MELISSA BLOCK, host:
Along the coast of Maine, people have been wondering: Is it just me, or is the earth shaking? It has been shaking. As many as 30 tiny earthquakes were detected over the first week of May.
Geologists call this a micro-quake swarm. And the director of the Maine Geological Survey, Robert Marvinney, joins me to explain why it's happening.
Robert Marvinney, we don't really think of Maine as an earthquake zone, with active fault lines. What's going on up there?
Mr. ROBERT MARVINNEY (Director, Maine Geological Survey): Well, no, we don't think of Maine has being active seismically, but we do have small earthquakes every year, and it's unusual to have so many in such a short time span.
But the Earth is dynamic, and this is just an expression of that dynamic nature. There's a couple reasons why we experience earthquakes in Maine. One is that the entire crust of the Earth is made up of mobile plates that move, and even though Maine is right in the middle of a plate, we still experience some stresses and responses to that movement in the form of small earthquakes.
And the second reason is that only, well, about 13,000 years ago, where I'm sitting, this area was covered with between 5,000 and 10,000 feet of ice, the last Ice Age, and that weight of that ice was enormous, and it depressed the crust in Maine and New England.
And the ice melted in a very short time span in terms of geologic time, and the crust is still responding to the loss of all that weight.
BLOCK: Really? So it's sort of - it's springing back up, in a sense.
Mr. MARVINNEY: That's right. It takes some time for the crust to respond and spring back, and these small earthquakes are a consequence of that.
BLOCK: Now, I understand that police in Maine were getting phone calls from people who thought they heard the sound of gunshots or blasting. So it actually - it's an audible thing that's going on?
Mr. MARVINNEY: Yes, these small earthquakes are very shallow, and because they are shallow, the seismic waves, which move through the rock, can actually generate waves in the air, and people hear that as a loud report of some kind or a rumbling.
And for people that are right over the location where these very small earthquakes occur, that's typically what they hear.
BLOCK: Why would there be a swarm of dozens of tiny earthquakes like these?
Mr. MARVINNEY: Well, that's probably the hardest question for us to find an answer to because we've had very few of these swarms over the years that we've been recording earthquakes in Maine.
There was a swarm like this in 2006 around Bar Harbor, another one in 1967 around the Augusta area. And because they're so rare and they're so small, we really don't have a good answer for why a spate of earthquakes will occur like this in one location.
BLOCK: And this swarm that we've been talking about, this swarm of dozens of earthquakes, it's over, or are you expecting some more?
Mr. MARVINNEY: Well, it could be over. The last time, in 2006, when we had a similar series of earthquakes, there was a large group of them in a short period of time and then a quiet period and then a few more later on. So that's possible, although it's quite probable that this event is over with at this time.
BLOCK: Is there something unique to Maine and the geology of Maine that produces these swarms that we're talking about? Or would this happen in a lot of places around the country?
Mr. MARVINNEY: I think it could happen in many places, although Maine's just got great geology, and this is just an example of it.
BLOCK: What do you mean great geology?
Mr. MARVINNEY: Oh, just - you know, it shows the closing of the prior ocean basin that - and then the opening of the Atlantic Ocean, that's all recorded in the rocks and then the fantastic glacial history that produced our landscape.
BLOCK: You see that all when you drive around? Do you notice that?
Mr. MARVINNEY: That's what I think about all the time.
BLOCK: Yeah?
Mr. MARVINNEY: Yeah. It's - well, it's just - it's everywhere. The geology is everywhere, and if you know how to read it, you can learn a lot of interesting things.
BLOCK: Robert Marvinney, thanks very much for talking with us.
Mr. MARVINNEY: You're welcome.
BLOCK: Robert Marvinney is director of the Maine Geological Survey. He spoke with us from Augusta. Transcript provided by NPR, Copyright NPR.