Instead of occurring at a plate boundary along an area called a subduction zone, where one tectonic plate is diving beneath another, this earthquake occurred in the middle of an oceanic plate, where the faults in the crust essentially moved from side to side instead of up and down.
But on that fateful Friday, when the North American plate slid over the Pacific plate along a subduction zone running 130km (80 miles) off the Pacific coast of northern Japan, the shock leapt from the first segment to a second and on to a third, extending the fault zone some 400km and increasing its intensity more than 30-fold.
Although they are sometimes produced by landslides on the seafloor, significant tsunamis are typically created by subduction earthquakes, when one massive oceanic plate suddenly lurches deeper beneath another plate, shoving up a huge section of the seafloor.