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A small earthquake was detected under the sea floor off the North Shore Tuesday night, officials said.

The 2.2-magnitude quake happened about 30 miles east of Rockport, according to the US Geological Survey.

“The earthquakes are happening in the rock,” said John Ebel, a researcher at the Weston Observatory at Boston College, which studies geophysical science.

“The rock doesn’t care if there’s water on top of it or not,” he said. “We say it’s offshore, but the rock doesn’t care where it wants to crack.”

USGS said the quake happened at about 10:30 p.m. Tuesday and was centered 6 miles below the surface.


Rockport and Gloucester police said the departments did not receive any reports from residents who said they felt the temblor.

“Some may have not known there was a quake,” Ebel said. “The odds of feeling it are low, but not impossible.”

It is more likely to have smaller magnitude earthquakes than larger ones, Ebel said. A 2-magnitude quake will happen a few times a year while a 1-magnitude quake may occur a couple of times a month, he said.

Earlier Tuesday, there was a quake near Franconia, N.H. The quake was originally graded a 1.5, but the USGS said Wednesday it was a 2.0-magnitude quake.

Mina Corpuz can be reached at mina.corpuz@globe.com. Follow her on Twitter @mlcorpuz.