BAR HARBOR – Monday’s earthquake, which rattled homes in more than 150 Maine towns and sent several large pieces of rock tumbling into the roads of Acadia National Park, may be the work of a widening fault in the Atlantic Ocean, according to a geologist.

It also may be creating a new fault line in the middle of Frenchman Bay, he said.

John Ebel, director of Boston College’s Weston Observatory, said Tuesday that scientists cannot say whether the 3.9 magnitude quake will be the last of a string of temblors that have shaken Mount Desert Island over the past 11 days or if more rumblings are on their way.

“We don’t really know what’s going on,” he said. “We can’t predict that at this point.”

The observatory, which monitors seismic activity in New England for the U.S. Geological Survey, initially indicated on its Web site that the 8 p.m. quake had a magnitude of 3.4. That figure has since been revised to 3.9, making it the strongest of the recent tremors felt in eastern Maine.

A 3.5 magnitude quake with an epicenter under the east side of Acadia’s Champlain Mountain struck early on the morning of Sept. 22. Six days later, Weston Observatory recorded two aftershocks, with magnitudes of 1.8 and 2.5, a few miles north of downtown Bar Harbor underneath Frenchman Bay.

Magnitude measurements represent the strength of earthquakes as calculated by seismographs, which detect and record ground motions. Magnitude is measured in exponential increments so that a difference of 1 – from 3.5 to 4.5, for example – represents a tenfold increase in strength. Magnitude 4 earthquakes are capable of causing moderate damage while those measured at magnitude 5 can cause considerable damage.

According to the Weston Observatory’s official Web site, tremors from Monday’s quake were felt as far away as New Hampshire and East Hampton, N.Y., at the eastern end of Long Island.

Ebel said the seismic activity may be the result of continental plates pushing from the middle of the Atlantic Ocean westward toward the Pacific. Just because there are no known active fault lines in the area, he said, doesn’t mean Maine is immune to global seismic forces.

There have been temblors in the Northeast and eastern Canada that have preceded stronger earthquakes, according to Ebel. He said a quake of magnitude 4.7 in northern Quebec in November 1988 was followed two days later by a quake of magnitude 5.9. But Monday’s temblor, centered in Frenchman Bay off Great Head, might be it for a while, he said.

“There may be a brand-new fault line that wasn’t there before,” Ebel said. “It’s a puzzle we’re still working out.”

Officials with Acadia National Park also were working Tuesday to determine the effects of the quake. Rangers and road crew workers closed part of the Park Loop Road near Champlain Mountain so they could remove rocks that had fallen from roadside embankments and knock down other pieces of rock that had come loose and were in danger of falling.

Several hiking trails near Champlain Mountain were closed Tuesday while rangers inspected them for possible safety hazards. Repairs are needed on Precipice Trail before it can be safely opened to the public again, park dispatcher Tony Linforth said, and East Face Trail needs to be checked further before rangers can determine whether it is safe to use.

A section of slab also fell onto the Homan Trail on Dorr Mountain, park officials said, but that trail will remain open and the debris will be removed at a later date.

Bill Gawley, an Acadia scientist who monitors air and water quality, said another point of concern was the effect the earthquakes are having on groundwater. Two USGS groundwater wells in Bar Harbor, one off Eagle Lake Road and another off Crooked Road, each had decreases in water levels when the latest earthquake struck, he said. So did Otter Creek, which flows between Champlain and Dorr mountains into Otter Cove, he said.

“Both of them showed a really pronounced drop,” Gawley said of the wells.

Boston College’s Ebel said changes to groundwater levels caused by earthquakes are not uncommon. Sometimes the effects are only temporary, he said.

“[Earthquakes] literally squeeze and stretch the rock, so that will affect the groundwater,” he said. “Sometimes they can be permanent.”

All three seismic events recorded since Sept. 22 have been centered within a few miles of The Jackson Laboratory, a world-renowned institution that specializes in mouse genetics and in breeding mice for scientific purposes.

Mark Wanner, spokesman for the lab, said Tuesday that none of the recent tremors has had a discernable effect on the lab’s Route 3 campus. All of the buildings have been inspected since Monday’s earthquake, and no structural damage has been found, he said.

“Some clocks fell off the walls and some shelves fell down,” Wanner said. “As far as we know, everything is fine.”

Wanner said the lab was not fully staffed at 8 p.m., when the latest quake struck, so they are not sure whether the mice had any immediate reaction to the tremors. As for longer-term behavior, it may take a few weeks for any changes to become apparent.

“It may affect reproduction,” he said, “but you won’t see any possible effect on that for a while.”

Earthquake facts

Earthquakes are caused by modern stress released occasionally along zones of weakness in the earth’s crust.

No significant amount of motion has been shown for any fault since the Ice Age, 20,000 years ago.

The largest earthquake recorded in Maine between 1747 and 1992 was near Eastport in 1904 with a Modified Mercalli intensity estimated at VII. (The scale runs from I to XII, with VII indicating a strong quake that causes considerable damage.)

Other strong earthquakes have been in Lewiston (1857), Sabattus (1905), Bridgton-Norway (1918), Milo (1928) and Portland (1957).

No Maine earthquake has caused significant damage.

Earthquakes have been reported by all Maine counties, with somewhat higher activity in central, eastern and southwestern parts of the state.

There is no obvious relationship between earthquakes and mapped faults in most areas. It is not clear whether mapped faults in the Northeast correspond to mapped earthquakes.

Between 250 million and 450 million years ago, the Northeast was the site of a “continental collision” in which the ancient African continent collided with the ancient North American continent to form the supercontinent Pangaea.

Earthquake hazard maps generally show that in most parts of New England, there is about a 1 in 10 chance that, in any given 50-year period of time, earthquake vibrations that may be damaging will strike.

Source: Maine Geological Survey, Boston College department of geology and geophysics.