Sumatra Quake's effect on Earth's Rotation?

[catmandoo]

On SKY TG-24 TV in Italy, Enzo Boschi, the head of the Italian National Geophysics Institute, stated that the magnitude 8.9-9.0 earthquake that happened beneath the Indian Ocean off of Indonesia this weekend had "disturbed the Earth's rotation."<br /><br />See story.<br /><br />Is this normal? There have been other 9.0-magnitude earthquakes in the past, did these also affect the Earth's rotation? Will we have to compensate somehow, by adding an additional second? Is the Earth's rotation permanently damaged by these events? What is the cumulative effect of all of these 9.0 magnitude earthquakes on the Earth's rotation (I can't believe these are a new phenomenon that the Earth had never experienced before)?<br /><br />Thanks.

 

[Maddad]

There will be no effect on rotation at all because there is no outside force acting on the Earth.

 

[meteo]

The earthquake changes the distribution of mass on the Earth. This changes the moment of inertia and for angular momentum to be conserved the angular velocity must change. It's like an ice skater pulling in their arms and spinning faster.<br /><br />The ice ages changed the length of day of the earth, so do mountains rising and changing sea levels. It's all in the micro seconds though doesn't make much of a difference.<br /><br />The earthquake in Chille 1960 changes the length of day by 23 micro seconds. I would imagine this would be a few times smaller. So yes it has happened many times in the past. It's pretty amazing though that an earthquake can change the length of day! <br /><br />I would think the random raising and lowering of land would cancle out the change in rotation; so earthquakes don't cause the rotation to decay or anything like that. It's all because the moment of inertia changes.

 

[mental_avenger]

You could say that the swing of Jupiter around the Sun causes a change in the orbit of the Earth, and you would be techincally corrrect. However, the change is so small it is negiligable, and as you say, averages out over time to be no change at all. IOW, Boschi's statement, while nanoscopically “technically correct”, is, for all practical purposes, an opportunistic media invitation and disingenuous attempt at publicity. <div class="Discussion_UserSignature"> <p style="margin-top:0in;margin-left:0in;margin-right:0in" class="MsoNormal"><font face="Times New Roman" size="2" color="#ff0000"><strong>Our Solar System must be passing through a Non Sequitur area of space.</strong></font></p> </div>

 

[paleo]

As a geologist I can say with confidence that the effect on the rotation of the Earth or any other macro geology is almost 'Zilch'. As much effect as a particle of dust landing on the Empire State Building.

 

[pizzaguy]

And it worked! The whole world knows this guy's name now.<br /><br /> <div class="Discussion_UserSignature"> <font size="1"><em>Note to Dr. Henry:  The testosterone shots are working!</em></font> </div>

 

[Maddad]

meteo<br />"<font color="yellow">The earthquake changes the distribution of mass on the Earth. This changes the moment of inertia and for angular momentum to be conserved the angular velocity must change. It's like an ice skater pulling in their arms and spinning faster.</font><br /><br />Nuh-uh. To invoke the ice-skater, you would have to make the case that a large mass moved closer to the center of the Earth. This did not happen; one plate slid by another horizontally.<br /><br />The next problem is that mass really did not change its distribution. In the long run mass moves up from mid oceanic ridges and similar mass moves down at plate boundaries. In the middle the mass moves laterally. Slowly, just a few centimeters a year, but over long periods it is a steady progression. This earthquake was part of that progression, so it represented no sudden change to the Earth. It's just part of what has always been.<br /><br />"<font color="yellow">The earthquake in Chille 1960 changes the length of day by 23 micro seconds.</font><br />Not.

 

[meteo]

www.ecgs.lu/pdf/jlg92/JLG92_Gross.pdf<br /><br />summary<br />"Earthquakes redistribute the Earth’s mass on a<br />global scale<br />• Change the Earth’s rotation and gravitational field"<br /><br />"Greatest earthquakes have greatest effect<br />• 1960 Chile<br />• 63 mas change in polar motion excitation<br />• 23 µs change in length of day<br />• Current observing systems are accurate enough to<br />detect changes caused by next great earthquake<br />• Polar motion accuracy about 0.05 mas<br />• LOD accuracy about 20 µs"<br /><br />from EUROPEAN CENTER FOR GEODYNAMICS AND SEISMOLOGY<br /><br />http://www.jpl.nasa.gov/news/features.cfm?feature=15<br />"The length of the day--how fast or slow the Earth rotates--depends on how Earth's mass is distributed. Its mass includes the atmosphere, the solid Earth and its fluid core. When the DISTRIBUTION OF EARTH'S MASS CHANGES, as during a major earthquake, for example, so does the speed of its rotation. "It's like an ice skater," says Gross, "who spins faster as she brings in her arms. She is changing her mass distribution.""<br /><br />"Nuh-uh. To invoke the ice-skater, you would have to make the case that a large mass moved closer to the center of the Earth. This did not happen; one plate slid by another horizontally. "<br /><br />Incorrect it is a subduction zone.

