Why are earthquakes continually hitting a small South Carolina town? The earthquakes started in December — and no one knows what is causing them or how long they will continue.

The U.S. Geological Survey has recorded more than 60 small earthquakes near the town since that first quake in December. The largest — a magnitude 3.6 — rumbled through in late June.

All the shaking has fascinated geologists, who've said this is the longest-running series of earthquakes in recent South Carolina history.

South Carolina's state geologist, Scott Howard, has been investigating these earthquakes with help from other experts. He said scientists refer to this phenomenon as an earthquake "swarm" — that is, a series of small earthquakes with no apparent mainshock.

South Carolina is on a minor fault line, Howard said, and the state has had swarms before. In the 1970s, a series of small earthquakes was traced to the creation of the Monticello Reservoir.

When the reservoir was filled with water, it put indirect pressure on the underground fault, Howard said, setting off the string of minor earthquakes.