Many occur each year due to the Earth's constant release of stress. The intensity of an earthquake is the damage caused by the seismic waves. The intensity is computed using the Mercalli Intensity Scale, which is a damage detector. An earthquake has a single magnitude but its intensity varies with distance. The maximum intensity occurs at the earthquake's epicenter, with intensities decreasing with distance.
Many factors affect the intensity, such as topography and near surface geology.
Mercalli Intensity Scale 1 Not felt. Recorded by seismographs. Felt indoors, like a passing light truck 4 Windows, dishes, doors rattle. Like passing train 5 Felt by all. Small objects upset 6 Books of shelves.