The 2016 Amatrice earthquake was used in the study.  Wikimedia Commons/Leggi il Firenzepost (CC BY 3.0)

When is a major earthquake a foreshock or an aftershock?

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Swiss researchers have come up with a system to help determine whether a magnitude 6+ earthquake is the main event, or simply a foreshock of a larger event to come. From the seismic activity of an aftershock sequence, you can derive a number called a 'b value'. If you only get smaller earthquakes after the first big quake, the b value increases over time. But if the b value decreases, they say it's likely to mean a bigger quake is coming - based on data from two earthquakes (one in Italy and one in Japan) where this occurred. When applied to historical earthquake sequences, their system was able to predict if quake was a foreshock or aftershock with 95 per cent accuracy.

Journal/conference: Nature

DOI: 10.1038/s41586-019-1606-4

Organisation/s: ETH Zurich, Switzerland

Media Release

From: Springer Nature

A simple traffic light classification system that can be used to indicate the probability of a subsequent larger event following an earthquake is reported this week in Nature. The approach may help with managing responses after an earthquake.

Large earthquakes are often followed by aftershocks that are difficult to predict and can sometimes be bigger than the original event. Currently, it is difficult to determine whether an earthquake was a foreshock of a larger event to come or was the main seismic event.

Laura Gulia and Stefan Wiemer analysed the average size distribution of aftershocks, termed the ‘bvalue’, following the 2016 Amatrice–Norcia earthquake sequence in Italy and the 2016 Kumamoto earthquake sequence in Japan. In most aftershock sequences following an earthquake of magnitude 6 or more, the b value will increase, meaning that small earthquakes become more common relative to larger events. However, the authors identified that in the Italian and Japanese earthquakes, the b value decreased after the initial quake; in both cases an earthquake of larger magnitude followed, revealing that the first was a foreshock. They suggest here that the b value could act as an indicator that a larger earthquake will follow the initial earthquake. They propose a traffic light system to classify the threat of aftershocks in real time, based on the changes in the b value. They applied this system to 58 historical earthquake sequences that were greater than 6 in magnitude, and were able to predict whether a specific event was an aftershock or the main quake with an accuracy of 95%.

The authors highlight that more research and investment in seismic monitoring worldwide will be needed to understand the full implications of the study.


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