The Yomiuri Shimbun
In addition to its own continuing aftershocks, the Great East Japan Earthquake has set off many other earthquakes in locations around the country.
Many experts say the unimaginably gigantic main shock of the quake on March 11 dramatically changed the crustal situation of the Japanese archipelago.
The government’s Earthquake Research Committee warns that a major earthquake could strike Japan at any time and in any place. Moreover, this state of heightened risk may last for years, the panel says.
Measures to minimize the possible damage from future quakes must be reexamined not only in the quake-hit areas where aftershocks are continuing but in other areas, too.
In just over a month since the March 11 quake, the number of aftershocks measuring magnitude 5.0 or greater topped 400. There were even five quakes recording magnitude 7.0 or greater. Normally, magnitude-5.0 or larger quakes occur about 150 times a year on average in this country.
Another huge tsunami possible
The reason for the current frequent occurrence of earthquakes is that the main shock of the great earthquake caused the Japanese archipelago to move as much as five meters to the east. Before March 11, the archipelago was being pushed westward from the Pacific side as the Pacific plate was plunging underneath Japan, causing strain to build up.
This huge strain was released in a burst when the main shock hit the country, causing faults across the nation, movement of which had been suppressed until then, to move more easily.
Especially worrisome now is the possibility of another huge tsunami. The crustal situation near the main shock’s focal zone has not stabilized yet. Therefore, the possibility of destructive crustal movements causing additional earthquakes is very high.
Quakes of this type, even if they are small in scale, are likely to generate big tsunamis, several seismic experts warned.
In the disaster-hit areas, measures to prevent tsunami damage are now weaker than they were before the great quake, for reasons including the destruction of protective embankments by the March 11 tsunami.
In Iwate, Miyagi and Fukushima prefectures, ground subsidence has been observed in a wide range of areas along the coastline. There are many areas on high ground where tremors have dangerously loosened the soil.
The number of people working near coasts has increased due to reconstruction activities. Some evacuees have left shelters to restart their lives at their homes. In this situation, they must secure evacuation routes for emergencies.
Entire nation must be ready
Even outside the disaster-hit areas, relatively strong earthquakes have been observed in 16 areas including the Tochigi-Gunma prefectural boundary and Tokyo Bay. The Meteorological Agency and other organizations have been on a special alert in monitoring the situation. Concern is mounting about an earthquake centered directly under the Tokyo metropolitan area. If such an earthquake occurs, it is predicted that about 13,000 deaths would result.
When the main shock of the great earthquake hit on March 11, a huge number of people in the Tokyo area faced difficulty getting home, for instance. Various weak points in the area’s disaster preparedness were revealed. Measures to cope with major disasters need to be formulated.
Western Japan is not free from crustal changes. According to the agency, seismic activity at volcanoes from Hokkaido to Kyushu has increased.
Japan is frequently hit by disasters such as earthquakes, volcanic eruptions and typhoons. Without being afraid of them, we should thoroughly prepare to strengthen the nation against future disasters.
(From The Yomiuri Shimbun, April 21, 2011)