Quake changed Earth’s shape, scientists say

<body leftmargin="0" bgcolor="#000060" text="#FFFFFF" link="#FFFF00" vlink="#FF9900"> <b> <p class="MsoNormal"><font size="4" face="Arial">Quake changed Earth’s shape, scientists say</font></p> </b> <p class="MsoNormal"><font face="Arial"><font size="1">Posted Jan. 12, 2005<br> Courtesy NASA<br> and World Science staff</font> <b> <font size="2" face="Arial"><br> <br> </font> </b> </font> <b><font face="Arial" size="2">Scientists using data from the Indonesian earthquake calculated it affected Earth’s rotation, decreased the length of day, slightly changed the planet’s shape, and shifted the North Pole by centimeters. The earthquake that created the huge tsunami also changed the Earth’s rotation, they said.</font> </p> <p class="MsoNormal"><font face="Arial" size="2">Benjamin Fong Chao of NASA’s Goddard Space Flight Center, Greenbelt, Maryland, and Richard Gross of NASA’s Jet Propulsion Laboratory, Pasadena, Calif., said all earthquakes have some affect on Earth’s rotation. It’s just they are usually barely noticeable. <o:p> </o:p> </font></p> <p class="MsoNormal"><font face="Arial" size="2">“Any worldly event that involves the movement of mass affects the Earth’s rotation, from seasonal weather down to driving a car,” Chao said. <o:p> </o:p> </font></p> <p class="MsoNormal"><font face="Arial" size="2">Chao and Gross have been routinely calculating earthquakes’ effects in changing the Earth’s rotation in both length-of-day as well as changes in Earth’s gravitational field. They also study changes in polar motion that is shifting the North Pole. The “mean</font><b><font face="Arial" size="2">,” or average</font></b><font face="Arial" size="2">, position of the North was shifted by about 2.5 centimeters (1 inch) in the direction of 145º East Longitude. This shift east is continuing a long-term seismic trend identified in previous studies.<o:p> </o:p> </font></p> <p class="MsoNormal"><font face="Arial" size="2">They also found the earthquake decreased the length of day by 2.68 millionths of a second. The reason is that the quake caused the Earth to become slightly more compact. This results in a faster rotation, the same way spinning skater will begin to turn faster if she draws her arms closer to herself. <o:p> </o:p> </font></p> <p class="MsoNormal"><font face="Arial" size="2">The quake also affected the Earth’s shape, the researchers found. They found Earth’s oblateness (flattening on the top and bulging at the equator) decreased by about one part in 10 billion. This continues a trend of earthquakes making Earth less oblate. <o:p> </o:p> </font></p> <p class="MsoNormal"><font face="Arial" size="2">To make a comparison about the mass that was shifted as a result of the earthquake, and how it affected the Earth, Chao compares 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 the length of day by only 0.06 microseconds and make the Earth only very slightly more round in the middle and flat on the top. It would shift the pole position by about two centimeters (0.8 inch).<o:p> </o:p> </font></p> <p class="MsoBodyText"><font face="Arial" size="2">The researchers concluded the Sumatra earthquake caused a length of day (LOD) change too small to detect, but it can be calculated. It also caused an oblateness change barely detectable, and a pole shift large enough to be possibly identified. They hope to detect the LOD signal and pole shift when Earth rotation data from ground based and space-borne position sensors are reviewed.<o:p> </o:p> </font></p> <p class="MsoNormal"><font face="Arial" size="2">The researchers used data from the Harvard University Centroid Moment Tensor database that catalogs large earthquakes. The data is calculated in a set of formulas, and the results are reported and updated on a NASA Web site. <o:p> </o:p> </font></p> <p class="MsoNormal"><font face="Arial" size="2">The massive earthquake off the west coast of Indonesia on December 26, 2004, 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 one hundred years and largest since the 1964 Prince William Sound, Alaska earthquake.<o:p> </o:p> </font></p> <p class="MsoNormal"><font face="Arial" size="2">The devastating mega thrust earthquake occurred as a result of the India and Burma plates coming together. It was caused by the release of stresses that developed as the India plate slid beneath the overriding Burma plate. The fault dislocation, or earthquake, consisted of a downward sliding of one plate relative to the overlying plate. The net effect was a slightly more compact Earth. The India plate began its descent into the mantle at the Sunda trench that lies west of the earthquake’s epicenter.</font><o:p> </p> <p class="MsoNormal"><font size="2" face="Arial"><a href="http://groups.yahoo.com/group/world-science">http://groups.yahoo.com/group/world-science</a></font> </p> </b> </body>