Plate tectonics
Earth’s outer shell is broken into about 15 major plates of rigid lithosphere, each up to ~100 km thick, gliding over the hotter, weaker asthenosphere at 2–10 cm a year — about as fast as fingernails grow. Almost every earthquake happens where plates meet.
Oceanic plate sinks beneath a continent. Coral flashes are earthquakes tracing the slab down to ~700 km.
Three kinds of boundary
At convergent boundaries plates collide: dense oceanic plate sinks beneath another in a subduction zone, producing the planet’s largest earthquakes and chains of volcanoes. At divergent boundaries, like the Mid-Atlantic Ridge, plates pull apart and magma fills the gap. At transform boundaries, such as the San Andreas Fault, plates grind sideways past each other.
Stick, then slip
Plate edges don’t slide smoothly. Friction locks them while the plates keep moving, so rock bends and stores elastic strain — sometimes for centuries. When stress exceeds the strength of the fault it ruptures in seconds, and the stored energy radiates as seismic waves. This is elastic rebound theory, first described after the 1906 San Francisco earthquake.
The Ring of Fire
Around 80% of the world’s large earthquakes occur around the Pacific, where oceanic plates subduct beneath Japan, Indonesia, the Andes, Alaska and New Zealand. Turn on Depth in the Globe tab to see subducting slabs traced by quakes hundreds of kilometres down.