Science Puzzle
Why Do Earthquakes Happen Where They Do?
Plot every earthquake of the last century on a world map and something startling appears: the dots are not scattered randomly. They form narrow lines snaking across the globe, tracing the edges of continents and the middles of oceans. Vast regions in between are almost silent.
Why are earthquakes confined to these lines?
The Answer
Those lines are the edges of the tectonic plates, the great slabs of rock that make up the Earth's outer shell. They creep about at roughly the speed your fingernails grow, and the middle of a plate rides along peacefully. All the trouble happens at the seams.
At a boundary, two plates try to slide past, pull apart or crash together, but friction locks them in place. The plates keep pushing regardless, so the rock near the fault bends and stores the strain like a ruler bent between your hands. Eventually the friction gives, the rock snaps back into shape, and decades of stored energy escape in seconds. That violent release is the earthquake.
This explains the geography perfectly. Japan, California, Chile and Indonesia sit on plate boundaries and shake constantly. Central Australia and central Canada sit deep inside plates and barely tremble. The map of earthquakes is really a map of the plates, drawn by the Earth itself, and it was one of the strongest clues that led scientists to plate tectonics in the first place.
The principle: Plate tectonics. Earthquakes concentrate at plate boundaries, where locked faults accumulate elastic strain until it is released suddenly as seismic waves.