Apr 29, 2015 1:29 PM

Joe Knows: Sanbornton earthquake registers a 2.2 on the Richter scale

NH1 Chief Meteorologist - NH1.com

Did you feel it last night? Around 4:11 a.m., a small earthquake happened near Sanbornton, just southwest of Laconia. It registered 2.2 on the Richter scale with a depth of about 3 miles in the ground. No damage has been reported.

So are earthquakes normal for New Hampshire? When you look at recent history dating back to 1975, you can see a flurry of seismic activity across New England. So small earthquakes are common.

But larger quakes can happen, too! New Hampshire’s largest earthquake happened on the morning of December 20, 1940. The Earth suddenly quaked near Lake Ossipee, New Hampshire with a magnitude of about 5.5 The tremors were felt all the way to New Jersey.

Without the direct help of collision or separation of plates, it is still a mystery what is causing these rumbles.

One explanation is that "ancient zones of weakness" from pre-existing faults are being reactivated in the present-day stress field of the Northeast.The bedrock of New England has been affected by many episodes of geological activity dating back to many millions of years ago.

During the past half billion years, the Earth's crust underlying New England has been the site of two major geological episodes, each of which has left its imprint on the New England bedrock.

About 350 million years ago, this area was the site of a "continental collision," in which the ancient African continent collided with the ancient North American continent to form the supercontinent known as Pangea.

Beginning about 200 million years ago, the present-day Atlantic ocean began to form as plate tectonic forces rifted the continent of Pangaea.

The last major episode of geological activity affecting the bedrock in New England occurred about 100 million years ago, during the Mesozoic era, when "continental rifting" led to the opening of the present-day Atlantic ocean.

Such an ambiguous relationship between geological features and earthquakes is typical of seismological studies in intraplate areas, which makes it very difficult to determine which parts of New England are more prone to earthquakes than others.

We can say with certainty that future earthquakes will occur in New England. But as for when and where…that is anyone’s guess.


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