The 6.1-magnitude earthquake that hit Haiti this morning was indeed a strong aftershock associated with last week's major temblor, even though it occurred eight days after the main event that devastated the island nation.

"Aftershocks are earthquakes," said Carrieann Bedwell, geophysicist with the U.S. Geological Survey (USGS) and the National Earthquake Information Center (NEIC). "When there's a large earthquake some like to say aftershocks occur because of a readjusting of the fault itself; it continually readjusts because it did have such a large energy release," Bedwell said.

With many major earthquakes, aftershocks are often felt primarily in the first 24 to 48 hours. But that's not always the case, and they can occur weeks or longer after the main earthquake.

Two main clues pointed to this morning's ground-shaking as an aftershock. "The aftershock refers to an earthquake that is close to the epicenter or within the rupture zone of a large earthquake, in this case the Haiti region where the 7.0 earthquake happened last week," Bedwell said.

Scientists define the rupture zone as the area in which energy was released from a major earthquake. And this 6.1-magnitude earthquake did occur within the rupture zone of last week's quake.

To tease out whether the shaking is considered an aftershock or a new quake, scientists also look at the depth where the new shaking started. And both quakes were shallow, occurring just below the surface.
 
"This one is shallow and that's another parameter we look at when we are determining whether this is an aftershock to a large earthquake or not," Bedwell said.

However, while earthquake analysts know it was a shallow quake, they don't know exactly how shallow. For the purposes of calculating other variables, the default is 10 kilometers, or 6.2 miles, below the surface. That's why this depth is reported for both the major earthquake and this aftershock.

"Depth is the hardest thing to actually determine on shallow earthquakes," said Don Blakeman, a geophysicist at the USGS and NEIC. "We have seismometer stations all over the world, but they are basically at the surface. We don't have any that are at depth. Most of the seismometers are no deeper than 200 or 300 feet."

He added, "It's a big deal to drill 10 kilometers," in order to insert a seismometer. "We can look at the recording of each of the waves at each station and even though we can't come up with an exact number we can see by the character of the waves that it's a shallow quake," Blakeman said.

A shallow quake also means energy gets released very close to the surface, which can cause violent ground-shaking.

Even though aftershocks are expected, with a cluster of aftershocks having occurred within this rupture zone, they can still take scientists and survivors by surprise.

"Even aftershocks are unpredictable," Bedwell told LiveScience. "We can predict that they are going to happen, but in terms of getting time and magnitude, that is unpredictable."

"This 6.1 earthquake was fairly widely felt, maybe not with the intensity or the shaking amount as the 7.0 earthquake," Bedwell said, adding the USGS has reports of the shaking being felt as far away as Santo Domingo in the Dominican Republic.

"A really rough ballpark is that the 7.0 earthquake that occurred last week is about 10 times stronger than this 6.1," Bedwell said. That's about 10 times more perceived shaking, she said