 

[Maddad]

Despite what this website said, the Earth's long term distribution of mass is not changing. If you invoke a subduction zone in support of the ice skater, then you're claiming that earthquakes move substantial mass downwards toward the center of the Earth. The hypothesis misses because the world's 70,000 kilometer mid oceanic ridge system continuously moves mass upwards, countering any such tendency. In fact, the reason that one landmass subducts at all is to make room for more mass rising at the mid oceanic ridges.

 

[bobvanx]

So there you have it. JPL geophysicist Dr. Richard Gross says the Earth's rate of rotation can change, and Maddad says he's wrong.<br /><br />Ladies and gentlemen, place your bets!

 

[meteo]

<font color="yellow">The hypothesis misses because the world's 70,000 kilometer mid oceanic ridge system continuously moves mass upwards, countering any such tendency. In fact, the reason that one landmass subducts at all is to make room for more mass rising at the mid oceanic ridges.</font><br /><br />Hypothesis? It's measured! Earthquake... then change in polar motion and or length of day.<br /><br />I think your problem is that your trying to think of the whole system. If an earthquake causes land to subduct that has no immediate effect on the mid ocean ridge. So there is a temporary change in the distribution of mass. I'm not saying it isn't eventually cancled out, I mentioned that it seems it would be in my first post.<br /><br /><font color="yellow">Despite what this website said, the Earth's long term distribution of mass is not changing.</font><br /><br />Again, were just talking temporary change. <br /><br />If you can find a reputable source that says that the rotation or polar motion can't change I would like to see it. I just started looking at this stuff recently.

 

[bobvanx]

I started noticing when I read a report that humans have changed the rate of the Earth's rotation by using reservoirs. We've gathered enough fresh water at high latitudes that we've decreased the length of the day by a few tens of microseconds.

 

[Maddad]

bobvanx<br />"<font color="yellow">JPL geophysicist Dr. Richard Gross says the Earth's rate of rotation can change, and Maddad says he's wrong.</font><br />Even a knowledgable person can be mistaken, and often are as a matter of fact. Before you insist that it must be right because you mommy told you so, do you have have a mechanism to offer us as to how this might be since the skater model doesn't work?<br /><br />meteo<br />"<font color="yellow">Hypothesis? It's measured! Earthquake.</font><br />Yes, the earthquake is measured. Show me where the Earth's rotation has been measured between Earthquakes over a timespan long enough to encompass more than one quake. That's the rub. Even if the Earth fluctuates <strong><em>very</em></strong> slightly at the moment of the quake, it would return during the followning interval. Did your mommy measure for that? Show me the data that you're citing as supporting evidence. Just quoting a famous name is insufucient.

 

[mcbethcg]

What a nitwit.

 

[odysseus145]

All these articles agree that the earthquake effects Earth's rotation:<br /><br />http://www.slate.com/Default.aspx?id=2111443&<br />http://www.newton.dep.anl.gov/askasci/env99/env275.htm<br />http://bowie.gsfc.nasa.gov/ggfc/sbm_cos.html<br />http://www.ecgs.lu/pdf/jlg92/JLG92_Gross.pdf - This one has actual calculations<br />http://geology.about.com/library/weekly/aa090797.htm<br /> <div class="Discussion_UserSignature"> </div>

 

[meteo]

Try and be more civil in future posts, okay?<br /><br />The polar motion measurements for the 1960 Chile earthquake were already given. It was written by the Gross from JPL in 2004.<br /><br />http://www.ecgs.lu/pdf/jlg92/JLG92_Gross.pdf<br /><br />Scroll down to page 27 and 28. It has the raw polar motion in x and y and the filtered polar motion to just show that effect from earthquakes. As you can see there is a huge jump with the 1960 Chile earthquake. <br /><br />The polar motion is measured with an accuracy of 0.05 micro-arc-seconds (probably two standard deviations), the Chile earthquake caused a polar motion excitation of 63 mirco-arc-seconds. The length of day seems to be less certain as it is measured to be 23 micro-seconds with an accuracy of 20 micro-seconds. Regardless there most certainly is a an effect on polar motion, which I belive is what some scientists on the news have been refering to when they say "wobble".<br /><br /><font color="yellow">Even if the Earth fluctuates very slightly at the moment of the quake, it would return during the followning interval.</font><br /><br />Not sure what you mean by following interval. I think Gross's paper shows this is because of the "overwhelming" static from oceans and the atmosphere not because of a tectonic response. The earthquake filtered polar motion does not go back to "earthquake filtered normal". However, I'm not sure.<br /><br /><font color="yellow">do you have have a mechanism to offer us as to how this might be since the skater model doesn't work?</font><br /><br />Well it makes sense to me. The distribution of mass changes on a rotating object, this changes the moment of inertia and for angular momentum to be conserved the earth's angular velocity must change. <br /><br />Polar motion is measured using Very Long Baseline Interferometry (VBLI). From other articles on using VBLI to measure earthquakes effect on

 

[bobvanx]

Nice data.

 

[Maddad]

It still hasn't measured rotation speeds between quakes, and that's the issue, not just the moment of the quake. The data still hasn't been independently verified and peer reviewed. And you and your mommy have still not given us a mechanism by which it should slow down.

