Tuesday, October 8, 2013
Water Between Plates Added To Strength Of '11 Quake
TOKYO (Nikkei)--The temblor that rocked eastern Japan in 2011 was so powerful because water that seeped in between tectonic plates in areas near the Earth's surface acted as a lubricant, according to researchers who drilled in the ocean floor.
The findings by researchers from the Japan Agency for Marine-Earth Science and Technology and other institutions are scheduled to be published Tuesday in the European journal Earth and Planetary Science Letters.
Major earthquakes that occur at sea, including the Great East Japan Earthquake of March 2011, are caused when stress building up between the continental crust and the oceanic crust that moves beneath it reaches a limit and this pent-up energy is released. Although it was widely believed that such events occur in deep areas where plates are locked tightly together, plates in shallow areas had slipped as well in the 2011 quake, resulting in severe shaking and huge tsunamis.
Researchers aboard the Chikyu deep-sea drilling vessel extracted rock samples from areas near the plate boundary some 850 meters below the ocean floor at a location 220km off the coast of Miyagi Prefecture. They found that clay minerals made up roughly 70% of the samples, compared to 40-50% for nearby sites. The high proportion of clay means that water is more easily trapped in the plate boundary.
When pressure is released in deep areas, friction between plates in shallow areas acts as a brake on the slipping between plates. Since a larger slipping force results in larger friction, the conventional wisdom was that earthquakes do not spread to shallow areas.
The researchers concluded that in the 2011 case, water that accumulated between the plates expanded due to frictional heat when the plates started to slip, reducing the friction between the plates and allowing them to slide more easily.
Hoping to cast light on how huge earthquakes occur, the researchers plan to analyze data from thermometers placed at the drilling area to gain a better understanding of friction at plate boundaries. Furthermore, the Chikyu will drill 5,200 meters under the ocean floor at the Nankai Trough next fiscal year. The researchers hope to use the latest findings to improve the accuracy of earthquake and tsunami forecasts.
(The Nikkei, Oct. 8 morning edition)