What's the News: When residents living on the central coast of Maine experienced nearly 30 small earthquakes in early May, some phoned their local authorities to report gunshots and unexpected blasting. That's because Maine lies far from any active faults and rarely experiences more than two earthquakes a year. Measuring less than 2 on the Richter scale, these small tremors were actually vestiges of the most recent Ice Age. As mile-high slabs of ice plowed their way across most of North America 25,000 years ago, they compressed Earth's crust hundreds of feet, and ever since the ice melted away around 14,000 years ago, the land beneath our feet has been decompressing, much like a (very slow-moving) bed springs back to an equilibrium position when you get up in the morning. “The crust of the Earth is constantly moving," Maine’s Bureau of Geology director Robert Marvinney told Wired. "We just don’t think about it that way, because it seems stable during our lifetimes."How the Heck:
Around 25,000 years ago, a lobe of the Laurentide ice sheet, a massive sheet of ice that eventually covered most of North America, "flowed down from the northwest, ending on what is now the Georges Bank," says Marvinney.
The encroaching ice sheet, which was nearly 2 miles thick in parts, was so heavy that it weighed down Earth's crust, compressing it roughly 500 feet. As the crust pushed against the underlying mantle, it moved down and forced some of the mantle to flow away from the compressed area.
When the ice sheet melted 14,000 years ago, the viscous mantle started flowing back to the once-decompressed area, pushing the crust upward over 200 feet in the first 3,000 years. The rashes of earthquakes in Maine continue today because the mantle and crust are still adjusting back toward their previous positions. Scientists say we'll continue experiencing Ice-Age-induced earthquakes until the upward force of the mantle matches the downward force of the crust (a situation known as isostasy).
What's the Context:
Earthquake swarms are rare in Maine, the last one occurring near Bar Harbor in 2006, when over 30 earthquakes triggered a few minor rock falls. The largest quake during this event measured 4.2 on the Richter scale.
Before that, the last series of earthquakes occurred in 1967, when at least 12 quakes (one with a magnitude of 3.9) disturbed the Augusta area.
In northern latitudes like Maine, there are some "earthquakes" that aren't really earthquakes: A cryoseism, or frost quake, "is a natural phenomenon that produces ground shaking and noises similar to an earthquake, but is caused by sudden deep freezing of the ground," according to the Maine Geological Survey.
Put Down the Duct Tape: The recent Maine quakes were all under 5 on the Richter scale, which means that they were well below the threshold at which earthquakes inflict damage to people or buildings. Image: The seismic waves measured between April 29 and May 5 in Maine. Courtesy of Maine Geological Survey.