 

[mcbethcg]

Maddad, the mechanism that everyone but you seem to be aware of is the effect that occurs when an ice skater pulls thier arms in, or when the ice skater puts their arms back out. A rotating object changes the speed of rotation when the distribution of mass changes.<br /><br />Earthquakes cause massive changes in the location of mass.<br /><br />Earths plates move, permanently, as much as inches or feet.<br /><br />Undersea mudslides occur, moving millions of tons of mass hundreds of feet.<br /><br />Anything that moves mass from a low elevation to a high elevation slows the earths rotation.<br /><br />And vice versa.<br /><br />Anything that moves mass towards the equator slows the earths rotation.<br /><br />And vice versa.<br /><br />If a plate in an equatorial region shifts up, while a plate away from the quator shifts dow, the earth slows.<br /><br />Earthquakes occur because large masses are moving.<br /><br />When I go upstairs in my house, it slows the earths rotation. By a zillionth of a second. <br />When I walk south to my toilet, it slows the earths rotation. By a zillionth of a second. <br /><br />These minor effects are counteracted by millions of other masses moving around the world.<br /><br />But earthquakes are bigger.<br /><br />When a plate moves a tiny bit, it changes the earths rotational speed by nanoseconds or microseconds.<br /><br />I have no idea why these forces are hard for you to understand or agree with- the changes are minor, and measured, and make perfect sense.<br /><br />If it is your view that the redistribution of mass on a rotating object has zero effect- I have no idea what to say about it. You need to study some more, or something.<br />

 

[meteo]

http://www.cnn.com/2004/TECH/science/12/29/quake.wobble.reut/index.html<br /><br />I'll try and find a "common experience" example of polar motion after I'm done taking down xmas lights.

 

[bobvanx]

<blockquote><font class="small">In reply to:</font><hr /><p>you and your mommy have still not given us a mechanism by which it should slow down.<p><hr /></p></p></blockquote>Since my mom is not a geophysicist, I'm not sure that anything she could tell us about this would be of much value.<br /><br />However, she <i>could</i> discuss how to face up to news that disturbs you, how to integrate distressing new science concepts into your worldview, and how to play nice with others.<br /><br />She's pretty wise in lots of ways, and I'm glad she's my mom.

 

[meteo]

Polar motion changes are a little more difficult to understand than length of day changes, I think this is the jist of it. A change in the distibution of matter causes the "moment of inertia axis" to be displaced from the rotation axis. I think you can best illustrate polar motion as a the wobble of a spinning top. <br /><br />Mostly on ocean and atmopshere's effects on polar motion but still useful.<br />http://www.agu.org/revgeophys/wilson01/wilson01.html<br /><br />

 

[Maddad]

mcbethcg<br />"<font color="yellow">Maddad, the mechanism that everyone but you seem to be aware of is the effect that occurs when an ice skater pulls thier arms in, or when the ice skater puts their arms back out. A rotating object changes the speed of rotation when the distribution of mass changes.</font><br /><br />What you are unaware of is that your mechanism doesn't work because there is no <strong><em>net</em></strong> sinking of mass into the Earth during an earthquake. It is not just the change of distribution of mass; it is the net change of distance to the center of the Earth that allows you to invoke your skater. Since it ultimately is rising mass that causes sinking mass, your skater effect doesn't work.<br /><br />Can you understand that?

 

[bobvanx]

Ah, I see maddad's blindspot now.<br /><br />Look at the Earth as a globe, and it's pretty clear that <i>over time</i> there is no net change in the distribution of mass from the center of the planet. Therefore, since the spin rate is determined by the overall mass, there can be no change.<br /><br />Maddad, that's not the important bit of info. The rate of spin is not determined by the distance to the center of the Earth (which is a point), but by the distance to the spin axis (which is a line). So if you take a mountain from the equator, which is 4000km from the axis, and move it to the North or South Pole, it is then 0km from the axis, and all of its angular momentum gets conserved and the planet speeds up.<br /><br />Obviously, nobody moved a mountain. But a plate shift of this magnitude did displace mass towards or away from the Earth's axis of rotation, and so the length of the day has changed. We're all just waiting to see by how much. A bigger change has been the alignment of that axis. Again, someone is at work measuring it, and soon we'll know how much it moved.

 

[meteo]

Angular momentum is equal to (moment of inertia)*(angular velocity). Moment of inertia = (mass)(radius)^2. Note the radius is squared.<br /><br />radius is distance from moment of inertia (principal) axis<br /><br />So if you dig a hole and make a hill that is a net INCREASE of the moment of inertia of the earth, since angual momentum must be conserved the angular velocity decreaces (AT THE SAME LATTITUDE). If a mountain forms this causes an increace in the moment of inertia. As Bob pointed out you can also move mass to different lattidudes which would decreace the moment of inertia. I don't know which is typically going to have the bigger effect. <br /><br />This causes the very slight change in length of day. The skater is just an illustration of the same physcis it's not the same problem. The math for the polar motion change is probably more difficult.