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information about Fukushima published in English in Japanese media info publiée en anglais dans la presse japonaise

Ranking the dangers of active faults

August 16, 2016

Danger of active faults to be ranked



A Japanese government task force that fosters earthquake research is set to review the method for evaluating the dangers posed by active faults.

Experts have pointed out that the task force failed to properly represent risk in its long-term assessment of a fault line zone that may have caused a major earthquake in southwestern Japan in April.

The task force has published information on 97 major active faults across Japan, including the probability that each will trigger earthquakes.

The probabilities are generally small, because such quakes usually occur within a span of several thousand years.

Among them is the Futagawa fault line zone, which is believed to have triggered a major earthquake in Kumamoto, southwestern Japan in April.

According to the previous assessment, the probability of a temblor being triggered by part of the Futagawa zone within 30 years was between slightly above 0 percent and 0.9 percent.

Some have criticized such small numbers, saying they failed to represent danger and made people feel safe.

The task force drew up a new plan to rank the active faults on a scale of 4, according to how likely they are to cause earthquakes.
It will designate the rank of S, meaning that the probability is quite high, or greater than 3 percent, that a fault could trigger an earthquake within 30 years.
It will designate the rank of A for a range of 0.1 to 3 percent. And the rank of Z is for probability of less than 0.1 percent. The rank of X means that there is not a clear probability, but a jolt could be triggered in the near future.

Officials say when applying the new ranking system to 97 major fault zones, about one-third of them will be labeled S, or the rank of the greatest threat.

They are scheduled to convene a committee meeting on Friday to finalize the new system.





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