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 were published in the journal Science Advances.
Earthquakes are the Earth's stress relief mechanism. The friction between two tectonic plates moving past each other builds up stress. Once it becomes too much, the energy overflows 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," says 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 catalogue of earthquakes spanning from 1976 to 2024 to search for possible clues. They were particularly interested in the earthquake growth probability or 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 catalogue 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 explains, "it is surprising that such a clear relationship between earthquake growth probability (b-value) and fault dip angle has previously been overlooked."
This, however, was in contradiction with 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 reflects on the influence of their findings.
"Our findings underscore the critical importance of tectonic stress orientation. Monitoring stress orientation in potential future earthquake regions will significantly improve the probabilistic forecasting of giant earthquakes."
