From Nature:
The devastating earthquake that struck the Indian Ocean on 26 December was so powerful that it has accelerated the Earth’s rotation, geophysicists have declared. They estimate that the shockwave shortened the period of our planet’s rotation by some three microseconds.
The change was caused by a shift of mass towards the planet’s centre, as the Indian Ocean’s heavy tectonic plate lurched underneath Indonesia’s one, say researchers at NASA’s Jet Propulsion Laboratory in Pasadena, California. This caused the globe to rotate faster, in the same way that a spinning figure-skater accelerates by tucking in her arms.
I don’t know which is more impressive, that the earthquake could cause enough mass to move to change the period of rotation, or that we can actually measure this change?
But why would they use the terminology in the first paragraph that the “shockwave” caused this change? The change would only have to do with the static redistribution of mass (a thousand km by 30 km chunk of land moving up by a few meters).
Silly imprecise science writer. I think I would be horrible at science writing because I would make all sorts of careless mistakes like this.
OK, here I’ll show my ignorance of all thing geological. Can I think about the change as a 1000km by 30km by X km block moving (where I don’t know what X is)? What are the 1000km by 30 km’s are they the length of the fault or the distance away from the fault or depths of the block or…? Also, isn’t the mass that is moving, moving down (closer to the center of the earth) so that the moment of inertia goes down and hence the angular velocity goes up?
Err… I kind of mispoke and it’s more complicated than that. In fact, there’s a whole discussion about what causes the region around a given thrust fault (a subduction zone is a thrust fault) to move up or down with respect to sea level, the geoid or any other elevation marker over long timescales. What DID happen during this earthquake for sure was that the crust became temporarily thicker in the region of the earthquake due to the stacking of one plate on top of the other, and region around it thinned. The short term behavior is mainly elastic, but over the long term the viscoelastic crust will relax and some of these mass redistributions will be evened out.
So imagine a silly putty ball, spinning in space. Imagine someone pinching part of it and pulling outward. It’s moment of inertia will now be different.
I’m still missing why the moment of inertia went down (in order to increase the angular velocity and hence shorten the period as claimed in the article.) Moment of inertia is like mass times distance from axis squared, so if I move the mass away, the moment of inertia increases.
I understand that what you are saying is one plate slid over the other and so on one side of the fault the crust increased in thickness but on the other side it got thinner. But did the thicker part sink (i.e. get closer the axis of rotation)?