The Yomiuri Shimbun
In the month since the March 11 earthquake, there have been frequent aftershocks at and around the initial quake's focus, including many strong quakes with a magnitude of 5 or higher. Experts say there are more shocks to come, possibly as strong as magnitude 8.
The phenomena can be attributed to the enormous changes in the force exerted in the Earth's surface in the eastern part of the nation due to the Great East Japan Earthquake, which measured magnitude-9.0 on the open-ended Richter scale, according to seismologists.
On Tuesday, a magnitude-5.6 earthquake occurred in northern Nagano Prefecture, while on the same day a magnitude-6.4 earthquake struck offshore from eastern Chiba Prefecture. On Monday, a magnitude-7.0 earthquake took place in eastern Fukushima Prefecture.
The Metrological Agency warned, "Earthquakes have come to occur frequently as the geological dynamics of eastern Japan have changed due to the March 11 earthquake."
"We have to expect more earthquakes in the magnitude-7.0 class for the time being," it warned.
On March 12, northern Nagano was jolted by a magnitude-6.7 earthquake whose focus was 20 kilometers north of that of the Tuesday earthquake.
Among seismologists, the area was already known for ground distortion in many places. But the March 11 earthquake increased the pressure the ground there is subjected to. Earthquakes measuring 1 or higher on the Japanese seismic intensity scale of 7 have occurred at least 430 times in the region since March 12.
The apparent southern boundary of the aftershock zone from the March 11 earthquake is off the shore of eastern Chiba Prefecture. In that area, the Pacific plate and the Philippine Sea plate slide under the landward plate that includes eastern Japan. Tuesday's earthquake off Chiba occurred because faults in the landward plate became active.
Movements of the Philippine Sea plate affect seismic activity in Tokyo Bay. But only a few earthquakes at the magnitude-3 or magnitude-4 levels have been recorded there since March 11, according to the agency. The level of seismic activity has not changed much.
However, Prof. Koshun Yamaoka of Nagoya University pointed out that seismic activity in northern Chiba and southern Ibaraki Prefectures, which is also linked to the Philippine Sea plate, has increased.
"An earthquake could hit the Tokyo metropolitan area with its focus directly below the city. We'll have to observe the situation carefully," Yamaoka added.
The professor anticipates the rise in seismic activity in eastern Japan will continue for four or five years. He said magnitude-7 quakes "are likely to occur once or twice in the next month."
"After that, they can be expected to happen once in two months. The occurrence of such huge earthquakes will gradually decrease to once in a half year. But the possibility remains that a magnitude-8 earthquake could happen at any time," Yamaoka warned.
Yoshimitsu Okada, president of the National Research Institute for Earth Science and Disaster Prevention, said, "Aftershocks of the March 11 earthquake occurred in shallow places in the Earth's crust and triggered other aftershocks."
"The frequency of earthquakes has increased by aftershocks caused by aftershocks of the March 11 earthquake," Okada added.
(Apr. 14, 2011)
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