about 100 fatalities. The recent quake will likely have a much greater casualty toll because it hit a more densely populated region.

Why was the Haiti earthquake so devastating?

While magnitude is important, it's the intensity that matters to those affected by a natural disaster.

"In general, earthquakes have different characteristics whether they are in the ocean or on land and depending on the geologic setting they are in," Bedwell told LiveScience. "A mountainous and rocky setting is more characteristic of not as much ground shaking, opposed to abundant sediments and not as rocky where there’s a potential for higher ground shaking. Haiti would be a more sediment type, more severe ground shaking geologic setting."

Depth is also important, as the source of the Haiti quake was 6.2 miles (10 km) below the Earth’s surface.

"The depth of this earthquake in Haiti was very shallow, meaning that the energy that was released is very close to the surface, which can also be another characteristic that causes some violent ground shaking," Bedwell said. "An earthquake that’s very deep – that energy has a chance to go through the Earth's crust before reaching the Earth’s surface and possibly not causing as much shaking of the ground."

Unofficial USGS reports suggest the shaking lasted anywhere from 35 seconds to up to a minute, Bedwell said. "That's a pretty long amount of time for the ground to be shaking."

All of these effects get magnified when the infrastructure is shoddy and not built to withstand shaking. "Unfortunately, Haiti has a rather poor economy and not a wonderful building style for earthquake resistance, so we would expect that we would see quite severe and widespread damage from this earthquake," Michael Blanpeid, associate coordinator for the USGS Earthquake Hazards Program, said in a podcast released today.

A potentially similar effect was seen when a 7.9 magnitude earthquake struck China's Sichuan province, taking tens of thousands of lives. Earthquake engineers speculated the adobe and masonry buildings and homes, many of which were probably not reinforced with steel as building codes dictate, added to the earthquake damage, especially in more rural areas.

What is the potential for future aftershocks in Haiti?

The threat is not over. "So far we have monitored over 40 aftershocks ranging from 4.5 all the way up to 5.9," Bedwell said. About 14 of those aftershocks were magnitude 5.0 or larger.

And they expect more in the coming weeks, she said. There is no way to predict whether one aftershock will be stronger than the next, as they will come in no particular order, according to Bedwell, but typically range between 4.0 and 5.5 magnitude.

The Port-au-Prince earthquake is not believed to pose a tsunami threat because it happened on land as opposed to out in the deep ocean.

"The only positive thing about this earthquake is that because it did occur on land, it did not generate a tsunami, and so that is one hazard that is quite a severe one in the area that was not faced by the people due to this earthquake," Blanpeid said.

The USGS initially sent out a tsunami alert but as more information about the quake came in, the alert was cancelled.

"A destructive widespread tsunami threat does not exist based on historical earthquake and tsunami data," according to a message posted on the USGS Web site.

The threat of mudslides is also on scientists’ radars. "Wherever there are steep slopes or coastal areas there’s likely to be landsliding, and that can bury homes, or block streams, rivers, block roads," Blanpeid said.

What was the world’s deadliest earthquake?

While the death toll in Haiti is still unknown, the deadliest earthquake in history struck that struck Shaanxi, China, in 1556, killing an estimated 830,000 people.

• The Deadliest Earthquakes in History
  
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• Gallery: Deadly Earthquakes Past and Present