# Thread: Chilean Earthquake May Have Shortened the Length of a Day on Earth

1. ## Chilean Earthquake May Have Shortened the Length of a Day on Earth

Yikes! Just how big was the magnitude 8.8 earthquake in Chile? One scientist says the shaking may have affected the entire planet by shifting Earth on its axis. This possibly may have shortened the length of each day on Earth by about 1.26 microseconds. Using a complex model JPL research scientist Richard Gross [...]

More...

2. Originally Posted by Fraser
Yikes! Just how big was the magnitude 8.8 earthquake in Chile? One scientist says the shaking may have affected the entire planet by shifting Earth on its axis. This possibly may have shortened the length of each day on Earth by about 1.26 microseconds. Using a complex model JPL research scientist Richard Gross [...]

More...
There is something wrong in their quote. A 2.7 mas (milliarcsecond) shift does not equal 1.26 microseconds in rotational time for Earth.

2.7 mas equates to ~ 180 microseconds.

The Earth spins 15 mas in one millisecond, so a 2.7 mas shift gives us the 180 microsecond time difference.

[More at IERS ]
Last edited by George; 2010-Mar-02 at 03:48 AM. Reason: articulation

3. Seems I remember a similar thing being determined with the quake in Sumatra a few years ago. Any more big one's like this and I am going to have to clone myself to have time to get anything acomplished in my day.

4. Originally Posted by George
There is something wrong in their quote. A 2.7 mas (milliarcsecond) shift does not equal 1.26 microseconds in rotational time for Earth.

2.7 mas equates to ~ 180 microseconds.

The Earth spins 15 mas in one millisecond, so a 2.7 mas shift gives us the 180 microsecond time difference.
You're mixing apples and oranges!

Changes in length of day are a result of the changes of the degree two, order zero shape of the earth, whereas changes in the figure axis are related to changes in the degree two, order one shape of the earth. They're independent--a change in one doesn't affect the other.

So the article is just reporting the earthquakes effect upon two orders of the degree two shape of the earth. There is a degree two, order two component, but it is symmetrical about the axis, in both directions, and might not be much affected by earthquakes, which are closer to the surface.
[More at IERS ]
That's a link to the IERS website. Do they also have a discussion of Richard Gross's calculations?

5. Originally Posted by hhEb09'1
You're mixing apples and oranges!

Changes in length of day are a result of the changes of the degree two, order zero shape of the earth, whereas changes in the figure axis are related to changes in the degree two, order one shape of the earth. They're independent--a change in one doesn't affect the other.

So the article is just reporting the earthquakes effect upon two orders of the degree two shape of the earth. There is a degree two, order two component, but it is symmetrical about the axis, in both directions, and might not be much affected by earthquakes, which are closer to the surface.
Oops, I should have read the article. I assumed the giant westward mass flow of water would be the issue at hand, which would alter temporarily the rotation rate.

So did the axis shift or are you saying the Earth's shape changed, as well as, shift the axis as a result?

After the wave bounces around, is there a near cancellation or is the mass of water not part of the issue?

That's a link to the IERS website. Do they also have a discussion of Richard Gross's calculations?
It simply shows the relation I gave of 15 mas for every 1 ms time change in rotation.

6. Originally Posted by George
So did the axis shift or are you saying the Earth's shape changed, as well as, shift the axis as a result?

After the wave bounces around, is there a near cancellation or is the mass of water not part of the issue?
Yes, that water is not the issue. As you say, movement of the water west is offset by its return east, more or less.

The shape of the earth is affected, but small localized (!) events like this earthquake can be thought of with the old skater analogy. A net movement of mass away from the axis will slow the rotation, more or less permanently. If the movement were right at the equator, it wouldn't pull the axis either direction.

7. Originally Posted by hhEb09'1
Yes, that water is not the issue. As you say, movement of the water west is offset by its return east, more or less.

The shape of the earth is affected, but small localized (!) events like this earthquake can be thought of with the old skater analogy. A net movement of mass away from the axis will slow the rotation, more or less permanently. If the movement were right at the equator, it wouldn't pull the axis either direction.
Ok, and I see it is not the north/south axis but the "figure axis" that is used for shift angle, which seems to be the more accurate axis for the Earth's c.g., if my quick georgeeze interprets correctly what little is said of it.

I simply associated the angular change with the period change, which is appropriate georgeeze and the media should understand me by now. My averageness alone should have made them stop and hold up a globe to point out the axis shift, right?

