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Japan quake caused 400km seismic slip

The magnitude 9 earthquake that occurred in Japan in March this year caused a 400km long land rupture, releasing centuries of pent up energy caused by two tectonic plates pushing slowly against each other, researchers have found.

Measuring the degree of land deformation, known as coseismic slip, caused by big quakes can help scientists better understand where strain has been released – which helps inform where quakes are not likely to occur again for some time.

Using data collected from a network of GPS stations called GeoNet, the Geospatial Information Authority of Japan determined that the land deformation occurred over an area that was 400km long and 200km wide due to a sudden slip at the spot where the Pacific tectonic plate slides underneath the Okhotsk tectonic plate, on which northern Japan sits.

The results were published in the journal Nature, which also featured an analysis of the data by Jean-Philippe Avouac, an earthquake expert from the California Institute of Technology in the U.S.

“Over the 15 years preceding the 2011 event, the GeoNet data had revealed the slow accumulation of strain across Honshu, with the Pacific plate squeezing and dragging down the eastern edge of Honshu,” Professor Avouac wrote.

Dr Paul Tregoning from the Australian National University’s Research School of Earth Sciences likened the pre-quake strain to two lumps of jelly stuck along an adjoining edge.

“If you keep pushing the lumps of jelly together from a distance, then you are building up strain and eventually it breaks. When it breaks, that’s the earthquake,” said Dr Tregoning, who was not involved in the study. The quake relieved some of the tension, he said.

“The likelihood of another really big quake happening in exactly this place is now very much decreased. It will happen sometime in the future but that might be 100, 500 or 1000 years time,” he said.

The 400km coseismic slip may seem compact compared to the 1200km land rupture caused by the Sumatran earthquake that triggered the Boxing Day tsunami, but the Japan quake was much deeper.

The Japan quake may have released some of the strain in that area but that has probably increased strain at other parts of the fault line – similar how to a car wind shield crack relieves pressure in one spot but increases it in another.

“So where is the next earthquake going to be? It’s where the last one wasn’t,” said Dr Gary Huftile, an earthquake expert from the Queensland University of Technology.

“[For] this one, the slip stopped at the south, at about Tokyo. Well, in 1941 you had an 8.1 earthquake south of there so it’s probably not going to go [there] yet now either,” he said.

“So then you look north of the rupture area, say east of Hokkaido. Maybe that’s where the next one will be. We don’t know.”

The strain may have migrated further along the fault line, possibly even beyond Japanese borders, he said.

“Now, I want to know where the next crack is going to be on this massive fault system, which goes from Alaska all the way to Taiwan.”

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