A combination of tiny faults and an ice age hangover caused Maine's swarm of tiny earthquakes earlier this month. The state was hit by as many as 30 minor tremors from the last day of April through the first week of May.
These tiny quakes , all of which were below magnitude 2, were caused by the Earth's crust continuing to readjust from the Ice Age, Wired reported. The quakes weren't big enough to be felt, but they sounded like gunshots which residents reported to the police because they were so shallow.
"That's not unheard of," said John Bellini, a geophysicist with the U.S. Geological Survey. "Very small quakes are often heard but not felt. It's very rare to feel something below a 2.5."
Maine isn't synonymous with earthquakes, but it has small ruptures every year. Maine sits in the middle of the North American Plate, and was once covered with huge glaciers. When these ice sheets melted around 13,000 years ago, Maine's crust began to rise. The earth buckled because of the released weight, just like a stick that is bent with two hands.
That stress is released today from tiny faults across the state. These little faults are about 100 feet (30 meters) long and about a mile deep, and can be found from the coast to the mountains.
Unlike the big fault lines that cut across California, Maine's micro-faults are too small to map, Bellini said, but once in a while they spring to life. Maine has also recorded quake swarms in 2006 and 1967. Bellini said there's no hard definition for a quake swarm; a swarm can range from a few to a thousand earthquakes.
Such "midplate" earthquakes are much less common than earthquakes occurring on faults near plate boundaries, such as Japan's massive earthquake in March , according to the U.S. Geological Survey.
Micro-quake swarms are even less common, which makes them hard to study, and scientists don't have a good theory on why swarms strike, Robert Marvinney, of the Maine Geological Survey, told NPR.
The most recent quake swarm began on April 30 and appears to have ceased on May 2, although the swarms can have quiet periods before becoming active again, such as in 2006, Marvinney said.