Feel the force of the world’s biggest earthquakes and discover how these seismic events rock our cities and settlements

WORDSSCOTT DUTFIELD

DID YOU KNOW? Japan has around 1,500 noticeable earthquakes every year

With the power to topple towers and bring entire cities to their knees, earthquakes are one of the most destructive forces in nature. To understand why the ground shakes, you must first look at Earth’s outermost layer, the crust. Also known as the lithosphere, the crust is a 25-mile-thick layer of solid rock that’s split up into 15 tectonic plates that fit together in one Earth-sized jigsaw puzzle. However, the plates aren’t static and are continually on the move. Driven by heat emanating from Earth’s centre, the plates, along with everything on top of them, move at around 1.5 centimetres each year. But where are they moving to?

At the boundary where two plates meet, one will subduct under the other. The heat and pressure generated by the collision cause the crust to melt and drive it back towards the core. At the same time that plates are being destroyed at this boundary, more crustforming rock is being released along other plate boundaries called oceanic ridges.

This creates a kind of crust-forming conveyor belt. However, the motion of tectonic plates creates a great deal of ‘stress’ between them. Tectonic stress, also known as shear stress, is the force between two plates that causes them to deform. Over time, shear stress can build between the two plates, until one day the pressure becomes too much and rocks suddenly slip past one another. The waves of seismic energy released from the slip can be strong enough to shake the ground, causing an earthquake.

In 2024, researchers studying the most powerful earthquake in Japan’s history, the magnitude 9.0 to 9.1 Tōhoku earthquake, calculated that the shear force at the plate boundary was equal to around 100 megapascals. For comparison, one megapascal is around the same force exerted by a 100-tonne rock resting on an area of one square metre. The force of the quake itself not only brought widespread destruction, but also created one of the most devastating side effects of an earthquake: a 40-metre-high tsunami. The waves of the Tōhoku tsunami were created by the sudden movement of the ocean floor, which in turn displaced enormous amounts of water. Earthquakes are isolated to plate boundaries, but also occur where two rocks press against one another, divided by a crack or ‘fault’. These fault lines are commonly found following plate boundaries, but can also be in the middle of a plate – the 2008 magnitude 7.9 quake in Sichuan, China, was generated at an intraplate fault line.