Scientists Measure Gravity Change From Earthquake

Science Daily is reporting that scientists were able to use satellite data to watch changes in the Earth's surface caused by a massive earthquake. These changes had two major measurable effects on the region. The massive uplift in the seafloor changed GPS measurements, and the density of the rock beneath the seafloor changed which produced a detectable change in gravity.

Comments

Feels lighter

[mortonda]

I thought I felt lighter this morning...

So, how much is it now?

[lheal]

I have a lot to keep track of, what with my checkbook, blogs, email, vehicle oil changes and tire rotation, bills, and keeping various client networks running.

So I'd appreciate it if someone could keep track of this whole gravity situation, and just give me a summary. Let me know if we're all about to go floating off into LEO, but otherwise, keep the announcements to a minimum.

Top-notch editing

[MustardMan]

Brought to you by the Department of Redundancy Department...

Detecting "major" quakes - those measuring a magnitude of 7 to 8.9 - which occur frequently is being investigated. NASA's planned extension of the current mission, dubbed GRACE 2, and its enhanced instrumentation should aid in that effort.

However, Han is hopeful that NASA's planned expansion of the current mission, dubbed GRACE 2, and its enhanced instrumentation, might allow the detection of "major" quakes - those measuring a magnitude 7 to 8.9 - which occur frequently.

magnitude of the change

[Vandilizer]

Really bland article I mean I can change the gravitation fields on my desk my moving my glass of water around and I do believe that is measurable. (Maybe not by a satellite.) So anyone out there have an idea of the magnitude of the change. Will athletes gain a boost there by training in a higher gravity environment? What are the effects of the lower gravity environment or is it so insignificant that who cares.

Or more interesting dose anyone have a map of the earth and differences in gravity in different areas? (I smell a new google map)

Just my 2 cents

Re:magnitude of the change

[Anonymous Coward]

|Or more interesting dose anyone have a map of the earth and differences in gravity in different areas? (I smell a new google map)"

And all the fat people can move to neighborhoods with lower gravity, Voila! Instant weight loss.

Re:magnitude of the change

[imsabbel]

The effect is well in line with the natural distribution of the local gravity constant.
Meaning it should be in the 0.01 m/s^2 range.

For the simple reason that if it were anymore, the earth would deform to counter that imbalance (molten core, you know).
That, btw, also limits the height of mountains to about 10-12Km on earth (compare to mars, where to lower gravity constant allowed much larger volcanos)

Re:magnitude of the change

[munpfazy]

So anyone out there have an idea of the magnitude of the change. Will athletes gain a boost there by training in a higher gravity environment? What are the effects of the lower gravity environment or is it so insignificant that who cares.

The full paper as well as a very nice layman's introduction in the Perspectives section is in this month's issue of Science. (Sorry - subscription only. But you may be able to find the text on a preprint server. I'm no geologist, but I haven't been able to find it in any of the obvious places.)

Basically, they map out a change of 15 microgals (1 gal = 1 cm/s^2) or around 1.5e-8 of the average gravitational field on the earth.

By comparison, the variation in g with latitude (at constant elevation) is around 0.5 percent, or 300'000 times as much. Variation associated with local geology is around 100 times smaller, but still swamps this earthquake signal.

What's cool about this measurement isn't that they're measuring something big enough to have any effect on humans, but rather that they're able to measure such a tiny effect at all.

There are all sorts of processes going on in the earth and in the oceans that involve movements of comparable amounts of mass: changes in glacier and polar icecaps, ocean-atmosphere gas exchange, deep sea current and temperature changes, movement and depletion of underground water, fast moving magma associated with volcanos, slow tectonic changes, etc. And now it seems like it's also helpful in trying to construct detailed models of an earthquake.

Incidentally, if you were an athlete trying to cash in on lower gravity, you'd be better off training in the Chilean highlands and competing in Puerto Rico - but it still wouldn't help you much, especially compared to biological effects and day to day variation in performance. (http://www.csr.utexas.edu/grace/publications/fact _sheet/3.html)

Death knoll for 'intelligent falling'?

[jnana]

Let's hope this is the final nail in the coffin for the theory of 'intelligent falling' proposed as an alternative to gravity.

GPS is relative to exactly where?

[r00t]

With everything moving all over and us trying to define property lines (including international borders) reliably, we sure do have a mess.

If GPS is tied to some NAVY building in Maryland and the building moves, do we then declare that the building DID NOT MOVE because it is by definition in a particular place? Everybody else moved?

(I do not in fact know: it could be an Air Force cave in the Rocky Mountains, etc.)

If half of the Earth moves relative to the other half, which set of property owners has a problem?

Re:GPS is relative to exactly where?

[AdmiralSpearmint]

If half the earth moves relative to the other half, I'd think we'd have bigger problems.

Re:GPS is relative to exactly where?

[mesri]

When you get a position fix from the GPS system, you are combining the information from several satellites, each of which is transmitting a signal of the form, "My name is GPS _X_, I sent this message at time _Y_ from the approximate location _Z_" A GPS receiver triangulates a position by calculating the distance to several satellites, using Center-Of-Earth coordinates.

It's true that your position is calculated relative to the satellites, but in order for the satellite to know where it was when it sent the message, there has to be pretty accurate data about its precise orbit, which depends intimately on the shape and mass distribution of the earth ("Geosynchronous" is only approximate), so that the final location in Latitude.-Longitude.-Altitude can be given relative to the center of the earth. A big quake could certainly shift things around enough to alter the orbit, which is probably what these researchers were talking about.

On the other hand, for the purposes of surveying on earth, its certainly conceivable that one could define property lines in relation to the locations of particular GPS antennae, fixed into bedrock or something, and that if those moved, things would be all kerflooey. But that's not new to GPS surveying, since its always been done relative to the location of particular fixed monuments. :)

Re:how does this work?

[kfg]

Is it in one spot . . .

Yes.

Put a cement block on the floor in front of you. Now stand on it.

Ta da! Instant local gravity increase, because there is now more mass underneath you.

KFG

Other gravity changes

[massivefoot]

This reminds me a little of a physics practical I did this year. It was supposed to be the first practical where we would get a decent accuracy, measuring g using a pendulum to about 6 significant figures.

We were also told at the end of the practical about far more accuarte ways of measuring g, and that a university in Germany several decades ago had used this regularly as experimental training for graduate students. However, when the experiment was performed at different times of the year, a small but definte increase in g was noticed during the winter. More accurate measurements showed a sudden spike near the start of winter, followed by a slow decrease until the summer.

Professors were baffled, until someone remembered that the lab in which the experiments were carried out was above a coal cellar used to store a huge quantity of coal for burning during the winter.