Schematic representation of the study area. Credit: Credit: JAMSTEC / IODP
Reading aloud Japan on March 11, 2011: The earth is starting to shake - basically nothing unusual in the earthquake-stricken island kingdom. But what now developed, broke all standards: the so-called Tōhoku earthquake reached a magnitude of nine on the magnitude scale, making it the strongest earthquake ever measured in Japan. The shocks brought countless buildings to collapse - but not only that: then a tsunami rolled over the rubble landscape. The consequences: Over 16, 000 deaths and the nuclear disaster in Fukushima. So far, it was unclear why the quake could reach such an extent. Drilling near the epicenter of the earthquake off the coast of Japan has now been shown: an unusually thin and slippery layer allowed for the enormous slippage of the two continental plates colliding here.

A so-called subduction zone is responsible for the earthquakes in Japan: Here, the Pacific continental plate moves at a speed of about ten centimeters per year under the Philippines and forms the Japangraben. It is not a uniform conduct, but it builds up tensions that dissolve in the form of earthquakes. The plates jerk forward as it were. The size of a particular jerk depends on the built-up tension and the material that counteracts the movement. In the case of the quake of 2011, the fault was huge: the ground moved 30 to 50 meters. This caused the devastating wave that broke over the shores of the earth over the coast of Japan.

To find out which geological features were responsible for the extent of the fault, an international research team has launched the "Japan Trench Fast Drilling Project". The geologists drilled three holes from a drillship into the Japangra valley to study the fracture zone material from the 2011 earthquake. The researchers have now published the results in the science magazine "Science". display

A slippery layer caused the continental plates to slide heavily

They found that the boundary layer of the two continental plates in the area of ​​the epicenter is extremely thin: only about five meters. "To our knowledge, this is the thinnest plate boundary on earth, " says Christie Rowe of McGill University in Montreal. For example, the corresponding layer in the case of the California San Andreas Fault several kilometers thick. The scientists also discovered evidence that there was only relatively little friction during the shift as part of the earthquake: Once in motion, it slid heavily. This was the result of analyzes that allowed conclusions to be drawn about the heat generated during friction during the shift.

The reason for this was found in the analysis of the material of the thin separating layer: It consists of an extremely fine sediment. "It's the slipperiest sound imaginable, " says Rowe. If you rub it between your fingers, it feels like a lubricant. Therefore, during the earthquake movement there was very little frictional resistance and all the stress could discharge in one fell swoop. Robert Harris, one of the participating researchers from Oregon State University, compared the effect of gliding cross-country skis on snow: "At rest, the skis stick a bit to the snow and it takes a certain amount of power to get them moving. But then heat is generated and the subsequent sliding movement results in less resistance, "explains the geologist.

According to the researchers, there could be similar geological features at other plate boundaries as in the Japangraben and thus a high risk for strong quakes or tsunamis. "Finding such a slippery tone could give us an opportunity to predict where the cataclysms are, " says Casey Moore, one of the scientists involved at the University of California at Santa Cruz.

Original papers by the researchers:

  • Science, edition of 5.12.2013
© science.de - Martin Vieweg
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