The earthquake that shook northern Japan on Friday brought more than seismic waves and a devastating tsunami; it also moved the coastline and changed the planet's rotation.
Global positioning stations close to the epicenter jumped 13 feet to the east, and Ross Stein, a geophysicist at the U.S. Geological Survey, said Japan is now wider than it was before. According to estimates by NASA scientists, the earthquake also shortened the day by a couple millionths of a second and caused the Earth's axis to tilt.
Part of Japan sits atop the North American plate, which is colliding with the Pacific plate. The Pacific plate is moving about 3.5 inches per year in a west-northwest direction, diving under the North American plate in what is called a subduction zone.
Last week's 8.9 magnitude offshore earthquake released some of that tension, causing the land to jump back to the east. As the land unbuckled, Japan's coastal altitude dropped by about 2 feet, paving the way for the tsunami to cause more damage once it reached land.
This unbuckling during major earthquakes causes a shift of mass toward the planet's center. Like an ice skater pulling her arms toward her body to gain momentum in her spins, the inward shift of mass causes the Earth to spin faster, thus shortening the day.
Other great earthquakes have altered the Earth's axis and shortened the day. The 2004 earthquake in Sumatra shortened the day by 6.8 millionths of a second, compared to 1.8 millionths of a second by last week's earthquake, said Richard Gross, a scientist at NASA's Jet Propulsion Laboratory.
"The Earth is always wobbling, and the length of the day is always changing," Gross said.
Stein said the event has reminded many of the need to re-evaluate the effect of large earthquakes in the region. Nothing larger than a 8 magnitude earthquake had hit the Japan subduction zone, leading to underestimates of the size of a tsunami that might strike the coast.
"It did them a great disservice," said Stein of the geological survey that led to the underestimate (Kenneth Chang, New York Times, March 13). -- PK
