December 10, 2013 – Decoding a Tsunami’s Source: In the powerful earthquake off Japan in 2011, the size of the destructive tsunami it spawned caught scientists by surprise. But they soon learned why the waves were so large: measurements showed that part of the seafloor along the fault moved as much as 50 meters, or 55 yards, to the east, displacing an enormous amount of water.
That degree of movement, the largest ever measured for an earthquake, surprised scientists, too. The quake occurred in a subduction zone, where one of the planet’s tectonic plates dives beneath another. It was thought that the largest slip would occur at greater depths, and that shallower parts of the fault, nearer the seafloor, would move less.
Now, following an unusual drilling expedition in deep water off the northeastern coast of Japan, Dr. Fulton and about two dozen other scientists have explained why there was so much movement at such shallow depths. The fault, they say, was weak and slippery.
The findings may help scientists understand the destructive potential in similar areas, like the Cascadia fault zone off the northwestern United States and Canada. “We want to know if Cascadia and other places have a risk of such a large tsunami,” Dr. Fulton said.
In three papers published last week in the journal Science, scientists from the United States, Japan and other countries describe the analysis of data and cores from boreholes drilled about a half-mile into the ruptured Japan fault, in water greater than four miles deep. Using sensors placed in one of the boreholes, the researchers took the fault’s temperature, something that had never been done in a subduction zone. By measuring how the temperature fell over the nine months the sensors were in place and working backward in time, they were able to determine how much frictional heat was generated in the earthquake. That gave them the answer to how much resistance the fault had to moving: not much.
Pictured above, a view of the destruction left by the tsunami that hit Japan in March 2011.
“It was extremely slippery,” said Dr. Fulton, the lead author of a paper describing the temperature work.
Normally a scientific drilling expedition like this takes years to organize and finance. Because of the need to measure the fault zone before it cooled off too much, this project, which consisted of two voyages on a special Japanese drilling ship, the Chikyu, was planned in little more than a year following the March 2011 quake that left almost 20,000 people dead.
Other researchers analyzed core samples taken from one of the holes, and data from instruments that were sent down another hole to study the surrounding geology. The analysis revealed why the fault was so weak — it consisted of fine clay sediments, which offer little frictional resistance, and was very thin. “We know it’s less than five meters (16 feet) thick,” said Frederick M. Chester, a geophysicist at Texas A&M University who was the lead author of another of the papers.
Kelin Wang, a Geological Survey of Canada scientist who was not involved in the project, praised the scientists’ work, particularly that of Dr. Fulton’s group. “The most remarkable discovery is the temperature measurements,” said Dr. Wang, who wrote a perspective article accompanying the papers.
Drilling into the rupture zone in such deep waters and getting usable cores and other data was a remarkable achievement, he said. “Before they drilled I was pretty sure they would fail,” he added. “I was thinking the rupture would be such a mess. But it’s such a clear zone.”
The Master of Disaster