But I'm still a bit befuddled. Look at this from Yahoo(currently their lead story).
The Chile earthquake was much smaller than the Sumatran temblor, but its effects on the Earth are larger because of its location. Its epicenter was located in the Earth's mid-latitudes rather than near the equator like the Sumatran event.
So seismic shifts further away from the equator generates greater axis shifts? [My mind is still tembloring. ]

8. Originally Posted by George
So seismic shifts further away from the equator generates greater axis shifts? [My mind is still tembloring. ]
Axis shift, which is independent from the effect on rotation rate.

Think about it this way, if a mass is added at the equator, which way does it pull the axis? Which end would you expect it to pull more on, the southern end, or the northern end? Neither, since it's halfway between the two.

The latitude and geometry can be a factor in another way. For instance, in the rotation rate affect, a mass rising straight up will have a stronger affect at the equator than at pole. However, masses moving sideways on the earth, a transform fault, might have an effect at the pole, but not at the equator.

9. Originally Posted by hhEb09'1
Axis shift, which is independent from the effect on rotation rate.

Think about it this way, if a mass is added at the equator, which way does it pull the axis? Which end would you expect it to pull more on, the southern end, or the northern end? Neither, since it's halfway between the two.
I was thinking in terms of the eccentricity. The more mass is located further from the axis the greater the load on the axis. A mass shift very close to the axis will have little effect.

But I see your point. I would guess that there is a sweet spot located in the mid-latitudes that gives the greatest axial deflection. Just a wild guess, of course.

10. I'm no expert, but I'm pretty sure it is just a function of the degree two, order one spherical harmonic, which I'm sure has its maximum at 45 degrees, or thereabouts (the earth is not a perfect sphere, the response is complicated, etc). You can imagine a sine or cosine wave wrapped around the world twice, with zeroes at the north pole, equator, south pole, and equator again.

11. Established Member
Join Date
Feb 2007
Posts
1,123
I heard him interviewed on NPR and was not impressed. He said that the change would be permanent. However, since subduction is basically offset by spreading centers, the effect should not be permanent.

12. Permanent, in geological terms, since everything changes, probably means over the years, rather than over the next few days.

Enough to affect leapseconds.

Wow, that reminds me. There has been a long period of no leap seconds. Essentially, in a short period of time, the earth had sped up to wipe out a hundred years of slowing. This big guy took us back to where we were overnight. Maybe we should have seen this coming.

13. We better get to the Moon quick before it gets away from us.

14. Established Member
Join Date
Feb 2007
Posts
1,123
Of course, the other fact that came out on the NPR program is that his team also predicted about a 6 millisecond decrease in the day from the Indian Ocean earthquake. He also admitted that neither prediction had measurements , in fact, verified the prediction. This may be because the tools required don't have sufficient accuracy, or because the predicted changes did not actually occur.

15. Originally Posted by TheHalcyonYear
Of course, the other fact that came out on the NPR program is that his team also predicted about a 6 millisecond decrease in the day from the Indian Ocean earthquake. He also admitted that neither prediction had measurements , in fact, verified the prediction. This may be because the tools required don't have sufficient accuracy, or because the predicted changes did not actually occur.
6 millisecond?

16. Established Member
Join Date
Feb 2007
Posts
1,123
Originally Posted by hhEb09'1
6 millisecond?
OK microseconds. milli, micro, what's a few orders of magnitude

17. Originally Posted by TheHalcyonYear
Of course, the other fact that came out on the NPR program is that his team also predicted about a 6 millisecond decrease in the day from the Indian Ocean earthquake. He also admitted that neither prediction had measurements , in fact, verified the prediction. This may be because the tools required don't have sufficient accuracy, or because the predicted changes did not actually occur.
I vaguely recall that it was 6 milliseconds. I also recall going to IERS site (see above post) at that time and saw the greater variation in the daily rotation rate. It seems, however, that the greater cause of rate change comes from air mass movements.

18. Established Member
Join Date
Feb 2007
Posts
1,123
Originally Posted by George
I vaguely recall that it was 6 milliseconds. I also recall going to IERS site (see above post) at that time and saw the greater variation in the daily rotation rate. It seems, however, that the greater cause of rate change comes from air mass movements.
There were some press reports that stated milliseconds, but the report was, in fact, microseconds.

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•