Earth's figure axis.

[Ouroboros]

Would someone explain the concept of the earth's figure axis please.  The term came up in a forum discussion elsewhere about the affect of the tsunami that hit Japan a few years ago that supposedly shifted the earth's figure axis by 8 inches. 

I googled but didn't find much. I understand it's to do with the earth's distribution of mass. I can see that if the earth's mass shifts, because of an earth quake say, then the centre of mass can shift, and therefore the axis of rotation can shift. What surprises me is that the shift is expressed in units of distance not angle. So is the earth's figure axis parallel to the earth's north south axis through the poles?

[vlaiv]

First time I've heard of it, but this article might help:

https://news.nationalgeographic.com/news/2011/03/110316-japan-earthquake-shortened-days-earth-axis-spin-nasa-science/

It says, and I quote:

Quote

Instead the quake shifted what's called Earth's figure axis, an imaginary line around which the world's mass is balanced, about 33 feet (10 meters) from the north-south axis.

So Earth's figure axis is an imaginary line that represents mass balance rather than rotational axis. It's very close to rotational axis, and I guess it is parallel to it.

[Ouroboros]

Thanks.  That link doesn't seem to work on my iPad. Nevertheless going by your quote I am surprised that the axis of rotation is not the same as the axis around which the mass is balanced. 

[vlaiv]

3 hours ago, Ouroboros said:

Thanks.  That link doesn't seem to work on my iPad. Nevertheless going by your quote I am surprised that the axis of rotation is not the same as the axis around which the mass is balanced. 

In principle it is, but center of mass is constantly shifting - tides, polar ice, tectonic movements, .... so those two are very close, but as you can see - not perfectly aligned and there are changes to this distance.