What causes earthquakes and why do they happen repeatedly?

Earthquakes are caused by the movement of tectonic plates, the large slabs of rock that make up Earth's outer shell. These plates are constantly moving, driven by heat convection in the mantle below. When plates collide, separate, or slide past each other, stress builds up along fault lines. When that stress exceeds the strength of the rock, it releases suddenly as seismic energy, producing an earthquake. They happen repeatedly because plate movement never stops. Stress continuously rebuilds along the same fault lines after each rupture, eventually triggering another quake. This is why regions like California, Japan, and Turkey experience earthquakes regularly. Aftershocks occur because the initial rupture redistributes stress to nearby fault segments, causing additional smaller adjustments in the surrounding rock.