On the last day of April and first five days of May, dozens of tiny earthquakes caused Maine’s eastern coast to tremble. What could have shaken this geologically quiet region, located in the middle of a tectonic plate, far from any active faults?
The last ice age, say geologists. Like a trampoline’s surface after liftoff, Earth’s crust along the eastern seaboard is still springing back from the pressing weight of a massive ice sheet that has since melted. The earthquakes are a present-time reminder of processes that are prehistoric at a human scale, but from a geological perspective still ongoing.
“This action is still taking place,” said Robert Marvinney, director of Maine’s Bureau of Geology. “Five or ten thousand feet of ice weighs a lot.”
All the quakes measured below magnitude 2, and many were too small to feel. Early notification came from residents’ calls to local authorities, reporting the sound of gunshots and unexpected blasting. It was actually the sound of Earth’s crust moving.
Maine experiences several earthquakes a year, but swarms are rare. The last took place in 2006, and before that in 1967. All were part of this rebound, said Marvinney.
Twenty-five thousand years ago, part of the massive Laurentide ice sheet flowed down from the northwest, ending on what is now the Georges Bank. The ice was a mile thick, two miles in some places, and so heavy that it pushed Earth’s crust down 500 feet.
Maine’s modern coast is a tapestry of features produced by that ice’s advance and retreat over underlying rock that had been contorted by eons of continental plate collisions. “We had tens of millions of years of mountain building, and then a couple hundred million years of erosion. The culmination was the last glacial period,” said Marvinney.
By the time the Laurentide sheet melted 14,000 years ago, the crust was so depressed that coastal Maine sea levels — the measure of how far sea reaches up on land — were 230 feet higher than today. The crust rebounded so violently that within 3,000 years, sea levels were some 200 feet lower than today’s.
The process is now approaching its end stages, but a bit more rebound is to be expected. “The inflection point, where shoreline goes from rising to falling, is somewhere up in Newfoundland,” said Marvinney. “The crust of the Earth is constantly moving. We just don’t think about it that way, because it seems stable during our lifetimes.”
Top images: An overlay of seismic recordings made between April 29 and May 5 in Down East Maine(Maine Geological Survey).