Facts About the Haiti Quake

What's the difference between this and the 2004 Indian Ocean quake?
The 2004 Indian Ocean earthquake off the coast of Banda Aceh, Indonesia, was caused by a thrust fault: a tectonic plate that was sliding beneath an adjacent plate moved suddenly, forcing that second plate upward and displacing enough water to cause a tsunami. Last night's earthquake in Haiti occurred along a slip fault: two adjacent plates moving in opposite directions slipped, releasing a cataclysmic amount of energy. Strike-slip earthquakes tend to be less severe than ones that occur along thrust faults, but this one occurred just six miles beneath the earth's surface, leaving little room to absorb the energy before it reached the surface.

Why didn't the earthquake extend to the Dominican Republic?
The slip and release of tension occurred near Port-au-Prince, but the Enriquillo fault line extends straight through the southern end of the island of Hispaniola, and Haiti's neighbor may very well experience its own quake in the coming days. When tension is released in one segment of a fault line, it is often displaced down the line to another segment. That tension can then be released as an earthquake. If enough tension was displaced in this way during last night's quake, the Dominican Republic could be in some serious trouble. Scientists are assess-ing the situation this very moment.

What's the Richter scale and how high is a 7 ?
The Richter scale is not a machine (as many people assume) but a mathematical formula used to quantify the amount of energy released by an earthquake. The scale is logarithmic, meaning that each number represents an order of magnitude jump in the amount of energy re-leased. So an earthquake of magnitude 7 is 10 times stronger than an earthquake of 6, and 100 times stronger than an earthquake of 5. The 2004 earthquake near Banda Aceh was a 9 on the Richter; it released 100 times more seismic energy than last night's quake in Haiti.

What causes the aftershocks, and how long will they last?
Aftershocks are an area of active research for scientists who study earthquakes. A 2006 Nature study found that they are caused by seis-mic waves that emanate from the original earthquake. Like ripples from a stone tossed in a pond, those waves get fainter the farther they are from the source and the more time goes by. The same study linked the amount and severity of aftershocks from a given earthquake to the amount of shaking caused by the original quake. Like most earthquakes, the aftershocks from last night's quake may last for months but will probably taper off in both frequency and intensity in the next few days.

How rare are earthquakes in the Caribbean?
Earthquakes can occur anywhere there is a fault line. They are actually fairly common in the Caribbean, but they are typically much smaller—magnitude 4 or 5—and they usually occur along the eastern edge of the Caribbean plate, where it pushes against the North Ameri-can plate. Last night's quake occurred along the southern edge of the plate; this particular fault has not experienced a significant earthquake since 1770.