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Aftershock Outlook

The Japan Meteorology Agency announces aftershock outlooks including the following information.

In addition, probability of large aftershocks which may occur within the period of 3 days will be posted as a reference.

Q. How does JMA calculate “Probability of aftershocks”?

There are two kinds of characteristics in aftershocks. We focus on these characteristics, and calculate the probability of aftershocks, and make an aftershock outlook.

One of the above two is that the number of the aftershock is the most right after the main shock but gradually decreases over time.

Number of Aftershocks vs Elapsed Days

The other nature is that earthquakes with larger (smaller) magnitudes occur less (more) frequently.

Number of Aftershocks vs Magnitude

Considering the above two characteristics, we may well describe aftershock generation process as follows. Immediately after the mainshock, aftershocks occur frequently including intense aftershocks; however, their frequency will be lowered over time, and will be settled eventually. The above two natures will be expressed by the statistical equations well-known in seismology, the combination of which enables the forecast of aftershock probabilities. In this way, we are able to calculate the probability of aftershocks that are beyond a certain size in scale in the form of percentage such as “the probability of aftershocks exceeding magnitude-5 within the next 3 days is 30 percent”. “Probability of aftershocks” is calculated in this way.

Q. Under what circumstances will JMA issue the “Probability of aftershocks”?

It is issued when there is a fear that aftershocks may aggravate the damages brought by the main shock. Concretely speaking, the followings are the cases.

The first information is generally issued about a day after the occurrence of the earthquake, after the type of the earthquake is identified as “main-shock type”.

Q. How often is "Probability of aftershocks” updated?

As for inland-earthquakes, it will be announced in every three days for the first seven days, and will be weekly thereafter. As for marine- earthquakes, it will be announced in every three days for the first ten days, and from thereafter it be weekly updates likewise.

Note that aftershock generation process differs between inland and undersea earthquake. As for inlands, the largest aftershocks occur approximately within 3 days from the main shock, and within 10 days as for the undersea which is a little longer.

Q. How should we interpret that event probability of aftershocks with magnitude of 6 or greater within 3 days is 30%?

The probability of getting each number, for instance “1”, on a dice is about 16% (1/6). The probability of 30% that aftershocks with magnitude of 6 or greater occur within 3 days is, therefore, higher than that of getting “1”. Concretely speaking, when JMA issues outlook that probability of aftershocks with magnitude of 6 or greater within 3 days is 30%, such aftershocks actually occur within 3 days 3 times out of 10.

Q. Until when should we be cautious about aftershocks?

Overall, the number of aftershocks gradually decreases over time, sometimes accompanying large aftershocks. In the case of earthquakes causing some damages, special attentions to aftershocks will be needed for the first seven to ten days (for about a month in the case of aftershocks being active). Generally, it takes a couple of years until activities of aftershocks completely settle.