Immediately after a large earthquake, the main question asked by the public and decision-makers is whether it was the mainshock or a foreshock to an even stronger event yet to come. So far, scientists can only offer empirical evidence from statistical compilations of past sequences, arguing that normally the aftershock sequence will decay gradually whereas the occurrence of a forthcoming larger event has a probability of a few per cent. Here we analyse the average size distribution of aftershocks of the recent Amatrice–Norcia and Kumamoto earthquake sequences, and we suggest that in many cases it may be possible to discriminate whether an ongoing sequence represents a decaying aftershock sequence or foreshocks to an upcoming large event. We propose a simple traffic light classification to assess in real time the level of concern about a subsequent larger event and test it against 58 sequences, achieving a classification accuracy of 95 per cent.
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Data and code availability
The datasets generated and analysed during the current study, as well as the Matlab codes written for the analysis, are available at https://doi.org/10.3929/ethz-b-000357449.
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The figures were produced with the Generic Mapping Tool (http://gmt.soest.hawaii.edu). b-value maps were created with ZMAP (http://www.seismo.ethz.ch/en/research-and-teaching/products-software/software/ZMAP/).
The authors declare no competing interests.
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