How giant earthquakes can form at fault planes where theory says they should not

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A research group led by Satoshi Ide from the University of Tokyo has demonstrated that classic earthquake generation theory does not hold in areas where the angle at which a tectonic plate dips under another is sufficiently low. The discovery explains why giant earthquakes can form in such areas, providing a theoretical basis to extend observation efforts to previously overlooked features. The findings are published in Science Advances.

Earthquakes are Earth's stress relief mechanism. The friction between two tectonic plates moving past each other builds stress. Once it becomes too much, the energy is released and dissipates as seismic waves, which cause havoc in their wake. To mitigate the destruction, scientists work tirelessly to refine their forecasts of where and when especially dangerous giant earthquakes might occur.

"Giant earthquakes have often been observed near low-angle faults," said Ide, the first author of the paper. "But according to classic theory, the conditions are actually not optimal for giant earthquakes to form in these areas, leaving their formation unexplained."

The researchers used a global catalog of earthquakes spanning 1976–2024 to search for possible clues. They were particularly interested in the earthquake growth probability, (b-value). This value represents the ratio of small to large earthquakes in a given area. Consequently, a small b-value means a higher probability of large earthquakes forming. The statistical analysis of the catalog revealed that fault planes, areas where tectonic plates meet, with low dip angles were much more likely to have a low b-value.

"Given that these catalogs are standard rather than exceptional," Ide said, "it is surprising that such a clear relationship between earthquake growth probability (b-value) and fault dip angle has previously been overlooked."

This, however, contradicted the received view called Andersonian fault theory. According to this theory, there is not enough stress built up at low dip angles for giant earthquakes to form. To resolve this contradiction, the researchers turned their attention to an additional feature: the orientation of the stress field. Calculations with optimal orientations showed that enough stress could indeed be generated for the formation of giant earthquakes. This demonstrated that Andersonian theory does not hold at sufficiently low dip angles.

Ide reflected on the inf