Chile Quake Shortens Length Of Day
MELISSA BLOCK, host:
The power of this weekend's earthquake in Chile can be measured in the hundreds of deaths there, the millions left homeless. It can also be measured in an infinitesimal change in the length of the day. That's according to Richard Gross, who is a geophysicist at NASA's Jet Propulsion Laboratory in Pasadena. He specializes in Earth rotation.
Welcome to the program.
Dr. RICHARD GROSS (Geophysicist, Jet Propulsion Laboratory, National Aeronautics and Space Administration): Oh, thank you. I'm glad to be here.
BLOCK: And you say that the force of this earthquake in Chile means that each day on Earth is now a tiny bit shorter. How much shorter is it?
Dr. GROSS: Well, my calculation shows it should be about a microsecond shorter. That's about a millionth of a second.
BLOCK: A millionth of a second every day.
Dr. GROSS: That's right. This happens because the earthquake moved a lot of mass around on the Earth. So just like a spinning ice skater, as she pulls her arms closer to her body, this earthquake moved the Earth's mass a bit closer to the Earth's rotation axis and made the Earth rotate a bit faster, just like the ice skater rotates a bit faster.
BLOCK: Wow. So that means we're spinning faster, the day gets shorter.
Dr. GROSS: Exactly.
BLOCK: By a tiny, tiny bit.
Dr. GROSS: Yeah, in fact, it's so small that I think it will be a challenge to detect this in our measurements.
BLOCK: Now, how do you come up with that calculation?
Dr. GROSS: Well, I use seismic determinations of the fault motion that was caused by the earthquake, so the location and the dip and how much slip occurred on the fault and what direction, I take that information, and I compute how the mass everywhere within the Earth should've changed and therefore how the Earth's rotation should have changed. So it's really based upon seismic information.
BLOCK: Should have changed. So if you find out more about this earthquake, if you get more data, then maybe your projection could change too.
Dr. GROSS: That's right. As the seismologists refine their estimate, my estimate of how the Earth's rotation changed will also change. But now that we have preliminary calculation, we can start looking at the data we have, the observations of the length of the day, and start trying to find it in our observations.
BLOCK: Now, did this happen with other powerful earthquakes, too, that you projected that the days got either a tiny bit shorter or a tiny bit longer?
Dr. GROSS: Yes, in fact, the 2004 Sumatran earthquake was, you know, much larger than this earthquake, and my same model calculation showed that for that earthquake, the length of the day should've gotten shorter by about eight microseconds. But unfortunately, that was still too small to see in the data. The data I looked at at the time just didn't show any such change.
BLOCK: Is this now a permanent change, in other words, our day now is always going to be one-millionth of a second shorter?
Dr. GROSS: Yes, that's right. The change in the mass distribution of the Earth caused by the earthquake is permanent.
BLOCK: So one-millionth of a second shorter of a day. Pretty soon, that adds up, and you're talking about real time, I guess, Richard Gross?
Dr. GROSS: Yeah, well, of course, it takes you know, it would take a million of those to add up to a second. So it really is quite a small effect.
BLOCK: Well, use it wisely.
Dr. GROSS: Thank you. Yes, we will.
BLOCK: Richard Gross, thanks for talking to us about it.
Dr. GROSS: Oh, thank you for your interest.
BLOCK: Richard Gross specializes in Earth rotation. He's a geophysicist at NASA's Jet Propulsion Laboratory in Pasadena.
NPR transcripts are created on a rush deadline by Verb8tm, Inc., an NPR contractor, and produced using a proprietary transcription process developed with NPR. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.