Japan Quake Released Hundreds of Years of Strain

by Brett Israel

Date: 15 June 2011 Time: 01:15 PM ET

The magnitude 9.0 earthquake that shook Japan on March 11 released hundreds of years of tectonic strain, and has provided researchers with clues about where such large quakes could strike in the future, according to a study published today in the journal Nature that provides more information on how the epic quake occurred.

The March 11 earthquake is the fourth-largest ever recorded in the world. The quake struck off the coast of the Tohoku region of Japan, triggering a deadly tsunami that may have killed nearly 30,000 people.

The rumbling didn't end with this massive rupture, and it hasn't stopped today. Nearly 75 aftershocks of magnitude 6.0 or greater have struck the region. In the new study, scientists used Global Positioning System (GPS) stations from a network that has been in place for 15 years to document the shaking. [Related: Listen to Japan's Huge Earthquake]

The quake deformed a stretch of ground 250 miles (400 kilometers) long and 124 miles (200 km) wide, "a remarkably compact area," according to Jean-Philippe Avouac, a geophysicist at Caltech, who was not involved with the study.

The Japanese earthquake ruptured along the Japan Trench near the boundary between the Pacific and North American tectonic plates — huge, moving slabs of the Earth's crust. The quake was a megathrust earthquake, where the Pacific plate dove underneath Japan at the Japan Trench. The seafloor was pushed away from Japan sending waves roaring toward Hawaii and the U.S. West Coast.

The data suggests that the ground between the two tectonic plates slipped as much as 165 feet (50 meters), twice the slip of other giant quakes such as the magnitude 9.4 off Sumatra in 2004 and the 9.0 in Chile in 2010. This massive movement is one reason why the Japan quake produced such a large tsunami. The slip was also shallower than might have been expected, and was in a notable location.

"A striking result is that the earthquake occurred at a place where these data were showing rapid strain buildup," Avouac told OurAmazingPlanet. "It says that places where strain is building up rapidly, more rapidly than it has been released by historical and recent earthquakes, are places of potential future earthquakes."

More analysis must be done to pinpoint specific places of rapid buildup, Avouac said. Yet despite worries that this year's quake could mean increased danger for a major quake near Tokyo, the data suggests that a plate interface south of the rupture zone shows "no indication of a major zone of strain buildup on that portion of the plate boundary that might threaten Tokyo," Avouac wrote in a commentary that accompanies the study.

The GPS data suggest that a significant amount of strain continues to leak via a process called afterslip, which is a nonviolent movement of the ground that follows an earthquake.

Reach OurAmazingPlanet staff writer Brett Israel at bisrael@techmedianetwork.com. Follow him on Twitter @btisrael.