Why do earthquakes happen along tectonic plate boundaries?

Earth’s outer shell is broken into about a dozen major tectonic plates and many smaller ones. At plate boundaries, stress builds up as plates collide, pull apart, or slide past each other, until the rocks suddenly fracture or slip. That rapid release of stored elastic energy causes an earthquake. The theory of plate tectonics was developed in the 1960s, and it explains why quakes cluster around the Pacific Ring of Fire and other active margins. Some of the largest earthquakes, such as the 1960 Chile quake and the 2011 Tōhoku earthquake, occurred on major plate boundaries.