Japan quake released immense stress – study

Samples of rock drilled from depths of the Pacific seafloor have revealed the tremendous release of tectonic pressure that occurred during the 2011 Tohoku-Oki Earthquake. The findings could have important implications for the understanding of fault boundaries and tsunami risk near New Zealand, says a Kiwi geologist involved the research.

The new study, published today in the journal Science, found nearly all the stress built up at the plate boundary near the epicenter was released in the tsunami-generating magnitude 9.0 quake.

Researchers on board the scientific drilling vessel Chikyu drilled boreholes into the fault zone, 850 meters below the sea floor and almost seven kilometers underwater (video of scientists aboard the Chikyu identifying the fault zone in drill core samples can be seen to the right). The stress rock samples were under at various parts of the earth’s crust was measured using electrical currents to detect the extent of fracturing.

The results of the study indicated a nearly complete drop in stress following the earthquake. Such a large release is unusual, according to University of Otago geologist Dr Virginia Toy, who was aboard the Chikyu representing an Australia-New Zealand International Ocean Drilling Consortium (IODP) consortium.

“This is significant because most earthquake faults only release a small portion (typically 10%) of the stress in the crust around them, not nearly 100% as in this case,” she said in a media release from the University of Otago.

“Also, such a high proportion of stress was probably released because the fault materials were particularly frictionally weak or slippery.”

She adds that the results suggest that subduction zone faults in other locations, including around New Zealand, need to be more carefully examined. New Zealand has “so much of its coastline exposed to the Pacific Ocean, which is ringed by subduction zones, for example in Tonga-Kermadec, Hikurangi and Chile.”

“If the materials in the fault planes are similar to those in the Japan Trench, it is likely they will also be very frictionally weak and therefore that we can also expect very large seafloor displacements when they slip,”

“It means that we should be prepared for other similar subduction zones to generate very large tsunami.”

The study has been widely covered in the media. Examples include:

Radio New Zealand: Scientists discover why Japan quake so big
NBC Science: Japan earthquake unleashed surprising torrent of energy
Popular Mechanics: How the Tsunami-Causing Quake Transformed Japan’s Fault
PhysOrg: New report illuminates stress change during the 2011 Tohoku-Oki earthquake
LiveScience: Japan Earthquake Unleashed Surprising Torrent of Energy