Earthquakes are the result of two of the Earth's crustal plates slipping past each other, otherwise known as plate tectonics. The vibrations caused by this sudden movement reverberate through the surrounding rock structures, and they are felt as tremors. Earthquakes are most common among the geologically active regions at the borders between plates of the Earth's crust, also known as fault zones.
The Earth's crust is made of several large plates that continuously grind against each other. At the borders between the plates, large sections of rock slide against, over or under each other. The process is not perfectly smooth, and static friction between two plates can temporarily arrest the movement. When this happens, tension builds up in the area as the plates continue to press against each other. Eventually, the border between the plates gives way and movement occurs. This shift in position can be lateral, as in a strike-slip fault, or it can be vertical, as in a subduction zone.
As the edges of the plates grind against each other, a series of shock waves travel away from the point of origin, known as the epicenter, thus shaking the ground. Depending on the magnitude of the specific occurrence, some earthquakes can be felt for hundreds of miles.