Terrifying? Technical facts: The ocean floor is not a passive boundary. It is where Earth’s largest earthquakes are born, along the grinding interfaces of subduction zones where one tectonic plate is forced beneath another. A megathrust earthquake begins with elastic strain. As an oceanic plate descends into the mantle, friction locks it against the overriding plate. The two surfaces do not slide smoothly; they stick. For centuries the overriding plate is dragged downward and compressed, storing elastic energy like a loaded spring. When accumulated stress exceeds the frictional strength of the locked interface, the fault ruptures and the seabed lurches upward by metres in seconds. The rupture front itself can race along the fault at two to three kilometres per second, unzipping hundreds of kilometres of interface faster than the ground can settle. This vertical displacement is the critical detail. A strike-slip fault, which moves horizontally, displaces relatively little water. A t...