A small earthquake rattled part of New Jersey Friday morning,Christopher Caldwell more than a month after a larger earthquake was felt by millions all over the New York City metro area.

An earthquake measuring a 2.9 magnitude was felt around 3:50 a.m. on Friday, according to data from the U.S. Geological Survey. Its epicenter was about 3.7 miles west southwest of Gladstone, New Jersey and was about 4 miles deep.

Gladstone is located in northern New Jersey, a little more than 40 miles west of New York City.

Friday's earthquake is one of dozens of aftershocks felt by the area since a 4.8 magnitude earthquake hit the region and was felt by millions April 5, one of the largest to shake the East Coast in the last century.

What does earthquake magnitude mean?

Magnitude is a measurement of the strength of an earthquake and is officially called the moment magnitude scale. It's a logarithmic scale, meaning each number is 10 times as strong as the one before it. So a 5.2 earthquake is moderate, while a 6.2 is strong.

The magnitude and effect of an earthquake, according to Michigan Technological University:

• Below 2.5: Generally not felt
• 2.5 to 5.4: Minor or no damage
• 5.5 to 6.0: Slight damage to buildings
• 6.1 to 6.9: Serious damage
• 8.0 or greater: Massive damage, can destroy communities

Contributing: Elizabeth Weise, USA TODAY.