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Tom Mazanec
2011-Mar-22, 11:15 AM
I read that the recent Japan earthquake speeded up the Earth's rotation. By how much, and how long will it take for tidal friction to slow it back down to pre-quake speed?

slang
2011-Mar-22, 11:39 AM
The day is now 1.8 microseconds shorter, according to the Wikipedia page on the 2011 quake (http://en.wikipedia.org/wiki/2011_T%C5%8Dhoku_earthquake_and_tsunami#Geophysica l_impact) [insert usual caveats on wikipedia accuracy].

Tom Mazanec
2011-Mar-22, 02:45 PM
How fast does the Earth slow down? If it is 1 sec/100,000 years we will be back to "normal" in a couple months or so, right?

Hornblower
2011-Mar-22, 04:00 PM
How fast does the Earth slow down? If it is 1 sec/100,000 years we will be back to "normal" in a couple months or so, right?

Yes, that looks about right. But remember, the roughly periodic variations are so much larger that the gradual tidal effect is undetectable over such a short period. I would expect effects from the earthquake to be easier to detect because they were sudden.

Tim Thompson
2011-Mar-22, 04:21 PM
The secular rate of slowing of Earth's spin due to the moon is about 0.0015 seconds per day per century. 1.8 Microseconds is 0.0000018 seconds. That is comparable to the day to day variability of Earth's spin as a result of friction at the core-mantle boundary, and the seasonal speeding and slowing of Earth as a result of atmospheric friction from shifting trade winds. If 0.0015 seconds per day per century is 0.000015 seconds per day per year, then clearly the secular slowing will erase this speed-up in less than a year.

slang
2011-Mar-22, 04:44 PM
But I suppose that large earthquakes on average cancel out each others influence on Earth's spin, won't they? Or is there a tendency to move in one direction (up?)?

astromark
2011-Mar-22, 07:23 PM
If the planet gets smaller then it accelerated... but just look at all those 0's that 'Tim Thompson' provided...Its so small a correction its almost unmeasurable.. and a note to the OP, speeded is or should be sped.:cool:

Bob Angstrom
2011-Mar-22, 09:06 PM
I read that the recent Japan earthquake speeded up the Earth's rotation. By how much, and how long will it take for tidal friction to slow it back down to pre-quake speed?One tectonic plate diving under Japan's continental shelf caused the shelf and the plate to "bunch up" pushing pushing them away from the Earth's center of gravity and slowing the Earth's rotation. When the shelf broke loose, the plate and shelf returned to their unstressed condition and the Earth's rotation sped up. This is a normal cycle that will happen again. Slowing by tidal friction is a gradual background to these local effects.