Most powerful earthquakes are called megathrust earthquakes. These happen at subduction zones where one tectonic plate is forced under another. If this happens suddenly, you get a massive earthquake. We understand these subduction zones poorly.
An unusual feature of megathrust quakes is that they are often followed by a series of other very powerful megathrust quakes several years later and with epicentres hundreds of kilometres away. Professor Anne Socquet, based at Universite Grenoble Alpes in France, wanted to look at these sequences of megathrust quakes and investigate the potential links between those great quakes.
Apparently, we can get a unique window on subduction zones as they were millions of years ago. There are a few places where rocks that have been through subduction zones are forced up to the surface. By analyzing these rocks we can determine the depths and pressures at which the subduction happened and build up a picture of how subduction works and potentially how megathrust earthquakes are triggered.
In these rocks are minerals with what’s called an inclusion crystal inside which are formed as two subducting plates squeezed each other at great depth. Professor Alvaro from the University of Pavia in Italy wonders if it is possible that the formation and deformation of the inclusions might act as the very first trigger of megathrust earthquakes. The idea is that these tiny changes set off cracks in larger rocks that eventually lead a fault to slip out of place.
https://phys.org/news/2021-04-earth-surface-powerful-earthquakes.html