日本地震使地球自轉加快1.8微秒
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 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。上週五(3月11日)襲擊日本東北部的巨大地震使地球每日時長略有縮短，還改變了全球的質量分佈。
據美國宇航局噴氣推進實驗室的地球物理學家理查德•格羅斯說，對日本發生的這次8.9級大地震的新研究發現，劇烈的地震使地球自轉速度加快，使原本24小時的一天縮短了1.8微秒。該實驗室位於加州的帕薩迪納市。
根據引發地震的斷層所發生的位移的新數據，格羅斯修正了他先前對日本地震影響的估計，他原先認爲地球日長縮短了1.6微秒。1微秒就是一百萬分之一秒。斷層位移將使地球質量分佈發生變化。
格羅斯在電子郵件中告訴網站說，“通過改變地球質量的分佈，日本地震將使地球自轉加快，使每日時長縮短約1.8微秒。”他說，在有關地震的更多新信息發佈之後，可能還會進行更多修正。
這一情景就像花樣滑冰運動員在冰上旋轉時，爲了讓自己旋轉得更快，她會將手臂內曲。在地震中，地球質量移動得離赤道越近，地球轉速就越快。
地球上的一天是24小時，或8.64萬秒。在一年中，由於地球質量分佈產生的季節性變動，例如高空急流產生的季節性變動，地球日長會發生約1毫秒(1000微秒)的差值。
據美國地質調查局的肯尼斯•哈得納特稱，最初的數據顯示，上週五的大地震使日本主島發生了約8英尺的位移。格羅斯還說，這次地震使地軸偏移約6.5英寸(合17釐米)。
地軸不同於空間裏的南北軸線。地軸每天圍繞南北軸線旋轉一次，轉速約爲1000英里/時(合1604千米/時)。地軸是地球質量平衡的軸線，偏離南北軸線約33英尺(10米)。
格羅斯說：“地軸位置的偏移將導致地球在旋轉過程中的搖擺有些不同，但不會導致地球軸心在空間中發生位移。只有外部作用力，例如太陽、月球和行星產生的引力才能使地球軸心發生位移。”
根據美國地質調查局，3月11日大地震是日本有史以來最大的地震，也是1900年以來全球五大地震之一。地震發生在距東京東北部231英里(373千米)、仙台市以東80英里(130千米)的近海處，引發的巨大海嘯摧毀了日本東北部的沿海地區。在主震過後，至少又發生了20次6級以上的餘震。
