Minor earthquakes, including three in Morris County during a 15-day span, are relatively common in the northeastern United States but puzzle geologists because the nearest active fault line is hundreds of miles away under the Atlantic Ocean.

"The cause of these earthquakes is really a big mystery because they really don't fit in with the geologist image of plate tectonics," said Catherine Riihimaki, a geologist who teaches at Drew University in Madison.

Morris CRiihimaki said that the 2.3-magnitude earthquake early this morning likely was the second of two aftershocks of the first Morris County quake Feb. 2. None of the earthquakes are believed to have caused any injury of property damage.

The first earthquake, with a 3.0 magnitude, was the largest of the three. The Lamont-Doherty Earth Observatory in Palisades, N.Y., said it was recorded at 10:34 p.m. Feb. 2. It was felt the strongest in Rockaway, Dover, Randolph and Denville.

A 2.2 magnitude earthquake last Saturday was felt primarily in Montville.

The epicenter of this morning's quake, recorded at 1:42 a.m., was just over a mile outside Dover and felt mostly by people in Denville and Randolph according to county office of emergency management coordinator Scott DiGiralomo.

Riihimaki said today there is no way of knowing whether today's quake will be the final aftershock but added, "I don't think there's any reason to be concerned."

She added that it is "very rare" for an earthquake in the Northeast to register above 4.0 - generally perceived as the minimum magnitude for causing actual damage.

More typically, earthquakes in the region are so minor that many people - as happened in each of the three Morris County quakes - report not even noticing what happened.

While the San Andreas Fault in California is an obvious hazard for a major earthquake, geologists have various theories about what could be causing comparatively tremors in regions that are distance from any active fault lines.

The active fault line along the Mid-Atlantic Ridge, hundreds of miles away, is the closest to Morris County.

Won-Young Kim, senior research scientist and seismologist at The Lamont-Doherty Earth Observatory in Palisades, N.Y., told the Daily Record last week that an east-west compressional force - caused by the force of the Mid-Atlantic Ridge pressing against the North American Plate - could cause instability.

Riihimaki said some theorize that older fault lines - such as along the Appalachian Mountains, for example - are "potential zones of weakness."

She added, though, that much will continue to remain a mystery.

"We have a hard time of predicting earthquakes in California," she said, adding that minor quakes such as the three in Morris County likely would be shrugged off by those living in the shadow of the San Andreas Fault.

"At least in New Jersey, it happens rarely enough that it makes the news," she said.

The Associated Press contributed to this story