March 12, 2011 -- The most powerful earthquake ever recorded in Japan. A tsunami that spread all the way across the Pacific, doing damage as far away as California. Two-dozen aftershocks of magnitude 6.0 or greater -- earthquakes that might have made headlines on their own if the original earthquake had not been an 8.9. A NASA geophysicist said the Japanese earthquake was so violent that the Earth's rotation was sped up by 1.6 microseconds.
Is this The Big One? The final catastrophe? A sign of more to come?
In a word, no, say scientists. They say the Japanese earthquake was terrible -- but not terribly unusual.
"We are not having more earthquakes than usual," said Lisa Grant Ludwig of the University of California, Irvine. "What we are having is earthquakes with a bigger impact as the world's population increases, and is concentrated in places where earthquakes are likely to strike."
Geoff Abers of the Lamont-Doherty Earth Observatory at Columbia University had a different take:
"In general, no, but it is true there have been more M>8 earthquakes [magnitude 8.0 or higher] per year in the last eight years than the two to three decades previous," Abers said. "There is some debate, currently, as to whether or not that increase is statistically significant, and if it is, why that should occur."
As of Saturday morning, Universal Time, which is five hours ahead of Eastern Standard Time, the U.S. Geological Survey showed there had been 365 earthquakes of at least 4.5 magnitude in the last seven days. Scientists said the number is about average.
The level of bad luck has been noteworthy, though. Last month's earthquake in New Zealand had a magnitude of 6.3 -- not unusual -- but it happened to be centered right beneath the city of Christchurch.
Last year's earthquake in Haiti was "large but not huge," in the words of Jian Lin of the Woods Hole Oceanographic Institution -- but it just happened to be centered beneath the impoverished capital city of Port-au-Prince. It also was on a fault line that had been relatively quiet for 200 years.
"This fault [under Haiti] was locked in a way that it didn't produce a lot of small quakes," said Art Lerner-Lam of Lamont-Doherty after the Haiti disaster.
Japan Earthquake: Awful but 'Not Unexpected'
The most powerful earthquake in U.S. history -- magnitude 9.2 -- hit Prince William Sound, Alaska, in 1964. The ones with the most widespread effects struck along the New Madrid fault in what is now Missouri -- but they came in 1811 and 1812, before the invention of the seismometer or any rating scale, and modern scientists debate from circumstantial evidence whether they would rate a 7.0 or an 8.1 today.
A team led by the late Paul Silver of the Carnegie Institution reported that small earthquakes seem to increase slightly in the autumn, and decrease in the spring. He surmised that barometric pressure may have a slight effect on the ground.
So the Earth is constantly restless, say geophysicists. The difference today is that human beings are there to notice it.
"Japan is a terrible tragedy," said Ludwig. "Hopefully there will be lessons to be learned from it."