'Japan quake shortened length of days by 1.8 microsecs'

Washington: The massive earthquake that struck Japan last Friday was so powerful that it accelerated the Earth's rotation speed, shortening the length of the day by 1.8 microseconds, a new analysis by NASA has claimed.

According to scientists at the US space agency, the 8.9-magnitude quake shifted the way the Earth's mass is distributed, which made the planet spin a little faster, cutting the 24-hour day by an estimated 1.8 microseconds. That is less than two millionths of one second.

The initial data suggested that the quake moved Japan's main island about eight feet and shifted the Earth's figure axis, around which the Earth's mass is balanced, by about 17 centimetres, said Richard Gross, a geophysicist at NASA's Jet Propulsion Laboratory in Pasadena, California.

Earlier, scientists have estimated that the quake has shortened the day by 1.6 microseconds.

But the revised estimate -- which is based on new data on how much the fault that triggered the earthquake slipped to redistribute the planet's mass -- showed that the day has actually been shortened by 1.8 microseconds, said Gross.

"By changing the distribution of the Earth's mass, the Japanese earthquake should have caused the Earth to rotate a bit faster, shortening the length of the day by about 1.8 microseconds," Gross was quoted as saying by the SPACE.com.

The scenario is similar to that of a figure skater drawing her arms inward during a spin to turn faster on the ice, Gross said, adding that the closer the mass shift during an earthquake is to the equator, the more it will speed up the spinning Earth.

One Earth day is about 86,400 seconds long and over the course of a year, its length varies by about one millisecond due to seasonal variations in the planet's mass distribution such as the seasonal shift of the jet stream.

The Earth's figure axis is not the same as its north-south axis in space, around which it spins once every day at a speed of about 1,000 mph (1,604 kph). The figure axis is the axis around which the Earth's mass is balanced.

"This shift in the position of the figure axis will cause the Earth to wobble a bit differently as it rotates, but will not cause a shift of the Earth's axis in space -- only external forces like the gravitational attraction of the sun, moon, and planets can do that," Gross said.

This isn't the first time a huge earthquake has changed the length of Earth's day. Major temblors have shortened day length in the past.

While the 8.8-magnitude earthquake in Chile last year also sped up the planet's rotation and shortened the day by 1.26 microseconds, the 9.1 Sumatra quake in 2004 shortened the day by 6.8 microseconds.

And the impact from Japan's 8.9-magnitude temblor may not be completely over. The weaker aftershocks may contribute tiny changes to day length as well, the scientists said.

According to the United States Geological Survey (USGS), the March 11 quake was the largest ever recorded in Japan and is the world's fifth largest earthquake to strike since 1900.

PTI