Here is an assessment of Japanese seismologists concerning why the frequent aftershocks since the mega earthquake occurred on 11 March and until when these will end.
First, we have to define some terms to avoid confusion.
The Japan Times said: The earthquake-prone country uses two scale systems to measure the strength of a tremor—the intensity scale and the magnitude scale. What's the difference?
Magnitude is the energy of a temblor. On the other hand, the degree of shaking or shindo is the degree of shaking in a specified place.
According to scientists, magnitude—which is measured on the Richter scale—is open-ended, in a report by the Yomiuri Shimbun. Meaning to say, the measurement is infinite depending on the tremor's energy.
In contrast, the Japanese seismic intensity scale has a maximum of 7. The intensity does not depend on the magnitude. It depends on the distance and depth from the epicenter.
The report also said that shindo cannot be determined from one quake if the interval between two temblors is short. Click here to see the Japanese Intensity Levels.
Going back to the 11 March magnitude 9.0 quake, the tremor recorded different shindo in various locations.
As an example, the magnitude 9.0 quake registered a higher shindo in Miyagi Prefecture (7) than in Tokyo (5) because the former is closer to the epicenter, which was approximately 72 kilometers (45 miles) east of Tōhoku at a depth of approximately 32 kilometers (19.9 miles).
Whew. Are you still with me? Let's move on.
The Meteorological Agency had recorded about 408 aftershocks with a magnitude of 5.0 or higher as of 15:00 Tuesday. Of these, 68 quakes had magnitude 6.0 and above, while five tremors registered at the 7.0 or higher.
On the other hand, there had been at least 430 tremors with shindo of 1 or more since 12 March.
My friends in Japan have been living on shaky grounds, literally. Come day or night, earthquakes are felt. But it is amazing that the subsequent shocks did not create any further significant structural damages. The Japanese technology must be doing its job. If the same happened in my country, I think I won’t be here to report it to you.
The Agency warned that due to the changes in the geological dynamics of eastern part of the country following the powerful quake, aftershocks have occurred frequently.
"We have to expect more earthquakes in the magnitude 7.0 class for the time being."
Many more earthquakes may be felt on the east coast of Japan.
There is a 10 percent chance that a magnitude of 7.0 or higher will happen within three days of 15:00 Tuesday. Another 10 percent might happen within three days of 15:00 Friday.
Yoshiro Ota of the Meteorological Agency said, "It is possible to have a shindo 7 if the epicenter of a magnitude 7 aftershock is on land and its focus is shallow."
Nagoya University's Prof. Koshun Yamaoka said, "An earthquake could hit the Tokyo metropolitan area with its focus directly below the city. We'll have to observe the situation carefully."
Yamaoka foresees the increased seismic activity in eastern Japan to continue within the next four or five years with the frequency continuously decreasing in the subsequent years.
Details of this report here.