New Earthquake Warning Framework in the Nankai Trough Subduction Zone in Japan and Scientific Rationale that the Society Need to Know for Effective Countermeasures

Abstract:

Short-term earthquake forecasts usually entail large uncertainties and had never been implemented for operational use, with an exception of aftershock forecasts. This situation has recently changed as the Japanese government introduced a new framework of relatively short-term (one to two weeks) earthquake warning for anticipated major earthquakes along the Nankai Trough subduction zone in western Japan. The warning is called the “Nankai Trough Earthquake Extra Information”. The content of the warning is that the probability of having a great earthquake has become higher than normal (without indication of the probability) and the society should take appropriate measures.

The Nankai Trough Earthquake Extra Information is issued, as one case, when an earthquake of Mw larger or equal to 7.0 occurred in the subduction zone. The level of information is “advisory” when Mw was larger than or equal to 7.0 and smaller than 8.0 (hereafter called M7-class earthquake), and is “warning” when Mw was larger than or equal to 8.0 (hereafter called M8+ earthquake).

In order for the stakeholders in the society to take appropriate and effective countermeasures, they should be aware of the range of possible scenarios and a rough idea of their probabilities. Although the real probability cannot be known, it is useful to make clear and share with them what is expected statistically from the global earthquake catalogues.

We calculated the probabilities of a M7-class and a M8+ earthquake triggering a subsequent M8+ earthquake using the ISC-GEM ver 6.0 and the ANSS comprehensive earthquake catalogues. An earthquake located within 500km and 160km from a precedent earthquake of M8+ and M7-class, respectively, was counted as triggered. We considered different periods of 1, 3, 7, 14 days and 3 years. We also calculated the probability gains assuming the Poisson model or the Brownian Passage Time (BPT) model.

The results illuminate the rapid decay of earthquake probability in time, one-order smaller triggering capability for M7-class earthquakes, and the model dependency when calculating the probability gain. Communicating these results to the stakeholders in an easily comprehensive way helps them to foresee the range of possibilities and better prepare for the cases of warning information release.