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Tsunami and earthquake explainer

What’s up with the whirlpool, the “supermoon,” the Indonesian tsunami … and is the Pacific really shrinking?

Japan earthquake tsunami 03 11 11Enlarge
Commuters make their way after train services are suspended after an 8.9-magnitude earthquake on March 11, 2011 in Tokyo, Japan. (Koichi Saito/Getty Images)

For answers to your earthquake and tsunami questions, we asked Dr. Ray Coish, professor of geology at Middlebury College.

GlobalPost: Some people on the internet are blaming the tsunami on a “supermoon” — a full moon that’s closer than usual to the earth. A supermoon has been predicted for March 19, and they point out that the tsunami off Indonesia also occurred just before a supermoon. Does the moon have anything to do with this earthquake and tsunami?

Dr. Ray Coish: The moon’s orbit is elliptical, so it’s not unusual for the moon to come closer or go farther from the earth. But that has nothing to do with an earthquake or tsunami. The lunar effects are just so minute compared to the energy needed to move the earth that the moon has no impact at all.

The earthquake is caused by the movement between two plates. Japan lies above what we call a “subduction zone,” where an ocean plate is sinking underneath a continental plate. This is happening around much of the Pacific rim, where the ocean is getting smaller — of course, at a very slow rate, over the course of millions of years.

The tsunami is caused by the sea floor moving. The tidal effect from the moon won’t affect a tsunami either. There's no connection.

GlobalPost: Can you explain the giant whirlpool off Japan’s coast?

Coish: It’s hard to say for certain at this point, but it’s probably related to the shape of the sea bottom where it was found. Any wave can end up as a whirlpool. It’s a large-scale eddy, similar to what you might see on a river or a smaller bay.

In this case, the amount of water sloshing around is tremendous, which is causing this giant eddy.

GlobalPost: What about climate change, does that have any effect?

Coish: No. The earthquakes are from miles below the surface. Climate has no impact.

GlobalPost: How far down?

Coish: This one is from a little less than 20 miles down. That’s fairly shallow for an earthquake, which explains why there’s so much damage. The closer to the surface, the more the buildings, roads and people will feel the shaking.

For example, at 7.0 on the Richter scale, the 2009 Haiti earthquake was much smaller in magnitude, compared to this 8.8 earthquake today in Japan. But the Haiti earthquake was shallow, only about 8 miles down, so it caused a lot of damage. As a rule of thumb, an earthquake will cause about 30 times more energy to be released per numeric increment — for example, as you go from 5 to 6 and 6 to 7 on the Richter scale.

Likewise, in California, almost all the earthquakes are shallow, because there, two plates are moving past each other, and this happens at a relatively shallow depth. California’s earthquakes would be much more damaging, but the state has excellent building codes.

GlobalPost: For decades no one had heard of a tsunami. Then in 2004 there was a tsunami off the coast of Indonesia. Now it’s Japan. What’s going on here, and how often do these occur?