CO – A swarm of at least 15 earthquakes reaching up to 2.1 magnitude rattled Ridgely, Tennessee — a small town near the Mississippi River — over a two-day period, the U.S.

Geological Survey reports. The other quakes in the swarm ranged from 1.1 to 1.5 magnitude, according to the USGS. A series of quakes, including the 2.1 temblor, began shortly after 2 a.m. central time on Friday about 1.3 miles southwest of Ridgely, the USGS reported.

They continued hourly until just before 5 a.m. Another 1.1 quake hit around 7:30 a.m. More followed on Saturday,

the last of which struck at 2:11 p.m. central time, according to the USGS. All 15 were within roughly two miles of Ridgely. The two largest 2.1 quakes at 4.2 and 4.5 miles deep hit at 2:06 a.m. and 3:10 a.m. central time, the USGS reported. But only one person reported feeling a tremor to the agency.

Ridgely is home to just 1,657 people, according to the U.S. Census Bureau, and sits less than four miles from the banks of the Mississippi River. It’s also part of the New Madrid Seismic Zone — which the Missouri Department of Natural Resources refers to as “the most active seismic area in the United States, east of the Rocky Mountains.”

Hundreds of earthquakes every year are reported in the New Madrid Seismic Zone, but the state agency said most go undetected by humans.

Magnitude measures the energy released at the source of the earthquake, the U.S. Geological Survey says. It replaces the old Richter scale.

Quakes between 2.5 and 5.4 magnitude are often felt but rarely cause much damage, according to Michigan Tech.