The massive earthquake that struck northeast Japan Friday (March 11) has shortened the length of Earth's day by a fraction and shifted how the planet's mass is distributed.
A new analysis of the 8.9-magnitude earthquake in Japan has found that the intense temblor has accelerated Earth's spin, shortening the length of the 24-hour day by 1.8 microseconds, according to geophysicist Richard Gross at NASA's Jet Propulsion Laboratory in Pasadena, Calif.
Gross refined his estimates of the Japan quake's impact – which previously suggested a 1.6-microsecond shortening of the day – based on new data on how much the fault that triggered the earthquake slipped to redistribute the planet's mass. A microsecond is a millionth of a second.
"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 told SPACE.com in an e-mail. More refinements are possible as new information on the earthquake comes to light, he added.
The scenario is similar to that of a figure skater drawing her arms inward during a spin to turn faster on the ice. 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 24 hours, or 86,400 seconds, long. Over the course of a year, its length varies by about one millisecond, or 1,000 microseconds, due to seasonal variations in the planet's mass distribution such as the seasonal shift of the jet stream.
The initial data suggests Friday's earthquake moved Japan's main island about 8 feet, according to Kenneth Hudnut of the U.S. Geological Survey. The earthquake also shifted Earth's figure axis by about 6 1/2 inches (17 centimeters), Gross added.
The Earth's figure axis is not the same as its north-south axis in space, which it spins around 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 and the north-south axis by about 33 feet (10 meters).
"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.
The March 11 quake was the largest ever recorded in Japan and is the world's fifth largest earthquake to strike since 1900, according to the USGS. It struck offshore about 231 miles (373 kilometers) northeast of Tokyo and 80 miles (130 km) east of the city of Sendai, and created a massive tsunami that has devastated Japan's northeastern coastal areas. At least 20 aftershocks registering a 6.0 magnitude or higher have followed the main temblor.
(Read by Renee Haines. Renee Haines is a journalist at the China Daily Website.)
| 上周五（3月11日）袭击日本东北部的巨大地震使地球每日时长略有缩短，还改变了全球的质量分布。
据美国宇航局喷气推进实验室的地球物理学家理查德•格罗斯说，对日本发生的这次8.9级大地震的新研究发现，剧烈的地震使地球自转速度加快，使原本24小时的一天缩短了1.8微秒。该实验室位于加州的帕萨迪纳市。
根据引发地震的断层所发生的位移的新数据，格罗斯修正了他先前对日本地震影响的估计，他原先认为地球日长缩短了1.6微秒。1微秒就是一百万分之一秒。断层位移将使地球质量分布发生变化。
格罗斯在电子邮件中告诉SPACE.com网站说，“通过改变地球质量的分布，日本地震将使地球自转加快，使每日时长缩短约1.8微秒。”他说，在有关地震的更多新信息发布之后，可能还会进行更多修正。
这一情景就像花样滑冰运动员在冰上旋转时，为了让自己旋转得更快，她会将手臂内曲。在地震中，地球质量移动得离赤道越近，地球转速就越快。
地球上的一天是24小时，或8.64万秒。在一年中，由于地球质量分布产生的季节性变动，例如高空急流产生的季节性变动，地球日长会发生约1毫秒（1000微秒）的差值。
据美国地质调查局的肯尼斯•哈得纳特称，最初的数据显示，上周五的大地震使日本主岛发生了约8英尺的位移。格罗斯还说，这次地震使地轴偏移约6.5英寸（合17厘米）。
地轴不同于空间里的南北轴线。地轴每天围绕南北轴线旋转一次，转速约为1000英里/时（合1604千米/时）。地轴是地球质量平衡的轴线，偏离南北轴线约33英尺（10米）。
格罗斯说：“地轴位置的偏移将导致地球在旋转过程中的摇摆有些不同，但不会导致地球轴心在空间中发生位移。只有外部作用力，例如太阳、月球和行星产生的引力才能使地球轴心发生位移。”
根据美国地质调查局，3月11日大地震是日本有史以来最大的地震，也是1900年以来全球五大地震之一。地震发生在距东京东北部231英里（373千米）、仙台市以东80英里（130千米）的近海处，引发的巨大海啸摧毁了日本东北部的沿海地区。在主震过后，至少又发生了20次6级以上的余震。
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发表于 2012/2/27 09:54:41