The energy of moving plates is transferred to rocks, which would groan at the strain if they could. After what may be hundreds or thousands of years, the built-up pressure causes a weak section of rock, known as a fault, to violently fail. We feel this huge release of energy as an earthquake. The spot at which rock first slipped against rock, called the hypocenter, is usually miles below the ground surface. Shock waves created by the rupture travel outward from the hypocenter in all directions like ripples from a rock dropped into a pond.
Seismologists mark the ground surface above the hypocenter with a dot known as the epicenter.
Aftershocks, smaller earthquakes that follow a larger one, occur as the stress from a fault is transferred to smaller rocks. The smaller rocks then crumble for a few days until friction causes movement along a fault to cease. Particularly strong earthquakes happen at plate boundaries, such as the Pacific coast of North America. As the Pacific and North American plates slowly collide, major earthquakes occur along their meeting place from Alaska to the California coast and southward.