AsianScientist (Feb. 11, 2013) – Researchers have discovered that the massive 2011 earthquake in Japan released nearly all of the stress that had built up along the plate boundary in that region.
Researchers had suspected that this release happened to some degree, based on observations that the sea floor moved nearly 50 meters during the earthquake.
To test the hypothesis the study authors, led by Weiren Lin from the Kochi Institute for Core Sample Research in Japan, analyzed fracture orientations they could see in “resistivity images” from three large boreholes drilled into the crust across the plate interface where the earthquake occurred.
The team obtained their material from an expedition last year on the drilling vessel Chikyu as part of JFAST, the Japan Trench Fast Drilling Project.
This project aimed to drill into the subduction fault that slipped in May 2011, generating a magnitude 9.0 scale earthquake, and the huge seafloor displacement that caused a tsunami wave that devastated the Sendai coast.
Dr. Virginia Toy, from the University of Otago’s Department of Geology and a member of the expedition, says the scientific aim of the project was to sample the fault materials and make measurements related to the frictional strength of the fault surface, to help understand why there was such a large displacement.
The study published in Science found that there was near total release of stress (i.e. pressure, or force, resulting from tectonic plate motions) in the crust in the wedge materials above the fault plane when the 2011 earthquake in Japan occurred.
“This is significant because most earthquake faults only release a small portion (typically 10 percent) of the stress in the crust around them, not nearly 100 percent as in this case,” she said. “Also, such a high proportion of stress was probably released because the fault materials were particularly frictionally weak or slippery.”
Toy, who represented the Australia-New Zealand International Ocean Drilling Consortium (ANZIC) on the project, adds that the results suggest that subduction zone faults in other locations, including around New Zealand, need to be more carefully examined.
“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,” Toy said. “It means that we should be prepared for other similar subduction zones to generate very large tsunami.”
The article can be found at: Lin W et al. (2013) Stress State in the Largest Displacement Area of the 2011 Tohoku-Oki Earthquake.
Source: University of Otago; Photo: JAMSTEC.
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