Asian Scientist (Dec. 11, 2013) – An international team of scientists has shown that the Tohoku fault is more slippery than anyone expected and this may explain why the March 2011 Tohoku-Oki earthquake triggered a devastating tsunami that led to the Fukushima nuclear disaster.
The scientists made the discovery after measuring the frictional heat produced by the fault slip during an earthquake for the first time. Their results, published in Science, show that friction on the fault was remarkably low during the magnitude 9.0 Tohoku-Oki earthquake that struck off the coast of Japan.
Because friction generates heat (like rubbing your hands together), taking the temperature of a fault after an earthquake can provide a measure of the fault’s frictional resistance to slip. But that hasn’t been easy to do.
“It’s been difficult to get this measurement because the signal is weak and it dissipates over time, so we needed a big earthquake and a rapid response,” said Professor Emily Brodsky, a geophysicist and co-author of three papers on the Tohoku-Oki earthquake published together in Science.
The researchers from the Japan Trench Fast Drilling Project (JFAST) expedition, comprising 27 scientists from 10 countries, drilled across the Tohoku fault in 2012 and installed a temperature observatory in one of three boreholes nearly 7 kilometers below the ocean surface.
The logistically and technically challenging operation successfully recovered temperature measurements and other data as well as core samples from across the fault.
According to the researchers, the low resistance to slip on the fault may help explain the large amount of slip-an unprecedented 50 meters of displacement-that occurred during the earthquake.
Their study of the core samples also revealed that a thin layer of weak, slippery clay material in the fault zone may account for the low friction during the earthquake.
“What the core samples show, for the first time, is that the fault, particularly near the seafloor, is composed of less than five meters of very fine volcanic sediment, highly altered to a special type of clay (smectite), which acted as an incredibly slippery lubricant and allowed the huge quake to occur,” said Dr Virginia Toy, a member of the research team.
“The extreme frictional weakness of this material facilitated the huge vertical and horizontal displacements of the seafloor during the magnitude 9 quake. It was the water displaced by this massive movement of rock that generated the much larger than anticipated tsunami waves which devastated Fukushima on the east coast of Japan.”
The discovery that the unexpected behavior of the Tohoku fault is attributable to the presence of high concentrations of this particular type of clay may help scientists to work out if other major faults around the Pacific Rim could generate similarly large tsunami due to very large slippage during future earthquakes.
The articles can be found at:
Fulton PM et al. (2013) Low Coseismic Friction On The Tohoku-Oki Fault Determined From Temperature Measurements.
Ujiie K et al. (2013) Low Coseismic Shear Stress On The Tohoku-Oki Megathrust Determined from Laboratory Experiments.
Chester FM et al. (2013) Structure And Composition Of The Plate-Boundary Slip Zone For The 2011 Tohoku-Oki Earthquake.
Source: University of California, Santa Cruz; Photo: Official U.S. Navy Imagery/Flickr/CC.
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