VOA常速英语:Scientists Explain Haiti Earthquake

Tuesday's earthquake hit Haiti hard, with extensive damage and many casualties.

Compared to the well-known seismic areas like the Pacific Rim, the Caribbean seems geologically quiet, but this area — where tectonic plates meet — has a long history of earthquakes, tsunamis, and volcanoes.

In 1946, the Dominican Republic, just across the border from Haiti, experienced a magnitude eight earthquake, which triggered a tsunami. Other major recorded earthquakes in the 17th, 18th, and 19th centuries may have been produced by movement of the same fault that caused Tuesday's disaster.

Earthquakes generally occur when the tectonic plates that make up the Earth's crust move suddenly. Eric Calais, a geophysicist, says that sudden movement can follow a buildup of stress as adjacent plates struggle to move past each other.

"If you have a fault that is being strained at a speed of 7 mm per year, which is the case here," says the Purdue University professor. "The last major event on that fault occurred 250 years ago, so if the fault snaps, it's going to snap by 1.7 meters, and that's about a magnitude 7 earthquake."

There are different kinds of faults. In this case, the damage was caused by movement of a "strike-slip fault."

"It's a horizontal motion of two pieces of the Earth's crust on either side of a vertical crack in the Earth," said Calais. "That is very similar to the San Andreas Fault in California, for example. That is the way that fault operates as well."

In other cases, the plates may move up and down, rather than from side to side. Either way, the release of energy can cause tremendous damage. How much damage depends on several factors.

"The main factor is the earthquake's size," said Stuart Sipkin, a seismologist at the U.S. Geological Survey. "But then there is also the earthquake depth. As you can imagine, the deeper the earthquake goes, the more of the energy is attenuated or spread out as it goes toward the surface. This was a very shallow earthquake, which means it would tend to be more destructive. And then there's the proximity to where people live."

And, in fact, people live very close to the geologic fault that caused this earthquake.

"The city of Port au Prince, the capital of Haiti, is essentially built on that fault," said Purdue's Eric Calais.

The center of the earthquake was a scant 10 kilometers beneath the surface, and just 15 kilometers from the capital.

Tuesday's quake has been followed by a series of aftershocks, which Stuart Sipkin of the U.S. Geological Survey says are pretty typical.

"The aftershocks will start off being fairly large and fairly frequent, and then sort of peter out as time goes on, although you will see aftershocks for weeks to months after the earthquake. They will be decreasing in size and frequency, but at this point they're still large enough to be of concern in hampering the rescue efforts."

Many of the aftershocks have been measured at over magnitude 5 - enough to cause additional damage, including toppling structures weakened by the original earthquake.