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Why quake rang like a bell

  • Professor Rowena Lohman: Quake due to plate movements, stress released by glacier melt
  • Lohman says big quakes are rare on East Coast and poorly recorded scientifically over time
  • She says they're felt over greater distances than in West because rock in East is harder
  • Lohman: Scarce historic data on faults make determining risk for building safety hard to know

(CNN) -- Earthquakes of the intensity felt Tuesday along a vast corridor of the East Coast don't come along too often in this region, geophysicist Rowena Lohman says. They are so rare "that it is very difficult for earth scientists to identify specific faults (in the area) that are 'active,' " she says, "where over time we would expect significant earthquakes to occur."

That doesn't mean quakes don't happen pretty regularly, she says. Smaller ones do, we just mostly don't feel them. But this time when the magnitude 5.8 earthquake hit Virginia, it resonated as far north as Toronto. That's partly because the East sits atop rock that is old and "cold," she says, and less able to absorb seismic waves than the malleable, younger rock of the West Coast's active earthquake zone. East Coast quakes, she said, tend to "ring like a bell."

Lohman, an assistant professor of geophysics and tectonics at Cornell University, spoke to's Opinion section about the earthquake Tuesday:

CNN: What caused the earthquake that shook the East Coast?

Rowena Lohman: There are two likely culprits: the motion of tectonic plates past each other, and the melting of large ice sheets and glaciers that used to cover North America during the last ice age around 14,000 years ago. When the ice sheet was on top of North America, it pressed down on the Earth's crust, causing it to sink lower. Since the ice melted and that weight was removed, the crust has been slowly rebounding back to its normal position, and that motion causes earthquakes.

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The effects of plate tectonics are felt much more strongly in regions that are close to plate boundaries, such as California or Japan, but the East Coast of the United States can experience smaller events, situated as it is essentially in the middle of a tectonic plate -- the North American Plate.

The ice sheet/glacier explanation is perhaps slightly more widely accepted, but in either case we are talking about very small motions of the Earth that result in relatively rare earthquakes. In fact, these earthquakes on the East Coast occur so rarely that it is very difficult for earth scientists to identify specific faults that are "active," that is, ones where over time we would expect significant earthquakes to occur.

If we were able to go back in time and record earthquakes in this region over many thousands of years, we would probably see that some areas had fairly regular earthquake activity, and we could then come up with a more solid explanation. In fact, that is what many people in the scientific community are trying to do -- they can't go and get the sorts of records that we receive from seismic instruments today, but they can go look for evidence of shaking in the past that tells us that there was a large earthquake in a specific region at a particular time. This is called paleoseismology and is a very powerful tool when combined with observations of current earthquakes.

CNN: I thought earthquakes were a West Coast phenomenon. Aren't these relatively rare here?

Lohman: Yes, they are rarer but result in relatively higher levels of shaking. The West Coast is a much more active region, with earthquakes, volcanoes and high rates of deformation overall and with a relatively warm, "squishy" young crust compared with the old, "cold" rock material underneath the East Coast. This means that the seismic waves that radiate outward from an earthquake in California are absorbed much more and are not felt as strongly as they would be for a similar earthquake here on the East Coast.

Scientists often say that the East Coast "rings like a bell" after an earthquake, with the seismic waves remaining strong over long distances, whereas in California the seismic waves are absorbed relatively quickly, so their effect is more like the thud you'd hear if you rang a wooden bell.

CNN: What's the risk for tsunami?

Lohman: There does not appear to be a risk for a tsunami from this earthquake at this particular time. One potential source would be if the shaking caused an offshore landslide -- and there is no evidence of that here. If the conditions are right, landslides can cause quite damaging tsunamis. This primarily depends on how large the landslide is and how quickly it moves. If the landslide moves slowly, then it doesn't transmit a lot of energy to the water, and we don't get a significant tsunami.

CNN: Is the magnitude 5.2 earthquake that struck Colorado on Monday night related to this quake?

Lohman: There isn't any indication of that right now, but that is certainly something people will look at.

CNN: When was the last time we had an earthquake on this side of the country? What happened?

Lohman: There have been quite a few large earthquakes on the East Coast, although since many of them occurred before we had the types of modern seismic instrumentation that we have today, it is difficult to determine their exact size.

There was one in 1897 very close to the location of today's event, in western Virginia, and there have been large earthquakes near Boston, Charleston (South Carolina) and other cities several times a century in the past that caused damage to churches, chimneys and other tall structures at the time.

Much smaller events, of the order magnitude 3 to 4, occur many times a year on the East Coast, but are usually only felt by the people who are directly on top of them. For reference, a magnitude 4 is about 33 times smaller than a magnitude 5, and 1,000 times smaller than a magnitude 6.

CNN: What's the risk for structures here? Like high-rises, nuclear power plants, reservoirs, water and transportation systems?

Lohman: When engineers discuss risk and hazard from earthquakes, they tend to need to know quantities that are quite difficult for earth scientists to provide. Earth scientists try to convert our understanding of how frequent earthquakes are likely to occur on a particular fault into numbers that reflect, for example, how likely it is that a particular spot will experience a given amount of shaking within the next 50 years. These are shown as "probability of exceedance" maps that allow structural engineers to determine the appropriate building codes that should be used for public structures, as well as private ones, in an area.

Since we have less information about earthquakes on the East Coast (since they don't occur as often), we've had to incorporate that uncertainty into our estimates. So the seismic hazard maps you can find online through the United States Geological Survey Hazards program reflect our understanding about all the faults we know about as well as an attempt to account for the ones we aren't aware of.

CNN: Is the East Coast ready for an earthquake?

Lohman: That's a question for a different kind of scientist or engineer. From what I saw today, the public response appeared to be very well-coordinated and appropriate. Sensitive facilities were shut down for further inspection, and agencies such as the USGS got information out to the public very rapidly.

This is a very good time for people to think about their own preparedness for any natural disaster, however. It's a good time to go through your house and make sure that your bookcases are attached to the walls (which is a good thing to do anyway) and that you have some emergency supplies and water on hand.