An earth tremor is another term for an earthquake, although it is most commonly used to describe earthquakes of low intensity. Earthquakes are measured with tools called seismographs, and their intensity is rated on a logarithmic scale, called the Richter scale. The Richter scale is not useful for earthquakes that rank higher than a level 8 earthquake.
Earthquakes occur when the earth’s tectonic plates move and contact other plates. Sometimes the plates rub against each other laterally, while other times one plate moves underneath another. When this happens, it is called a subduction zone or a subduction-caused earthquake. In some particularly violent earthquakes, two plates collide into each other. This type of collision may even cause mountains to form, which happened millions of years ago when the Indian subcontinent crashed into Asia, producing the Himalayas.
Earthquakes are rather common, though few are strong enough to cause much damage. People may not even feel very weak earthquakes. The center of the disturbance is called the earthquake’s epicenter, and epicenters usually occur along major fault lines, where two plates contact each other. Many large and active fault lines occur around the margin of the Pacific Ocean, which has given the region the name “ring of fire” for all of the earthquakes and volcanoes in the region.