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Devastating Earthquake and Earth's Rotation
Name: Luanne
Status: Other
Age: N/A
Location: California
Country: United States
Date: December 27, 2004
Question:
Hello - We are watching the News regarding the
devastating earthquake in Indonesia. I just read that it may have
disturbed the earth's rotation.
"All the planet is vibrating" from the quake, said Enzo Boschi, the head
of Italy's National Geophysics Institute. Speaking on SKY TG24 TV, Boschi
said the quake even disturbed the Earth's rotation.
http://earthquake.usgs.gov/eqinthenews/2004/usslav/
What does that mean and what can that do?
Replies:
The earth, although hard, is elastic. When some sort of high energy event
occurs (earthquake, volcano, explosion of a nuclear device) the earth can
"ring" -- a geological bell of sorts. Sensitive seismometers at numerous
locations around the world can detect these earth-vibrations. In fact, the
echoes of the vibrations can also be detected. There is no global danger
from such events (except maybe the atmospheric dust from a volcano), but
obviously from the news disaster can occur at a local level and even
hundreds of miles away.
Vince Calder
Thank you for your question, Luanne… This event is simply tragic,
and the loss of life unimaginable. In this age of preoccupation with
the threats of war and terrorism, it is easy (sadly) to forget that
nature can wield such destructive power, and wield it so
indifferently. I am not so sure that any line of reasoning could
ever produce substantial justification for the death of 40,000+
people, but perhaps some solace will be found in whatever progress
might be made, as a consequence of this disaster, in safeguarding
lives from future earthquakes.
Because earthquakes involve movements of tectonic plates (and large
quantities of water, in the case of submarine earthquakes), they
necessarily involve relatively rapid movements of large amounts of
mass. When this mass moves, the distribution of Earth’s mass
changes and results in slight deviations in the rotational motion
(e.g., angular velocity and axial tilt) of the planet. To illustrate:
a figure skater pirouetting with his or her arms extended has a
specific way in which their body mass is distributed with respect to
their rotational axis (the rotational axis, incidentally, is an
imaginary line that extends through the middle of the rotating object
in a direction that is perpendicular to the direction of rotation…
think of a line connecting the North and South poles, for example).
If the figure skater then pulls one of their arms slightly closer to
their body, the way in which their body mass is distributed is
altered, thus causing their rotation to change. The intensity of that
change is proportional to the degree to which the distribution of
their body mass is altered. A spinning skater who bends but a single
finger toward their body may not feel a significant change in their
rotation; a spinning skater who pulls an arm in completely flush with
their body may produce so great a change in their rotation as to fall
down.
In the case of earthquakes, the amount of redistributed mass is
comparably small to the mass of the entire planet. Likewise, the
distance over which that mass is moved is comparably small to the
size, or radius, of the planet. Thus, the mass redistribution is
effectively small, in the grand scheme of things. But, earthquakes
still do cause some minor change to Earth’s rotation; it is purely
an inescapable consequence of physics. If a large (read: unfathomably
big) enough earthquake (or any seismic event, for that matter)
occurred, such that very extensive portions of Earth’s crust moved,
causing the planet’s rotation to become significantly altered, the
global climate could potentially be affected. In the case of this
most recent seismic activity in Indonesia, however, Earth’s rotation
will not likely be altered to such a degree that it will have
significant, climatic consequences.
I hope this explanation helps.
Scott J. Badham
Department of Geology and Geophysics
University of Wyoming
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Update: June 2012
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