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Why so many aftershocks? Why so large?

by Minoru Matsutani

Staff Writer

A record number of powerful aftershocks have continued to jolt the already battered prefectures of Miyagi, Fukushima and Ibaraki after the 9.0-magnitude March 11 earthquake.

The aftershocks have halted reconstruction efforts in towns wiped out by the quake-triggered tsunami, as well as efforts to stabilize the crippled Fukushima No. 1 nuclear plant.

Why do we have so many aftershocks? Why are they so large?

And how long will they continue? Below are questions and answers on aftershocks.

Why do aftershocks occur?

Because tectonic plates and faults try to stabilize ground conditions that have been greatly altered by a huge earthquake.

Why do we have so many big aftershocks?

The magnitude of the largest aftershocks tend to be about 1.0 magnitude lower than the main quake, which was 9.0, said Yoshiro Ota of the Meteorological Agency.

“People tend to think aftershocks are small. But a magnitude 8 is huge. Magnitude 7 is also huge, so we should be very careful,” Ota said.

How many aftershocks have there been? How big have they been?

As of 5 p.m. Tuesday, there have been 408 aftershocks with a magnitude of 5.0 and above, 68 registering 6.0 and higher and five at the 7.0 level or higher, according to the Meteorological Agency, which does not count aftershocks smaller than those with a magnitude of 5.0.

“On average, we have about 10 quakes a year with magnitude of 7.0 or higher, and we already had six in the past month. One of them was the gigantic 9.0,” Ota said.

The number of aftershocks following the March 11 quake have been by far the most recorded in Japan. The magnitude 8.2 quake off eastern Hokkaido in 1994 had 117 aftershocks with a magnitude of 5.0 above, making it the quake with the second-highest number of aftershocks on record in the country.

The number of aftershocks is high because such a large part of the tectonic plate was affected.

Will we have any more huge aftershocks soon?

Maybe. The three largest aftershocks occurred within an hour of the main quake, as is the case with most earthquakes.

But that does not mean the worst is over. April 7 and 11 saw aftershocks with magnitudes of 7.1 and 7.0 respectively.

And the Meteorological Agency warned that many more large ones may be in store for eastern Japan.

The agency said the likelihood of an aftershock of magnitude 7.0 or higher is 10 percent within three days of 3 p.m. Tuesday, and also the same percentage within three days of 3 p.m. Friday.

On the Japanese seismic intensity scale to 7, the largest April aftershocks were an upper 6. The main quake topped the Japanese scale at 7 in Miyagi Prefecture on March 11.

“It is possible to have a ‘shindo’ 7 if the epicenter of a magnitude 7 aftershock is on land and its focus is shallow,” Ota said.

What is shindo? How is it different from magnitude?

Shindo, which literally means tremble degree, describes the degree of shaking in a particular place, while magnitude is the energy of an earthquake.

Therefore, the March 11 quake recorded different shindo in different locations.

For example, central Tokyo generally felt a shindo 5 after the magnitude 9.0 quake, while Kurikoma, Miyagi Prefecture, experienced a shindo 7 in the same quake. Shindo does not only depend on magnitude, but also the distance from the epicenter and the depth of the quake.

Also, if an interval of two quakes is short, shindo cannot describe the degree of shaking from one single quake.

How much longer will aftershocks last?

Who knows? “The Great East Japan Earthquake was so huge that we cannot make reasonable predictions from past quakes,” Ota said. Some experts say they could last at least a year.

Will other parts of Japan experience aftershocks?

Maybe. The aftershocks registering an upper 5 shindo or higher have so far occurred in three places that are not normally considered part of the likely aftershock area: northern Nagano Prefecture, eastern Shizuoka Prefecture and inland Akita Prefecture. The main quake may have caused stress in fault lines outside the coastal areas of northeast Japan, Ota said.