The major earthquake that struck Haiti Tuesday, January 12, 2010,  may have shocked a region unaccustomed to such temblors, but the devastating quake was not unusual in that it was caused by the same forces that generate earthquakes the world over. In this case, the shaking was triggered by much the same mechanism that shakes cities along California's San Andreas fault. 

The 7.0-magnitude Haiti earthquake would be a strong, potentially destructive earthquake anywhere, but it is an unusually strong event for Haiti, with even more potential destructive impact because of the weak infrastructure of the impoverished nation. 

While reports from the ground on the effects of the quake are spotty because of downed communication lines, geologists can use worldwide measurements of the event as well as their general knowledge of how earthquakes work to piece together a picture of what happened in Haiti. 

Sliding plates 

Earthquakes typically occur along the jigsaw-puzzle pieces of Earth's crust, called plates, which move relative to one another, most of the time at an imperceptibly slow pace. In the case of the Haiti quake, the Caribbean and North American plates slide past one another in an east-west direction. This is known as a strike-slip boundary. 

Stress builds up in points along the boundary and along its faults where parts of the crust stick; eventually that stress is released in a sudden, strong movement that causes the two sides of the fault to move and generate an earthquake. The fault system that ruptured to cause this quake is called the Enriquillo-Plantain Garden fault system. 

Major earthquakes are rare in this part of the world in part because the Caribbean is a minor plate, with a fault system that isn't as long as, say, the San Andreas, which is at the boundary between two of the world's largest plates - the Pacific and North American plates. 

The unusually high magnitude of Tuesday's quake for this region is part of the reason it has likely caused enormous damage to Haiti. 

Intensity and infrastructure 

Another factor in the damage that a quake can cause is it intensity. While magnitude is a measurement of how much energy is released by an earthquake, intensity is "simply an estimate or a measure of how strongly that earthquake was felt," said Don Blakeman, an earthquake analyst with the United States Geological Survey. 

One factor that influences earthquake intensity is the distance to the epicenter of those who feel the earthquake's effects. In the case of the Haiti quake, the epicenter of the quake was only 10 miles (15 km) southwest of the capital Port-au-Prince and just 6.2 miles (10 km) below the Earth's surface, "which for earthquakes is very shallow," Blakeman told LiveScience. 

"So everyone in Port-au-Prince is basically within 30 to 40 km [18 to 25 miles] of the earthquake," he added. 

"The depth of this earthquake in Haiti was very shallow meaning that the energy that was released is very close to the surface," said Carrieann Bedwell of the USGS and NEIC. 

In contrast, areas like the Fiji Islands in the South Pacific can experience earthquakes that originate hundreds of miles down in the Earth's crust, which would already put them hundreds of miles away from the earthquake, Blakeman explained. Earthquakes are much deeper in this the South Pacific because instead of two plates sliding past one another, one is descending deep into the Earth below the other, allowing earthquakes to originate much farther down below the surface. This is called a subduction zone. 

Another unfortunate factor in the intensity equation for Haiti is the infrastructure involved. 

The 1989 Loma Prieta earthquake that struck San Francisco just before Game 3 of the 1989 World Series was also about a 7.0-magnitude quake. While it killed scores of people and caused billions of dollars in property damage, the relatively high construction standards in the city kept the devastation much lower than what will likely be the case in Haiti. 

Haiti is a poor country with lax building standards and high population density, which makes buildings more likely to crumble, according to Blakeman. "Unfortunately that's going to be a lot of the factor here," he said. 

Experts have estimated that the death toll will likely reach into the thousands, with untold numbers homeless. 

Another problem is the relative rareness of major earthquakes in the area coupled with poor public communication and education, which likely means that most Haitians were not prepared for such a disaster, as many Californians might be. 

Waiting for answers 

Information from the earthquake will help scientists better understand the future quake threat that exists for Haiti and the rest of the Caribbean by providing information that isn't available from the previously known major quakes in the region, which occurred in the 18th and 19th centuries. 

Just how bad the destruction from the quake will be remains to be seen and likely will not be known fully for days, as reports trickle in from the crippled island nation. The United Nation and Red Cross have both mobilized emergency efforts. 

"There is no doubt that we are facing a major humanitarian emergency and that a major relief effort will be required," said United Nations Secretary-General Ban Ki-moon. 

Compounding the problems of the quake aftermath will likely be the numerous aftershocks that accompany any major earthquake. Though aftershocks are typically several orders of magnitude below the original temblor, they can still cause further damage, especially with the precarious building situation in Haiti. 

The USGS has already measured more than 40 aftershocks above a 4.0 magnitude (including a 5.9 and 5.5 magnitude) and many more below that, Bedwell said. More aftershocks are anticipated in the coming days and weeks as the restive fault continues to react to the jolt that set it off in the first place. 

"We like to think they are more repositioning of the faults in the area because of the larger earthquake," Bedwell said.