Aftershocks continue to rattle northeast Japan
Reviewed by Hitoshi URABE
"Aftershocks continue to rattle northeast Japan"
After three large quakes hit the area on 26th, and while still uncountable number of medium to smaller tremor were being felt, another large quake shook the Miyagi area on the morning of 28th, indicating that it is still not over.
Japan is one of the world's most earthquake-prone countries as the islands sit on top, at the tips, of four tectonic plates that form continents and oceans. As such, Japan experiences various types of earthquakes as well. One type of quakes originates directly from the activities of the plates, due mainly to frictions and rebounds when oceanic plate tries to creep under continental plate, are often large in magnitude, i.e. high on Righter scale, and affect relatively large area, but the size of damage it causes vary depending on the distance and other factors.
The other type of earthquakes is caused by small faults when they release their stresses that have been formed originally by pressures from movements of different plates. This causes a smaller quake in terms of magnitude, which is a measure of the energy released, but as it occurs in the shallow region of the land, severe damage is done in a relatively small area. The Great Hanshin Earthquake that attacked Kobe and Osaka in 1995 and killed more than six thousand was this type, though several faults, induced by the first and successive quakes, were involved to make the total damage enormous.
There are two ways to describe the severity of an earthquake. One is to state the energy level, often referred to as "Richter scale", and another to indicate the actual size of tremor as it is felt at a certain point on land, called (Japanese) seismic intensity scale. As can be seen from the explanation above, the quakes deriving directly from tectonic plates tend to show larger Richter scale figures while seismic intensity level may be relatively smaller as it often originates far out in the ocean. (It does, however, increase the risk of Tsunami to attack the shores.)
The quake this time in Miyagi was in a way typical of the shallow-type earthquake, where the damage was severe but confined to a small area. But this one was in many ways unique. One is that strong level of tremors occurred several times over a long period of time. Although it is well known that small quakes commonly follow a major one, a succession of large tremors of such magnitude within intervals of a few hours to a couple of days was unprecedented, which misled the authorities in analyzing the situation and advising the residents properly, resulting in the increased level of tension and anxiety among people.
With due sympathy to the sufferers, one aspect which drew people's attention this time was the fact that despite there have been more than 500 casualties, no fatality was reported. Also, while more than 400 houses were destroyed, there was no damage to larger buildings. This indicates that though there could be no real solution to avoid earthquakes, measures based on experience, in terms of legal and administrative endeavors as well as voluntary preparations by people may be finally taking some effect.