Sumatra Quake's effect on Earth's Rotation?

CalliArcale · Dec 29, 2004

Now I get it! Thanks, bobvanx! I'd been puzzled for the same reason as maddad, but that makes perfect sense now. Thank you! <div class="Discussion_UserSignature"> "People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff." -- The Tenth Doctor, "Blink" </div>

chew_on_this · Dec 29, 2004

Maybe Maddads' Mommy will make him apologize. I think Maddad admitting he's wrong is one of the signs of the apocolypse...

newtonian · Dec 29, 2004

maddad- you posted: There will be no effect on rotation at all because there is no outside force acting on the Earth. Remember our discussion concerning changing earth's rotation speed through selective directional harnessing of earth's tides? You still seem to have a mental block concerning the laws of motion.

Saiph · Dec 29, 2004

maddad: I'm afraid you're wrong this time. <blockquote>In reply to:<hr />There will be no effect on rotation at all because there is no outside force acting on the Earth<hr /></blockquote> Now, if you'd said there was "no effect on angular momentum because..." I'd agree. However you reduced it to rotation, or in this case angular velocity. Through the efforts of internal forces one can change the velocity of a rotating object. We'll take the ever popular figure scater. They change their rotation by exerting internal forces. At no time does an external force act on the system to change their rate of rotation (ignoring the drag and friction of course). Their angular momentum remains constant however, as it is a function of the moment of inertia (rotational mass) and angular velocity. The tricky part is the moment of inertia. Unlike linear momentum, this is not determined solely by the total sum of mass, but also by it's distribution. By redistributing mass, the moment of inertia can be altered. The angular velocity changes according to pick up the slack, leaving the angular momentum unchanged. As bobvanx as said, if the earthquake shifts mass towards or away from the poles (or even inwards, by changing densities and allowing things to sink...) the rotation rate will be altered. The angular momentum is the same though. So your conserving the wrong quantity. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

newtonian · Dec 29, 2004

Saiph- this is a bit of a tangent, however it is relevant. Can we change earth's rotation speed by selectively harnessing tides in one direction? I.e extracting energy which averages no net direction from one direction of tidal flow only. I had a separate thread on this on the old SDC but maddad and I locked heads ...... The reason would be to stabilize rotation speed over billions of years -I know the effect would be negligible in the short term

meteo · Dec 29, 2004

Also.... 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). Angular momentum is equal to (moment of inertia)*(angular velocity). Moment of inertia = (mass)(radius)^2. Note the radius is squared. radius is distance from moment of inertia (principal) axis

Saiph · Dec 29, 2004

newtonian: Perhaps, I'd have to think about it. You'd essentially be transfering the linear momentum of the tides to rotational motion of the turbines...but you might be able to do it. Meteo: Correct, any net shift of mass inwards or outwards from the core/spin axis will do the trick. Raising a mountain will increase inertia and slow rotation...assuming you have more mass further out than the mountain displaces downward into the mantle. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

igorsboss · Dec 29, 2004

Raising a mountain will increase inertia and slow rotation Correction: it increases the moment of inertia, not the inertia. The quantity of mass didn't change, so the inertia didn't change. The position of the mass changed, so the inertia's moment is what changed.

Saiph · Dec 29, 2004

Moment is an odd term to use. I usually end up making the slip of using plain old inertia in both linear and rotation cases, as the effect is the same. Both the moment of inertia and inertia resist acceleration. I could very well get away with saying rotational inertia. Anyway, the meaning can be derived from context that I'm refering to rotational "moment of inertia". Enough being defensive on my part. Tweak accepted. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

meteo · Dec 30, 2004

Since I=mr^2 it could be equal mass in the mountain or even a little less.

Saiph · Dec 30, 2004

1) The effect will always be greater for rotational inertia than for linear, since R must always be positive (in definition, and due to the square factor) 2) I'm not saying the value will be the same, merely their role in determining momentum. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

Maddad · Dec 30, 2004

bobvanx The blind spot is still yours. If that mountain moves poleward from the equator tectonic activity, the reason it moved poleward is that another mountain pushed it there and is now in its place. Ultimately, the first mountain rose from a mid-Atlantic ocean ridge closer to the equator, and this current mountain will eventually subduct. Therefore, there is no net change in mountain mass, either toward the Earth's center or towards the Earth's spin axis. Surface components will rearrange themselves, but the overall effect is that the mass distribution remains the same. You still lack a mechanism for involving earthquakes in a change of rotation period since your skater model is falsified. You also still lack a series of measurement over decades between earthquakes on the length of rotation. They probably exist, but you have not examined them.

