Much increased quake activity after 3/11

March 10, 2015

Earthquakes in some areas of Japan around 100 times as frequent as before 3/11

http://mainichi.jp/english/english/newsselect/news/20150310p2a00m0na015000c.html

Earthquakes in some parts of the country are around 100 times as frequent as before the Great East Japan Earthquake, according to a researcher's analysis.

Professor of earthquakes and geology Shinji Toda at Tohoku University's International Research Institute of Disaster Science says, "Even four years after the Great East Japan Earthquake, there are areas where the effects are lingering. We need to be yet more vigilant against a major earthquake caused by the increase in activity."

Toda looked at the rates of earthquakes of magnitude 1 or over occurring within 20 kilometers of the surface between March 11, 2013 and Feb. 18 this year, and compared them to the rates during the 10 years before the Great East Japan Earthquake. He left the two years after the Great East Japan Earthquake, when there were many aftershocks, out of the analysis.

The areas found to have earthquake rates some 100 times what they were before the disaster fall largely along a region including the Hamadori area of Fukushima Prefecture, Kuji, Iwate Prefecture, and Choshi, Chiba Prefecture. Each of these areas had comparatively few quakes before the disaster, but the effects of crust changes from the Great East Japan Earthquake appear to have left lasting effects in them. The results showed that there are areas of high activity further inland than the aftershock region for the Great East Japan Earthquake, which extends from off the coast of Aomori Prefecture to off the coast of Chiba Prefecture.

For the Tokyo metropolitan area, where there are fears of a massive earthquake striking, Toda expanded the scope of his analysis to quakes within 100 kilometers of the surface. He found that the rate of quakes over magnitude 3 for the past two years was around twice what it was for the 10 years before the 2011 disaster.

Areas including the Chuetsu region of Niigata Prefecture had a lower incidence of quakes than before the Great East Japan Earthquake, but this is believed to be because of a decrease in aftershocks from an inland earthquake that occurred there before the 2011 disaster.

Meanwhile, on March 9, the Japan Meteorological Agency announced its recent records for aftershocks from the Great East Japan Earthquake. Over the one-year period from March 11 last year to March 7 this year, there were 737 earthquakes registering 1 on the Japanese seismic scale of 7 or higher and detectable by humans in the aftershock region. The number was well above the average yearly number of 306 for that region in the 10 years before the Great East Japan Earthquake.

"The area remains active," the agency said.