Japan Earthquake May Have Shifted Earth's Axis

Richard Gross, of NASA's Jet Propulsion Laboratory, calculates that last week's earthquake off Japan's coast may have shortened the length of Earth's days by about 1.26 microseconds and shifted its axis by about 6.5 inches. Gross describes the findings and explains how changes in Earth's land masses affect the way it spins.

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IRA FLATOW, host:

Did you notice that the days are just a wee bit shorter now, eh? No, nothing to do with daylight savings time. They should be getting longer. Well, that Japanese - the giant 9.0 Japanese earthquake shook up the entire planet, and it shifted the Earth's axis, yeah, shortening the day just a teeny-weeny little bit.

According to the U.S. Geological Survey, the earthquake shifted some global positioning stations, you know, GPS stations in the country, and one near the epicenter. The one near the epicenter moved 13 feet. Wow. To put it in another way, Japan is wider now than it was before, said one USGS scientist.

Joining me now to talk more about it is Richard Gross. He's a research scientist in the Geodesy and Space Group at NASA's Jet Propulsion Laboratory in Pasadena, California.

Welcome to SCIENCE FRIDAY.

Dr. RICHARD GROSS (NASA Jet Propulsion Laboratory): Oh, thank you. It's my pleasure to join you.

FLATOW: What is that Space Geodesy Group about at NASA?

Dr. GROSS: Well, we use space geodetic measurements, such as from GPS, to study how the planet deforms and its rotation changes as a result of mass motion within and on the Earth.

FLATOW: And you could tell from the GPS, things around the countryside that you have placed around there?

Dr. GROSS: Yeah. There's a worldwide network GPS receivers that we use that continually monitor how the Earth is rotating.

FLATOW: And this was such a big earthquake that it changed the rotation of the Earth.

Dr. GROSS: According to a calculation I did, it should have. You know, it should have gotten - the days should have gotten shorter by about 1.8 microseconds. That's 1.8 millionths of a second.

FLATOW: One-point-eight millionths of a second - not something we really going to notice, but I would imagine to scientists, that's an important number.

Dr. GROSS: Yes. The Earth's rotation changes all the time. The length of the day changes mostly in response to changes in the atmospheric winds. And these winds - the effect of the winds can be 500 times larger than this effect of the earthquake. And we need to account for these changes in the length of a day at JPL because it affects how we navigate spacecraft to planets like Mars.

FLATOW: Huh. And what other things affect it? You know, I know we have melting glaciers and water sloshing and things like that. Would that also affect the rotation?

Dr. GROSS: Yeah, it certainly does. Anything that moves mass around will, in principle, affect the Earth's rotation. But in order for it to have a measurable effect, it has to be a lot of mass. So changes in the atmosphere, changes in the oceans, melting ice off Greenland, all of these can affect the Earth's rotation.

FLATOW: And what about the axis? Was the axis tilted any?

Dr. GROSS: Yeah. So, the Earth rotates about its rotation axis, but the mass of the Earth is not balanced about that axis. If the mass of the Earth were balanced about that axis, it would spin very smoothly, just like a well-balanced tire on your car spins smoothly.

But the Earth - the mass of the Earth is not balance about its rotation axis, and so it wobbles as it rotates, just like an out-of-balance tire on your car vibrates as it rotates. And it's that axis about which the mass is balanced is what shifted during this earthquake. The earthquake rearranges the mass of the Earth, and therefore the figure axis moved by about six-and-a-half inches.

FLATOW: So it's like somebody moved one of those little weights on your tire when they bang it in with the hammer?

Dr. GROSS: Exactly. And it's the global displacement of the mass of the Earth that was important for the Earth's rotation to change.

FLATOW: Mm-hmm. And what kind of instrument - you mentioned the space probes that NASA might send. Tell me about what kind of - what's out there now and what kinds of things might be affected. Are these things that are far away, near, you know, circulating Saturn, or are they stuff that's right near the Earth, here?

Dr. GROSS: Well, every satellite that is launched to the planets like Mars or Saturn or asteroids, when we navigate those planets, those spacecraft, to make sure that we target - you know, accurately approach the target body, we have to account for these kinds of changes in the Earth's rotation.

It turns out that if you were to simply assume that the Earth were rotating uniformly without, you know, any change, you could be off by kilometers in the position of where you want to land a rover on Mars.

FLATOW: Yeah. Now, you said that your - one of your GPS stations moved almost 13 feet. Is that common, something like that?

Dr. GROSS: For these really great earthquakes, stations close to the source of the earthquake can move many feet like that, many meters. Of course, the motion gets much smaller as you move farther away from the source of the earthquake. But even a few thousand kilometers away, you can still have displacements of a millimeter or so.

