Two years ago, the devastation wrought by the magnitude 9.0 Tohoku-Oki earthquake and resulting tsunami was laid out for the world to see: Japanese towns destroyed, nuclear reactors melting down, and a staggering loss of life. Geologists, though, are still learning about the underlying causes of the earthquake, and finding that the cataclysmic event had an unusual aftermath.

Studies done just after the disaster found that the 9.0 quake caused the ocean floor to slip nearly 50 meters (about 165 feet). In addition, according to Weiren Lin, head of the study at the Kochi Institute for Core Sample Research in Japan, pressure that built up along the fault line dropped to nearly zero after the earthquake (typically quakes release only a small amount of pressure at a time). Lin is the leader author on a new study in Science this week that not only confirms the weird pressure drop, but also explains why the quake was so peculiar: After the event, the fault transformed into an entirely different kind of fault.

"I was very interested to know what the stress state is in the vicinity of the ruptured fault," Lin tells PM, "and what role the stress played in the earthquake." Participating in an Integrated Ocean Drilling Program Expedition provided "a unique opportunity to access and study the fault, which lies below a very deep seabed in about 7000 meters of water depth."

The team got this in-depth look by drilling boreholes into the shallow portion of the subduction zone, or the place where one tectonic plate moves beneath another and sinks into the earth. While drilling, the researchers looked for what are called borehole breakouts—enlargements in the drilled core caused by too much stress for the area to maintain. These breakouts show researchers which parts of the plate are under pressure and by how much. Geologists use acoustic imaging tools to analyze the breakouts and fractures, and calculate the amount of stress they are under.

What happened during the Tohoku-Oki earthquake, researchers now know, was an unexpected change in fault formation. Before the quake, it was a thrust fault, one where older rocks are pushed up out of the earth to sit over new ones. But during the quake, the plates slipped so much that they're now in a normal fault position, with primarily horizontal rather than vertical movement. This transformation may be related to the unusual stress drop scientists now detect.

Lin's team isn't sure what this means for future earthquakes yet. During the next six months, they plan to collect temperature data from the boreholes they've drilled. It's possible, Lin said, that they could find evidence of a temperature anomaly related to the friction of the slipping plates. They also hope to test for other signs of unusual friction at the fault line, or at least something to help them figure out what caused such an unusual earthquake, and how to get a better idea of when the next one is coming.

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