Felt the ground tremble? Yup, earthquakes in New Hampshire are somewhat common

Earthquakes might be better associated with states like California. But central New Hampshire also saw some seismic activity in late December and early January.

Leslie Sonder, associate professor of Earth Sciences at Dartmouth, said Granite State quakes aren’t uncommon, even if they’re unexpected.

California sits where two tectonic plates meet and are ever so slowly sliding past each other, causing tension, stress and — you guessed it — earthquakes. The East Coast, on the other hand, doesn’t sit on a plate boundary — so we don’t get as many quakes.

But Sonder said that doesn't mean we don’t get them at all.

“New Hampshire has a certain degree of low level seismicity, as does a lot of the eastern U.S.," she said. "We just don't hear about it much because the earthquakes are small."

The earthquakes felt recently in Loudon and Concord measured 2.7 and 2 in magnitude on the Richter scale. Because they happened near a more densely populated area, lots of folks felt them. But if those quakes had hit in less populous regions, then people might have carried about their day as if nothing had occurred.

Some reports of the event said they felt like an appliance rupture. Other residents said their home shook loudly, if only for a short duration.

Sonder said the rocks beneath the Earth’s crust in our region haven’t had major geological activity for at least 300 to 400 million years, so our rocks are colder compared to California’s. That means our smaller earthquakes might register to us more because they pass on seismic activity without losing as much energy.

“They can be felt at somewhat longer distances than they might in California,” she said. “Again, that just means we perceive them more easily, but doesn't mean they're any worse or that we have to worry.”

Also, Sonder said that as earthquakes travel, the experience differs for those living further away. The length of the recent earthquakes may have felt different, for example, for those living in Manchester or Laconia — as the duration of an earthquake expands with distance.

“It's that seismic waves are composed of different frequencies,” she explained. “It turns out that they can travel at different speeds depending on their frequency. So even though they all are generated at the same time, some will arrive earlier than others. The farther away you are, the more time there is for that to spread across time.”

Sonder said New Hampshire's earthquakes might occur because of something called “glacial rebound” — where the weight of glaciers from tens of thousands of years ago pushed the Earth’s crust down; now, the crust is slowly readjusting and rising back up over time.

“That can make things creak and groan a little bit – cause a few small earthquakes,” Sonder said.

Another possibility, she said, is that there could be a plate boundary — also called a "subduction zone" or "convergent boundary" — in the works between the continent and the Atlantic Ocean. Those take millions of years to form, Sonder said, though the start of it could be happening now.