Earth's outer shell forms a lithosphere (rocky crust) about 100 km thick, divided into dozens to hundreds of large and small plates. These plates move at rates of several centimeters to 10 cm annually.
Where plates collide or rub, rock experiences constant stress. Rock is elastic and can deform somewhat, but when applied stress exceeds rock strength, sudden failure occurs. At breakage, accumulated energy releases abruptly as seismic waves propagating outward.
Earthquakes have three main causes: compressional earthquakes from plate collision, strike-slip earthquakes where plates slide sideways, and normal faulting from tension. Most large world earthquakes are subduction-zone earthquakes from plate collisions.
Heat-driven convection inside Earth is relevant too, with mantle convection driving plate motion. This process continues on geological timescales (millions of years), making earthquakes an inevitable natural phenomenon.