Maddad · Dec 30, 2004

Saiph "Through the efforts of internal forces one can change the velocity of a rotating object. Agreed. However, the internal forces mentioned so far are balanced, and so will not effect such a change. The mass distribution in plate tectonics equalizes the mass that rises out of the Earth in mid oceanic ridges with mass that sinks in subduction zones. The north-south orientation of the many subduction zones around the world, and the 70,000 kilometers of oceanic ridges are not balanced. However, whatever goes up, poleward, comes down - subducts. The fallacy of thinking that earthquakes cause a change in Earth's rotation speed comes because you are examining a snapshot of a decades-long event at a non-representative point. Suppose that a big block of mass moves during an earthquake, causing at that instant a change in rotation speed. During all the other instants, leading up to the next earthquake you will have a series of smaller changes in the reverse direction. Once you add all the instants together you get zero change.

bobvanx · Dec 30, 2004

Oh, okay, well I guess your superior ability to not follow the conversation trumps my meager ability to try to make it all clear for you. I was actually afraid for a moment that you were going to process the data and see where your mistake was, and then we'd have to face the apocolypse. Thanks for preserving us from that!

sloracer · Dec 30, 2004

This is a copy of a post I made earlier at another site, but it fits well here with some additional information that is worth your attention. An extensive special on ABC tonight had a lot of film, apparently collected from those who just happened to have cameras handy. They also were discussing setting up a series of electronic equipment that would feed information to surface buoys that would transmit to satelites to help in giving warning when Tsunami's are active. I wish I had better retention with details, but they talked about a group of Islands off Africa that are noted for active Volcanic action. In fact, they said they anticipated that one of them--I thought it was Los Palmas--was expected to collapse when the time came for it to erupt, and that the collapse would cause a Tidal Wave that would reach our East coast. They were predicting something like 30 to 50 ft. waves.

meteo · Dec 30, 2004

From the USGS http://earthquake.usgs.gov/eqinthenews/2004/usslav/neic_slav_faq.html Answer: Richard Gross at JPL has modeled the coseismic effect on the Earth's rotation of the December 26 earthquake in Indonesia by using the PREM model for the elastic properties of the Earth and the Harvard centroid-moment tensor solution for the source properties of the earthquake. The result is: change in length of day: -2.676 microseconds polar motion excitation X : -0.670 milliarcseconds polar motion excitation Y: 0.475 milliarcseconds Since the length of the day can be measured with an accuracy of about 20 microseconds, this model predicts that the change in the length-of-day caused by the earthquake is much too small to be observed. And, since the location of the earthquake was near the equator, this model predicts that the change in polar motion excitation is also rather small, being about 0.82 milliarcsecond in amplitude. Such a small change in polar motion excitation will also be difficult to detect./safety_wrapper>

Maddad · Dec 30, 2004

Well, that's an interesting idea, that the magnetic pole represents where the spin axis pole used to be. My first inclination is to disagree, but I'm not sure that I'd be right. The magnetic poles have wandered quite a bit in the last 400 million years or so. North magnetic used to be where Hawaii is now. Anyone have a link that might relate the two types of poles?

newtonian · Jan 1, 2005

Saiph- Shall I restart my old thread on this? Concerning my old (lost) thread on selective harnessing of tidal power to stabilize earth's rotation over billions of years, you posted: Perhaps, I'd have to think about it. You'd essentially be transfering the linear momentum of the tides to rotational motion of the turbines...but you might be able to do it. Yes, I may as well restart that thread.

claywoman · Jan 1, 2005

A question from someone who only had one class in geology and slept through geography (it was a 7 am class), I do have one small question. Isn't the slight glitch in the movement of the earth really insignificant because the earth weeble-wabbles all the time? I mean, the effects are so small that in the long run say over a millenium, will it make any difference at all? What about the water displacement like with the Tsunami, wouldn't that also have some sort of effect on the rotation or tilt of the earth? Remember guys, I'm a real neophite here!!

meteo · Jan 2, 2005

There is the Chandler Wobble which may be caused by the earth's buldge at the equator interacting with the moon, there are other explanations as well. The range in polar motion change with the Chandler Wobble is 0.37 arc-seconds over 14 months. Compared to 0.82 milli-arc-second (modeled) for the Sumatra earthquake. There's also cycles for obliquity, precession which change over thousands of years, and are MUCH larger. I think the Tsunami would have a neglidgeble (billionth of an arc second, probably much smaller) effect on rotation. Ocean currents or tides or a change in the ocean floor (volume of part of the ocean) can cause a measurable change in rotation by changing the moment of inertia, or having angular momentum (currents), I'm not positive about currents. A wave would "theoreticaly" change the moment of inertia, but so would a person going upstairs "theoreticaly", I think the effect during the tsunami was VERY negligeable but now would be EXTREMELY neglideable (an effect on currents, maybe).

