There was a 3.3 magnitude earthquake 11 miles southeast of Williston just before 6 a.m. Friday, according to geologists.
Mike Stickney, director of the Earthquake Studies Office at the Montana Bureau of Mines and Geology in Butte, Mont., said the earthquake was likely centered under the river or reservoir.
"It would be right on the borderline where somebody might have felt it," said Stickney. "It wasn't a large earthquake."
Fred Anderson, a geologist with the North Dakota Department of Mineral Resources in Bismarck, said anybody nearby might have felt their car move a little bit when the earthquake hit or, had there been a tall building in the vicinity, they might have seen lights swaying back and forth.
"Based on where it was, it's not likely that they would have noticed," he said.
Anderson said earthquakes in North Dakota are extremely rare because the state is in the continental interior, which is generally understood to be seismically quiet. While earthquakes here do occur, they are generally very low level.
Prior to Friday, the only other instrumentally located earthquake in North Dakota happened on July 8, 1968, near Bismarck, but that earthquake didn't exceed 4 in intensity either. Shocks have been felt in North Dakota from earthquakes that occurred elsewhere, such as Iowa, Minnesota, Montana, Nebraska and Canada, according to the U.S. Geological Survey.
When earthquakes do occur in North Dakota, they are most common in the northwestern part of the state. Stickney said there were two micro earthquakes near the border of Montana and North Dakota a couple of years ago that were only detected because the U.S. Geological Survey happened to be testing for earthquakes in the area at the time. They were a magnitude of about 1.3 to 1.5.
"The big story for this event is that it's so unusual," said Stickney. "It goes to prove that nowhere is completely immune from the possibility of earthquakes."
Both Anderson and Stickney said that any earthquakes that do happen in North Dakota are likely to be too low in intensity to cause any damage.
Though there is speculation that the increase in hydraulic fracturing along the Bakken Formation might be the cause of the earthquake, Anderson said that's mistaken. Anderson said Friday's earthquake would have occurred at an extremely shallow depth about 2,000 feet below the surface and that is well above the depths where hydraulic fracturing is occuring. Anderson said scientists also are agreed that there are no proven links between hydraulic fracturing and earthquakes.