"When the stress exceeds the friction at that fault, you'll have an earthquake," seismologist Paul Earle said.
In the last 24 hours, the United States Geological Survey's National Earthquake Information Center in Golden has kept tabs on seven different 5.0 or stronger earthquakes in the area around Japan.
"We had a 6.6 this morning. That's enough to seriously shake things up," Earle said. "This is a very large earthquake, and that's why we're seeing this very robust aftershock sequence."
Just last week, there was a 7.1 quake. And it appears the aftershocks may not be letting up any time soon.
"This can go on for several more months," Earle said.
He says it can even go on into next year. While the aftershocks have not caused the damage the original earthquake did (13,000 dead, more than 13,000 missing), they make life in the area very difficult.
"Just that main shock is very unnerving and you add to that the aftershocks and that only adds to the nervousness," Earle said.
Most of the aftershocks in Japan have been off the coast and have not been widely felt. Monday's earthquake did, once again, knock out power at the Fukushima Dai-ichi nuclear plant.
Seismologists like Earle get paid to do threat assessments for other parts of the world, namely the U.S. Earle says the Pacific Northwest could be vulnerable to a catastrophe similar to what happened in Japan.
"There's evidence of a very large earthquake in 1700 off the coast of Washington and Oregon that produced a large tsunami," he said.
For now, he and the rest of the scientists at the USGS will continue to look at the numbers coming out of Japan.
(KUSA-TV © 2011 Multimedia Holdings Corporation)