FLATOW: Well, you know, that's how mountains get pushed up, right? We have these earthquakes, these tectonic events - a little 13 feet here, a little 10 feet there, soon you got the Himalayas.

Dr. GROSS: That's right. Over time, all of the small displacements caused by earthquakes build up, and that's how mountains form. That's right.

FLATOW: It's just amazing to think about how many it must have taken to throw up these giant mountains, how long of a period of time, and how many different earthquakes must have happened.

Dr. GROSS: Yeah. Well, the tectonic plates that make up the crust of the Earth have been moving for, you know, millions, even hundreds of millions of years, causing these mountains to grow.

FLATOW: Now you've done a computer model on this. How can you verify that it's correct, your calculation? Is there any way to do that?

Dr. GROSS: Yes. We can look at the GPS measurements we have of the change in the Earth's rotation and try to see if we can detect what I calculated the earthquake should have caused. And this will be a bit challenging because it's a small signal, and there are these other mechanisms like the atmosphere and oceans that cause much bigger changes in the Earth's rotation.

So we'll take the observations and we'll remove these larger effects due to the atmosphere and oceans. And with any luck, we'll see this smaller signal caused by the earthquake.

FLATOW: Mm-hmm. This is NPR: SCIENCE FRIDAY. I'm Ira Flatow. And we're talking about the earthquake and what it did to the rotation of the Earth.

Have there been bigger ones than this that have shifted the wobble and the axis and stuff on the Earth?

Dr. GROSS: There have certainly been bigger earthquakes on the Earth. In fact, the largest earthquake that's been recorded was in 1960 in Chile. And I did a similar calculation for that earthquake, and according to my calculation, it should've changed the length of the day by about eight microseconds.

FLATOW: Is it? Wow. Is there any way you can run your computer model backwards to see, you know, past histories of where the earthquakes might have happened or the wobble may have changed eons ago?

Dr. GROSS: Well, I use seismic information for how the faults looked during the earthquake.

FLATOW: Right.

Dr. GROSS: So there's - so I need that information to do my model. And so I can only run my model on the earthquakes that I have that information, which only spans about the last 30 years or so.

FLATOW: And so your model doesn't have any predictive value about predicting when we might have another earthquake?

Dr. GROSS: Oh, no, of course not. This is - this model just computes consequence of the earthquake on the Earth's rotation.

FLATOW: Mm-hmm. And this earthquake has pretty - been pretty well monitored, has it not? I mean, you're in Japan. They have earthquakes there all the time. There are sensors all over the world, and all -certainly all over that Ring of Fire area. There must be an awful lot of data that scientists have been collecting on this earthquake.

Dr. GROSS: Yes, that's right. Japan is one of the best-instrumented countries in the world as far as seismometers and GPS receivers go. And so this is one of the best-recorded earthquakes. And hopefully, by analyzing all of this data in the future, we'll have a better understanding of how really great earthquakes like this occur.

FLATOW: And maybe we - somebody, some place puts this data together and might see a precursor that might have hinted - tell us when to expect another one.

Dr. GROSS: That is one of the things that the scientists are certainly looking at. With having such a wealth of data from this earthquake - not only during the earthquake, but before and after - they'll certainly be looking for those precursors.

FLATOW: And a quake like this, even though you talked about it moving, you know, Japan's just a little bit wider, does it move to the other -does the crust up on the other side of the earth change, also?

Dr. GROSS: In principle, it will, but by such a small amount that there's no hope of measuring it. You can measure the changes up to maybe a few thousand kilometers away, but that's about as far as you'll be to see the displacements.

FLATOW: Mm-hmm. And so where do you go with your research now? What would you like to do next?

Dr. GROSS: Well, actually, my next research project is what you mentioned, having to do with melting ice off of Greenland and Antarctica. I'm currently looking into the impact that that will have on the Earth's rotation.

FLATOW: Oh, you mean if global warming melts the ice off the continents and they start flooding into the ocean. We're going to have more liquid water.

Dr. GROSS: Exactly. As all that melt water flows into the oceans, it raises the level of the oceans, and that can change the Earth's rotation.

FLATOW: Wow. I mean, you know, we think about it, but we don't think of all the consequences. That's why you guys are out there doing it.

Dr. GROSS: Yeah. Well, for most people, these small changes in the Earth's rotation have no consequence. But it's important for us at JPL when we navigate spacecraft.

FLATOW: And there you have it. Thank you very much for taking time to be with us today. Good luck on your mathematics.

Dr. GROSS: Oh, thank you very much.

FLATOW: You're welcome. Richard Gross, a research scientist in the Geodynamics and Space Geodesy Group at NASA's Jet Propulsion Laboratory. Of course, that's in Pasadena.

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