Why earthquakes happen: plate tectonics explained
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Open EarthquakeIndex's map with plate boundaries switched on and the pattern is unmistakable: the quakes line up like beads on a string along the same lines. That is no coincidence.
Earth's shell is broken — by design
Earth's outer shell is cracked into about 15 large tectonic plates floating on the hot, slowly flowing mantle. The plates move a few centimetres per year — roughly the speed your fingernails grow. But they do not slide smoothly: along their edges they lock against each other, stress builds for decades, and when it releases all at once, the ground shakes.
Three kinds of plate boundaries
- Collision (convergent): Plates push together; one dives beneath the other. This is where the very largest quakes happen — e.g. along South America's west coast and Japan.
- Spreading (divergent): Plates pull apart, as along the Mid-Atlantic Ridge. Quakes are typically smaller and shallow.
- Sliding (transform): Plates slide horizontally past each other, as at the San Andreas Fault in California.
The Ring of Fire
About 90% of the world's earthquakes occur in the "Ring of Fire" — the horseshoe of plate boundaries around the Pacific, from Chile via Alaska and Japan to Indonesia and New Zealand. Look at the live map on any given day and you will see exactly that pattern.
Can earthquakes be predicted?
No — nobody today can predict the time, place and size of a coming quake, and anyone claiming otherwise deserves healthy scepticism. What science can do is compute probabilities over decades and build early-warning systems that give seconds of notice once a quake is already underway. That is why safety is about preparation — read what to do during an earthquake.