A North Carolina town was hit June 16 by its eight earthquake in just over three weeks, which means there’s an old fault line that’s now active, the U.S. Geological Survey says.

The quake was a 2.1, centered in a sparsely populated area about 2.4 miles north of West Canton. That’s the same general area where seven previous earthquakes have been recorded since May 23, ranging from a 1.8 to a 3.2, records show.

According to Yahoo News, Hundreds of witnesses have reported feeling some of the stronger quakes, but the latest had only one witness report as of midday Friday. That was filed by someone who felt weak shaking 8 miles away in the town of Clyde, the USGS says.

That means there is definitely an old fault in the area, the USGS says, but experts are at a loss as to why it has suddenly become active. They also don’t know if the “cluster” will continue or come to a sudden halt.

“Earthquakes are caused when rocks underground move. Sometimes, we have other earthquakes when those rocks come back to some kind of equilibrium,” USGS officials told McClatchy News.

Faults are cracks between two blocks of underlying rock that allow those “blocks to move relative to each other,” the USGS says. In some cases, the cracks can be several hundred miles deep.

“During an earthquake, the rock on one side of the fault suddenly slips with respect to the other. The fault surface can be horizontal or vertical or some arbitrary angle in between,” the USGS says.

All eight earthquakes were shallow and minor in magnitude, but that’s not to say stronger quakes aren’t possible, the USGS says.

The Appalachian Mountains are not on an active tectonic plate boundary — where earthquakes are common — but the region has sporadic minor quakes linked to old fault lines, geologists say.

Those faults date back hundreds of million years — to when the Appalachians were formed — and they can occasionally become active when stress builds in spots where the rock is weak, experts say.