The depth of the rupture caused by earthquakes in certain sectors of the tectonic plate is directly related to the possibility of generating tsunamis. Consequently, more than the magnitude of the earthquake would affect its depth.
The greater magnitude or strength of earthquakes is not the main factor that marks the increase in the possibilities of generating tsunamis, as was thought until now. Now, scientists at the University of Hawaii at Mānoa have shown that the key point is the depth of the earthquake: if the rupture caused extends shallowly in the less rigid part of the tectonic plate, the resulting tsunami is greater than if the rupture it is deeper.
According to a press release, earthquakes of similar magnitude can cause tsunamis of widely varying sizes. This phenomenon has been identified by specialists for many years, but until today it has not been possible to understand the dynamics that determine it, thus hindering reliable local tsunami warnings and prevention measures. The new study by American scientists could shed light on this: it was recently published in the journal Nature Geoscience.
Initially, the researchers worked with a special class of events identified in previous studies, which are described as tsunami earthquakes. They produce tsunamis of disproportionate power, depending on the magnitude of the initial earthquake. This was the starting point to determine the crucial role played by the depth of the cracks produced by earthquakes.
Apparently, although magnitude is an important point, the depth where two tectonic plates slide past each other and the stiffness of the plates involved would have a direct bearing on the potential size of a subsequent tsunami. This new knowledge is essential to create warning instances and anticipatory measures that reduce the impact of these phenomena on human societies.
Scientists believe the new research provides a quantitative framework to investigate the influence of rupture depth and plate stiffness in explaining the observed variability between the magnitude of subsequent earthquakes and tsunamis. The results of the study highlight the importance of a rapid estimation of the surface slippage on the plates caused by the earthquake: these geophysical data would be vital to carry out a reliable tsunami warning at the regional level.
waves that amplify
Why, if the depth of the break is less, the tsunami will be more powerful? The researchers explained that shallow, concentrated rupture generates a relatively weak ground tremor. However, what is recorded by seismographs has a different impact on the oceans, as determined by specialists.
After a shallow rupture occurs, displaced water in the overlying deep ocean has enhanced the energy of the rupture and produces shorter tsunami waves. They amplify at a high rate as they move towards the coast, determining more extreme and dangerous events.
The experts concluded that the practice of using earthquake magnitude to estimate the potential threat of a tsunami has led to deficiencies in the ability to predict the impacts of these events. However, the new approach provides a simplified numerical model to isolate the core parameters of the earthquake and assess their importance, thus defining the size of the generated tsunami.
Tsunami size variability with rupture depth. Kwok Fai Cheung, Thorne Lay, Lin Sun, and Yoshiki Yamazaki. Nature Geoscience (2021). DOI:https://doi.org/10.1038/s41561-021-00869-z