Japanese Scientists Warn of a Major Earthquake Off Northeastern Japan Coast
February 1, 2012 • 9:58AM

Japanese researchers say that the 2011 Tohoku earthquake, which was followed by a killer tsunami, has increased the risk of a major quake and tsunami east of the Japan Trench off northeastern Japan. The research was carried out by a group at the Japan Agency for Marine-Earth Science and Technology.

The Japan Trench is where the Pacific Plate begins to sink under the tectonic plate extending from land. Before the 2011 earthquake, normal-faulting earthquakes beneath the outer trench slope occurred only at depths shallower than 20 km, whereas those at depths of around 40 km had reverse-faulting mechanisms, scientists noted. These observations suggest that the stress regime in the Pacific plate was changed by the 2011 earthquake. The tensional stresses that now extend to depths of about 40 km may play an important role in the occurrence of large normal-faulting earthquakes.

The data cited show that the March quake apparently changed the dynamic force deep inside the Pacific Plate. Before the disaster, many of the deeper quakes involved faults that form when the plate is being compressed. But the research data showed that many of the post-March aftershocks involved a fault that forms when the plate is pulled apart.

This type of force is known to have caused a magnitude-8 earthquake about 80 years ago off northeastern Japan. The March 11th earthquake originated closer to land from the plate boundary.

The research group installed 20 seismometers on the seabed east of the Japan Trench to analyze aftershocks that occurred between late April and early July last year.