Aug. 23, 2011 -- The source of earthquakes on the East Coast, like today's 5.8 temblor in eastern Virginia, can be difficult to pointpoint, unlike faults in well-studied areas of the U.S. where there are major fault lines, according to the US Geological Survey.
"Based on the data, to really be able to point out what has happened and what fault line was responsible it is definitely going to require more research. It can take several months to a year to discover the fault line," said Rafael Abreu, a geophysicist at the U.S. Geological Survey.
"We have noticed, and it's a particular characteristics of the eastern U.S. [earthquakes] tend to be felt more widely than an earthquake of the same size in California or Colorado. It's an effect of the geology of the eastern U.S," said Abreu.
Tuesday's quake was unlike anything scientists would expect in this area, which has had earthquakes in the 2 to 3 range throughout recent history. "It takes just about everybody by surprise," he added.
"The previous time time we had a 6 on the East Coast was also in Virginia, again back in the 19th century over in the Adirondacks. So it's a rare event, but not unprecedented," said Dr. Lucy Jones, a Seismologist with the US Geological Survey.
"Earthquakes happen around the world. There are magnitude 5's several times a day, around the world. They're a very common phenomenon, actually," Jones continued.
But, they can still be dangerous.
"Earthquakes are a national hazard," said David Applegate, a seismologist with the U.S. Geological Survey.
On Monday, the strongest earthquake to strike Colorado in almost 40 years shook awake hundreds of people, toppled groceries off shelves and caused minor damage to homes in the southern part of the state and in northern New Mexico. No injuries were reported as aftershocks continued Tuesday.
People usually associate earthquakes with the West Coast, the Pacific Northwest and Alaska, but 39 of the 50 states -- including New York and Tennessee -- have moderate to high seismic hazard risk, Applegate said.
The New Madrid fault in the central United States is particularly dangerous. The fault is among the most active in the country, running from St. Louis to Memphis.
New Madrid Earthquake Caused River to Run 'Backward'
The New Madrid fault line is best known for some of the most violent earthquakes to ever hit the U.S: a series of four in 1811 and 1812. The quakes were estimated at magnitude 7.5 to 8.0, so strong the Mississippi River reportedly flowed backward. Damage occurred as far away as Washington, D.C., and Charleston, S.C.
Some New Madrid, Mo., residents saw large cracks open in the ground. The crew of a steamboat mooring overnight along a Mississippi River island reportedly awoke to find the island had disappeared below the water.
"We have about 200 small earthquakes per year that we record," said Gary Patterson of the University of Memphis Center for Earthquake Research and Information. "Compared to California, you're getting two to three thousand smaller quakes like that, but for eastern North America, this is a hot spot, the most active seismic area east of the Rockies in the United States."
As for whether there is a big earthquake on the way for the zone, Patterson said there have been major earthquakes in the area about every 500 years in the past 1,500 years. The earthquakes happened as a part of a sequence of events, not just one main shock, he said.
Ramapo Faults Spans N.J., N.Y. and Pa.
Earlier this year, several small quakes were centered near the New Madrid fault in southeast Missouri, including some that were felt in parts of Missouri, Arkansas, Tennessee and Kentucky, according to the U.S. Geological Survey. No damage was reported.
Another well-known fault line is located in New York City. It crosses Manhattan from the Hudson River to the East River, running approximately along 125th Street.
The Ramapo Fault, another New York Metro-area fault line, runs 70 miles through New Jersey, New York and Pennsylvania.
The fault has been quiet for about 200 years, which is part of the problem, seismologists say.
"We know in the future at some point an earthquake is going to occur but we don't really understand the rules of the game yet in this area, so we can't come with numbers and specifically tell you what the hazard is going to be," Leonardo Seeber, a research scientist at the Lamont-Doherty Earth Observatory at Columbia University in New York, said.
The Associated Press and Lyneka Little contributed to this story.