Planet Earth is composed of layers: the inner core, the outer core, the lower mantle, the upper mantle, and the crust. We live on the crust.

Earthquakes result from the movement of a fault, a zone of deformation in the uppermost layers of the crust.

The crust is made of massive pieces of solid earth called tectonic plates. These plates are in slow but constant motion. As the plates move towards or away from each other, the stress they create sometimes exceeds the Earth’s capacity to support the movement. This is when an earthquake happens. Earthquakes usually occur at plate boundaries. Less frequently, earthquakes occur in the interior of a plate.

There are different kinds of faults, and different kinds of earthquakes. A strike-slip fault creates horizontal movement and shaking. Dip-slip faults create vertical motion. In normal dip-slip faults the earth falls. In a dip-slip thrust fault, the earth is shoved upwards.

In Utah, the mountain ranges were primarily created by normal dip-slip faults. When we experience a "Big One," the fault line along the mountains will drop 3 to 20 feet at close to the speed of sound, sending shockwaves across the valley. The lakes may also tip and dams may break, causing flooding.