The surface of the earth is called the crust, and it is made up of plates, called tectonic plates, that move. Earthquakes happen when these plates bump, scrape or drag against each other.
Most of the earthquakes are too small or too far below the earth's surface for people on the surface to feel. Most earthquakes occur 50 or more miles below the surface of the earth. Some happen in the deep sea and are not felt by humans. Powerful earthquakes can be felt thousands of miles away and can cause landslides, tsunamis, flooding and widespread destruction. In heavily populated areas, death and property damage occurs. This is because the shaking causes structures to collapse and fires to erupt.
Earthquakes are measured using seismometers. Earthquakes often occur in volcanic regions due to the flow of magma in volcanoes and are an early warning of volcanic eruptions. Geologists rate earthquakes based on their magnitude, which is the amount of energy released during the quake. One of the largest earthquakes recorded was in Chile, with a magnitude of 9.5. An aftershock is an earthquake that occurs after a previous earthquake or main shock. It is usually smaller in magnitude than the main shock.