Wednesday, 12 January 2005

Earthquakes and Navigation

From NASA :
NASA scientists studying the Indonesian earthquake of Dec. 26, 2004, have calculated that it slightly changed our planet's shape, shaved almost 3 microseconds from the length of the day, and shifted the North Pole by centimeters.
Chao and Gross routinely calculate earthquakes' effects on Earth's shape and rotation. They also study changes in polar motion--that is, the shifting of the North Pole.

According to their latest calculations, the Dec. 26th earthquake shifted Earth's "mean North Pole" by about 2.5 centimeters (1 inch) in the direction of 145 degrees east longitude, more or less toward Guam in the Pacific Ocean. This shift is continuing a long-term seismic trend identified in previous studies.

The quake also affected Earth's shape. Chao and Gross calculated that Earth's oblateness (flattening on the top and bulging at the equator) decreased by a small amount--about one part in 10 billion. This continues the trend of earthquakes making Earth less oblate. Less oblate means more round.

They also found the earthquake decreased the length of the day by 2.68 microseconds. (A microsecond is one millionth of a second.) In other words, Earth spins a little faster than it did before. This change in spin is related to the change in oblateness. It's like a spinning skater drawing arms closer to the body resulting in a faster spin.

None of these changes have yet been measured--only calculated. But Chao and Gross hope to detect the changes when Earth rotation data from ground based and space-borne sensors are reviewed.
BTW this appears to be not quite enough to affect the WGS-84 standard spheroid, as navigation calculations using it routinely involve 9 significant figures. 1 part in 10 billion is too small to be significant. But only just. If the "1 part in 10 billion" refers to the flattening coefficient of the WGS-84 spheroid, then it will change the last 2 digits of the current value ( 1/298.257223563). How important is that? Well, until 1984 we used the WGS-72 Spheroid, and that had a "flattening" value of 1/298.26.

OK, that's probably lost you. The best explanation for the layman of all this Navigational stuff is provided by UNESCO.

Far more important in practical navigation are the different spheroids and datums used on different maps. The US military, who are interested in really accurate navigation in order to avoid unpleasant incidents involving misdirected weaponry, have a handy chart giving the various corrections needed for artillery systems.

More important still is the fact that the whole of the Andaman area must be re-surveyed, and new charts produced. We know that the Sentinel Islands have moved upwards 3-5 metres, meaning the local seabed has done likewise.

Had they sunk instead of risen, then a unique branch of humanity may have been lost forever. But that will have to remain for now as the subject for a future article.

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