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Where do the really large earthquakes occur? This is one of the biggest questions not only for earth scientists, but also for the nearly 4 billion people who live near active fault systems - potential sites for major earthquakes. Earth scientist Dr Ylona van Dinther of Utrecht University and colleagues from Italian and Swiss universities are bringing us a step closer to answering this question with new models they have developed. These models forecast how different characteristics influence the size of earthquakes and thus the seismic risk.

Scientists have already suggested that the location of giant earthquakes, those with a magnitude of more than 8.5 Mw, is partly determined by the sediment thickness on the incoming oceanic plate. This heaviest plate slides under a lighter continental plate into the earth's mantle, generating megathrust earthquakes at the interface between the two plates. Scientists made this suggestion based on observations from the past 50 to 100 years. However, observations cannot be used to determine whether this is really the case and how it will affect the seismic hazard now and in the centuries to come.

Thickness of sediment layer increases earthquake size

Van Dinther and her colleagues were able to examine this idea using their new models, which simulate both long and short time scales. The results show that the strongest earthquakes indeed occur in places where the incoming sediment layer is thickest. In regions with almost no sediment on the oceanic plate, the maximum magnitude in the models is as much as one step lower on the logarithmic magnitude scale than in regions where sediment is deposited over a depth of 8 km, accumulating to form a thick layer. This could be the difference between an earthquake that houses and tsunami walls are designed to withstand, and one that causes as much unexpected devastation as the 9 Mw earthquake in Japan in 2011. Van Dinther and her colleagues have concluded that this difference in size is due to the fact that the weight of the accumulating sediments bends the subducting plate upwards, making the sliding surface between the two plates flatter and increasing the area over which earthquakes can develop.

Above: Model results of how the thickness of the sediments on the oceanic plate (Tsed) affects the maximum size of an earthquake (Mmax) and the time between all earthquakes and major earthquakes. For comparison, the left hand side also shows the maximum observed values in nature for each subduction segment. Below: Diagram summarising the earthquake and tectonic characteristics of both situations.

Only small quakes for the time being

If we then look at the largest earthquakes observed for each segment in a subduction zone, we see that relatively small earthquakes occur in the three segments with thick sediments (Calabria, Aegean and Makran). However, we would also expect large earthquakes in these segments based on the new models. The models can explain this difference as well, because larger earthquakes are also less common in such areas. As a result, there may indeed be a long stretch of time in which only smaller earthquakes occur, though bigger earthquakes are still possible, too.

Large tsunamis in areas with thick sediment layer

Van Dinther's models additionally reveal more earthquakes in the sediments of the upper continental plate (see the figure above). This type of earthquake can also trigger larger tsunamis. The larger earthquakes that could occur in these sediment-rich areas in the future may therefore be accompanied by large tsunamis.

Publication

Silvia Brizzi, Iris van Zelst, Francesca Funiciello, Fabio Corbi, Ylona van Dinther*

Journal of Geophysical Research, 2020

* = from Utrecht University