One cold December night, I woke up to a rumbling. I swore about noisy plow trucks and rolled over.

But the rumbling went on way longer than it takes to block off the driveway. Man, this is a loud snowplow, I grumbled. The bedroom walls are practically shaking. And how come there are no flashing lights?

In the morning, my brother-in-law, who was visiting from San Francisco where they have a lot of experience with vibrating houses, said, “Did you feel the earthquake?”

“We don’t have earthquakes in Maine,” I said. But snowplows don’t tilt pictures and rattle glasses. He was right. It was a magnitude 4.0 earthquake centered a few miles north of Unity village where I lived in 1988. Yikes.

It turns out earthquakes are fairly common in Maine, though most are so small your house doesn’t notice them. Most, but not all.

The Weston Observatory of Boston College detected three earthquakes in Maine in December, and several in September and October off Mount Desert Island, plus foreshocks and aftershocks. The strongest was a magnitude 4.2 quake four miles southeast of Bar Harbor on Oct. 2 that shook houses all the way to Bangor.

Is Maine going to tumble into the sea?

The Earth’s crust is made of plates that shift slowly around. The places where they meet are called fault lines. When the plates grind against each other on a fault line, earthquakes occur.

Back in the anti-nuclear power days, there was a rumor – possibly prompted by a 4.0 quake near Bath in 1979, I don’t know – that the Maine Yankee plant in Wiscasset had foolishly been built on a fault, and a disaster could occur any minute. But the rumor was as false as the Maine-is-earthquake-free fallacy. Maine is not on an active fault. The geologic faults here have been inactive since the last Ice Age 20,000 years ago, and many since 300 million years ago.

So how could we be having earthquakes?

Earthquakes in Maine are caused by, um, well, no one knows. To quote a Maine Geological Survey summary: Beyond the fact that quakes must arise from weaknesses in the Earth’s crust, a “specific cause for the earthquake activity [in Maine] is not known.”

And yet, earthquakes occur, sometimes tilting more than pictures. The largest earthquake in Maine was an estimated magnitude 5.9 quake in the Calais-Eastport area in March 1904. Chimneys fell down, and the tremor was felt in Montreal. In 1929 a 7.2 quake about 250 miles off Newfoundland broke undersea cables, stopped clocks and broke dishes in southern Maine. In 1988 (a shaky year, I guess) a 6.0 quake near Chicoutimi, Quebec, was felt as far away as New York City.

Frequency maps show clusters of earthquake activity in the Eastport and Dover-Foxcroft areas, and to some extent in the Rumford area. Higher concentrations occur near Quebec City. In December there were two more quakes off Mount Desert, a 2.3 and a 3.1.

The most powerful earthquake on record occurred in Chile in 1960, registering as magnitude 9.5. In earthquake-speak, one order of magnitude indicates a tenfold increase in shaking of the earth. A 2.0 quake is 10 times more powerful than a 1.0 quake; a 3.0 quake is 10 times greater than a 2.0. The West Coast is far more earthquake-prone than Maine. The most quakes in the U.S. occur in Alaska, which is saying something because Southern California has about 10,000 earthquakes a year. Mostly unnoticeable. But still.

In Maine we have little rumbling to be grumbling about.

DWILDE@BANGORDAILYNEWS.NET