A half-million earthquakes rock the Earth each day. Most are too small or deep below the surface to be felt, but some can be so powerful that people hundreds of miles away feel their vibrations. Earthquakes occur when blocks of Earth’s crust slip past one another. The spot underground where this occurs is called a fault zone. The spot above the fault on the surface is called the epicenter.
Earthquakes can cause damage to buildings and their contents. The greatest risk is from aftershocks—much smaller earthquakes that follow the main earthquake, especially those with the same or larger magnitude. Buildings can also be damaged by poor construction technique or by having their foundations weakened by mining activities.
When an earthquake occurs, the shaking can last a few seconds or a few minutes. The longer the shaking, the greater the damage is likely to be. The intensity of the shaking varies depending on the geology where an earthquake occurs. For example, a quake in soft sediments may have longer and more intense shaking than the same quake occurring on solid bedrock.
The intensity of an earthquake is measured on a scale known as the Richter Scale. The magnitude of an earthquake—how strong it is—is determined by how quickly the Earth’s rock moves and by its size. There are a variety of ways that scientists determine an earthquake’s magnitude, including the use of seismometers and other seismic monitoring equipment.