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Name: Luanne
Status: Other
Age: N/A
Location: California
Country: United States
Date: December 27, 2004

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.

What does that mean and what can that do?

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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