Maddad · Jan 2, 2005

There is another reason that a wave would not change the rotation rate. Newton's Third Law of Motion says that for every action there is an equal and opposite reaction. Applied to this wave, it means that an equal force that made the wave go slosh in one direction works on the Earth in the opposite direction. The net change to the Earth is zero.

nexium · Jan 2, 2005

Some recent locations of the Magnetic pole may mark a previous location of the pole of rotation. It's my understanding that the magnetic pole has moved west more than 1000 miles in the past 50 years. The pole of rotation is also moving, more or less constantly, but less than one inch per year. Neil

catmandoo · Jan 3, 2005

Yes, sloracer, I saw the same program. The islands were the Canary Islands. However, the scientists asked about it were divided, some cited that landslide-induced tsunami are much more localized events and would not be transoceanic. I want to thank everyone who took time to answer my question, particularly meteo, whose answers were superb. I'd have to agree with him, that it's more reasonable to think of it using the Skater model. Suppose the plate on top were to jut upward by 1 mile (I know it was probably only feet, which is why the change is so miniscule), and the plate subducted thrust 1 mile deep. That's like increasing the Earth's diameter by a mile at that point, and it is like extending an arm (it may be easier to visualize by thinking of the movement in terms of a larger distance, like miles ... if a mountain shot up out of the water 15,000 feet high.. it's like an arm). There would be no counter-acting force from the Mid--Atlantic Ridge, maddad. The subducted rock would push more rock into the mantle, where it would liquefy, and circulate along the currents and eddies deep within the Earth. But the mantle circulates very slowly. So the time until some counter-acting event happens could be considerable. I guess I also disagree that taken over a long enough period of time (if you take enough samples, as maddad suggests), the effect must cancel itself out. Hasn't the Earth's rotation slowed down over the past several billion years? Can this effect be solely due to mass being added to the Earth by meteorites? Nobody really knows what's floating around deep within the Earth. I don't think there's any finding that the Mantle and Core must be made up of a distribution of materials with homogeneous density. Heavy elements could congregate and bob and weave and flow around, clos

Maddad · Jan 3, 2005

catmandoo Welcome to SDC. Sometimes the discussions get intense, as you have seen. "Suppose the plate on top were to jut upward by 1 mile . . . There would be no counter-acting force from the Mid--Atlantic Ridge, maddad. There has to be or else your plate stays where it was. The driving engine for the entire process is the Earth's internal heat. The magma expands, becomes less dense and therefore lighter, so it rises. While the cold plate about to subduct is in fact denser, the mantle is too vicous to allow it to subduct on its own. It has to be driven into the Earth by being pushed by material upwelling from an oceanic ridge. "the mantle circulates very slowly. So the time until some counter-acting event happens could be considerable. It would happen instantly on a geological timescale. Did you ever play with a Newton's Cradle? It had five string in a line with five hanging steel balls. The strings were spaced so that the balls just touched without the strings bending. When you pulled one back and let it strike the second, the fifth ball immediately bounced away, no waiting. " Hasn't the Earth's rotation slowed down over the past several billion years? It sure has, but now you are speaking about an entirely different process. The energy of the tides ultimately comes from angular momentum between the Earth and Moon. As we sap this energy, one of the effects is that the Earth's rotation slows. The process is very slow compared to the few decades needed to examine the tectonic effect on the Earth's rotation. About the homogenity of the Earth's interior, we use sound waves to track density, temperature, and phase changes. We see abrupt changes showing a solid metal inner core, a liquid outer core, plastic mantle, and of course we live on the solid crust. This gives us a very good model

mcbethcg · Jan 4, 2005

Of course, there would be a plate that moves down while another moves up. But the center of mass of the two plates would be in different lattitudes, effectively causing the roation of the earth to change, oh so slightly. A more equatorial mass moving up and a more polar mass moving down would cause a slight slowing.

Sumatra Quake's effect on Earth's Rotation?

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