BAR HARBOR – Another set of small earthquakes shook the area Sunday afternoon and evening, and there could be more on the way, according to one expert.
The quakes caused no reported damage and little reaction among residents, with the Bar Harbor Police Department logging only a handful of calls about the tremors.
“I guess people are getting used to it,” said one police dispatcher.
The four separate quakes on Sunday were normal aftershock activity from the quake measuring 4.2 on the Richter scale that hit the area on Oct. 2, according to John Ebel, director of the Weston Observatory at Boston College, which monitors seismic activity in New England for the U.S. Geological Survey.
“There were two small ones, one that registered 2.9 and another, 2.5,” Ebel said Monday. “The two larger ones would have been felt in the Bar Harbor area.”
Magnitude measurements represent the strength of earthquakes as calculated by seismographs, which detect and record ground motions.
Aftershocks are a reaction to the main quake activity, Ebel explained. The quake results from the movement of rock along a fault.
“The rock doesn’t move uniformly because it is not absolutely smooth,” he said. “Those areas that did not slide as much are trying to catch up with the rest of the fault. So, you get a little slip.”
Those slips are the aftershocks, which, he said, are actually little earthquakes. There is no way of predicting when they might happen, Ebel said, but there could be more in Bar Harbor’s near future.
“It would not surprise me if over the next several days or few weeks that we would see a couple more events of this size or smaller,” he said. “On the other hand, it wouldn’t surprise me if nothing more happens. There is a slight chance that we could see something bigger, but the odds are against it.”
The likelihood of aftershocks diminishes the more time passes after the main event, he said.
The first of the recent earthquakes shook the area on Sept. 22, with smaller aftershocks felt six days later.
A team from the Lamont-Doherty Earth Observatory at Columbia University plans to return to the region this week to gather data from four monitoring stations placed in and around Mount Desert Island after the first earthquake hit the area in September.
According to Won-Young Kim, a research scientist at the observatory, the stations do not provide real-time information about the local quakes, but should help scientists to pinpoint the quakes’ locations. They are considering adding another station that would provide instant information to researchers from the site.
The existing monitoring stations are located at Otter Creek, Pretty Marsh, Schoodic Point and Lamoine State Park, he said. Those locations are all within 10 kilometers of the epicenter, he said.
Although scientists have roughly located the epicenter to within several kilometers, the new data from the stations should pinpoint the epicenter to within 100 meters or less.
“We’re trying to understand where and how it occurred,” Kim said. “We should be able to tell how deep it was and if any faulting occurred.”
The Bar Harbor quakes are not related directly to other earthquakes that have struck locations in New England in recent months, including several in other sections of Maine. They do all stem, however, from the same movements of the North American tectonic plate, Ebel said.
The North American plate is rifting away from Europe and Africa on one edge while it is colliding with the Pacific Ocean plate near the West Coast. The pressure on one end of the plate and resistance on the other are squeezing the North American plate, resulting in small earthquakes in the East.
Ebel noted that within about two hours of the 4.2 quake in Bar Harbor, there was a 3.5 quake in South Carolina. They were not directly related, in the sense that one did not cause the other, Ebel said, but they were a response to the same plate pressures.
Historically, the New England region averages a magnitude 5 earthquake or greater once every 50 years. The 5 magnitude quake is significant, Ebel said, for that is the strength at which damage to structures can occur.
The 4.2 quake in Bar Harbor shook loose some rocks from ledges in Acadia National Park, but did not cause any damage to structures.