Volume: 03, Issue: 01 01/12/2005 
DigitalGlobe’s Quickbird satellite captured an image of Kalutara, Sri Lanka (top), on Dec. 26, 2004 at 10:20am local time, about an hour after the first waves hit, and Jan. 1, 2004 (lower) under normal ocean conditions. Credit: DigitalGlobe.
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The west coast of Sumatra near the capital of Aceh, Banda Aceh. Credit: Ikonos images copyright Centre for Remote Imaging, Sensing and Processing, National University of Singapore, and Space Imaging.
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This image acquired Dec. 28, 2004 by the MERIS (Medium Resolution Imaging Spectrometer) onboard ESA's Envisat Earth observation satellite shows the northeast coast of Sri Lanka and the southern coasts of India. Image courtesy ESA.
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This Moderate Resolution Imaging Spectroradiometer (MODIS) image was processed from data taken by NASA’s Terra Satellite Dec. 26, 2004 at 11:15am local time, about an hour after the tsunami struck the coast. Image courtesy NASA/NOAA.
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Other Articles in This Issue:
Deep Impact Heads for Comet Collision
Cassini Reveals Bulging Waistline of Iapetus
Hubble Providing Evidence of Possible New Planet
Participate in Professional Development Offerings

Tsunami-Causing Earthquake Changed Rotation of Earth

Using data from the Dec. 26, 2004 earthquake and tsunami that ravaged lands along the Indian Ocean, NASA scientists have discovered the disaster changed Earth’s rotation. The quake decreased the length of day, slightly changed the planet's shape, and shifted the North Pole by centimeters.

The earthquake, centered off the west coast of Indonesia, registered a magnitude of nine on the new "moment" scale (modified Richter scale) that indicates the size of earthquakes. It was the fourth largest earthquake in 100 years and the largest since the 1964 earthquake in Prince William Sound, Alaska.

The quake occurred as a result of stresses that developed as the India plate slid beneath the overriding Burma plate. The India plate began its descent into the mantle at the Sunda trench, which lies west of the earthquake's epicenter.

The tremendous force of the quake generated a tsunami--a series of extremely powerful, fast-moving waves that overwhelmed and destroyed many areas along the coast of the Indian Ocean. Approximately 150,000 people were killed by the tsunami, and thousands more are at risk for disease and starvation.

Dr. Benjamin Fong Chao of NASA's Goddard Space Flight Center and Dr. Richard Gross of NASA's Jet Propulsion Laboratory (JPL) reviewed data from the disaster and discovered that its impact reaches far beyond the visible wreckage. According to the team, the event actually changed the rotation of Earth. Chao and Gross said all earthquakes have some effect on Earth's rotation, but the changes are usually barely noticeable.

"Any worldly event that involves the movement of mass affects the Earth's rotation, from seasonal weather down to driving a car," Chao said.

Chao and Gross have been routinely calculating earthquakes' effects in changing Earth's rotation by examining length-of-day and gravitational field variations. They also study changes in the polar motion that is shifting the North Pole.

In the case of the recent devastating quake, the "mean North pole" was shifted east by about 2.5 centimeters (1 inch). This eastward shift is continuing a long-term seismic trend identified in previous studies.

They also found the earthquake decreased the length of day (LOD) by 2.68 microseconds. The quake caused the Earth to spin ever so slightly faster, resulting in the LOD decrease.

In addition, the quake affected Earth's shape. They found Earth's oblateness (flattening on the top and bulging at the equator) decreased by a small amount, about one part in 10 billion. This continued the trend of earthquakes making Earth less oblate.

To make a comparison about the mass shifted as a result of the earthquake and its effect on Earth, Chao compared it to the great Three-Gorge reservoir of China. If filled, the gorge would hold 40 cubic kilometers (10 trillion gallons) of water. That shift of mass would increase LOD by only 0.06 microseconds and make the Earth only very slightly less oblate. It would shift pole position by about two centimeters (0.8 inch).

For their research, the team used data from the Harvard University Centroid Moment Tensor database, which catalogs large earthquakes. None of these changes have yet been measured, only calculated, but changes are expected to be exactly measured when Earth's rotation data from ground based and space-borne sensors are reviewed.

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