http://tvnz.co.nz/view/news_world_story_skin/466502%3fformat=html
Earthquake speeds up Earth's spin
Dec 29, 2004
The deadly Asian earthquake may have permanently accelerated the Earth's
rotation -- shortening days by a fraction of a second -- and caused the planet
to wobble on its axis, US scientists say.
Richard Gross, a geophysicist with NASA's Jet Propulsion Laboratory in
California, theorized that a shift of mass toward the Earth's center during the
quake on Sunday caused the planet to spin 3 microseconds, or one millionth of a
second, faster and to tilt about 2.5 cm on its axis.
When one huge tectonic plate beneath the Indian Ocean was forced below the edge
of another "it had the effect of making the Earth more compact and spinning
faster," Gross said.
Gross said changes predicted by his model probably are too minuscule to be
detected by a global positioning satellite network that routinely measures
changes in Earth's spin, but said the data may reveal a slight wobble.
The Earth's poles travel a circular path that normally varies by about 10
metres, so an added wobble of an 2.5 cm is unlikely to cause long-term effects,
he said.
"That continual motion is just used to changing," Gross said. "The rotation is
not actually that precise. The Earth does slow down and change its rate of
rotation."
When those tiny variations accumulate, planetary scientists must add a "leap
second" to the end of a year, something that has not been done in many years,
Gross said.
Scientists have long theorized that changes on the Earth's surface such as tide
and groundwater shifts and weather could affect its spin but they have not had
precise measurements to prove it, Caltech seismologist Hiroo Kanamori said.
"Even for a very large event, the effect is very small," Kanamori said. "It's
very difficult to change the rotation rate substantially."
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