The term "earthquake" refers to the vibration of the Earth's surface caused by movement along a 'fault', by a 'volcanic' eruption, or even by manmade explosions. The vibration can be violent and cause widespread damage and injury, or may be barely felt. Most destructive earthquakes are caused by movements along faults. Earthquakes can occur at the surface of the Earth or as deep as 400 miles below the surface. An earthquake can trigger additional hazards such as 'landslides' or 'tsunamis'. Earthquakes occur all over the world and often occur without significant warning. These geohazards can have far-reaching affects on humans and on the surface of the Earth. Small, localized earthquakes may cause no noticeable damage and may not even be felt by people living in the affected area. In contrast, a large earthquake may cause destruction over a wide area and be felt thousands of miles away. The Earth is formed of several layers that have very different physical and chemical properties. The outer layer, which averages about 43 miles (70 kilometers) in thickness, consists of about a dozen large, irregularly shaped 'plates' that slide over, under, and past each other on top of the partly 'molten' inner layer. The plate boundaries are fault zones, and are where most earthquakes occur. In fact, the locations of earthquakes and the kinds of ruptures they produce help scientists define the plate boundaries. There are three types of plate boundaries: spreading zones, transform faults, and subduction zones. At 'spreading zones', molten rock rises, pushing two plates apart and adding new material at their edges. Most spreading zones are found in oceans; for example, the North American and Eurasian plates are spreading apart along the mid-Atlantic ridge. Spreading zones usually have earthquakes at shallow depths (within 19 miles (30 kilometers) of the surface). 'Transform faults' are found where plates slide past one another. An example of a transform-fault plate boundary is the San Andreas fault, along the coast of California and northwestern Mexico. Earthquakes at transform faults tend to occur at shallow depths and form fairly straight linear patterns. 'Subduction zones' are found where one plate overrides, or subducts, another, pushing it downward into the mantle where it melts. An example of a subduction-zone plate boundary is found along the northwest coast of the United States, western Canada, and southern Alaska and the Aleutian Islands. Subduction zones are characterized by deep-ocean trenches, shallow to deep earthquakes, and mountain ranges containing active volcanoes. Earthquakes can also occur within plates, although plate-boundary earthquakes are much more common. Less than 10 percent of all earthquakes occur within plate interiors. As plates continue to move and plate boundaries change over geologic time, weakened boundary regions become part of the interiors of the plates. These zones of weakness within the continents can cause earthquakes in response to stresses that originate at the edges of the plate or in the deeper crust. The 'New Madrid earthquakes of 1811-1812' and the '1886 Charleston earthquake' occurred within the North American plate. Earthquake strength is measured as both magnitude and intensity. Magnitude measures the relative strength of an earthquake and is recorded with the 'Richter scale'. Each earthquake only has one magnitude. People usually cannot feel earthquakes with magnitudes of 3.0 or less. Intensity measures the severity of an earthquake in terms of its effect on structures, and the land surface. The USGS usually uses the 'Modified Mercalli intensity scale' to describe earthquake intensity. The intensity of a given earthquake will vary from place to place. We tend to picture most earthquake damage as resulting directly from ground shaking, but there are many other related impacts from an earthquake. For example, ground shaking can result in soil liquefaction, damage to dams or levees with resultant flooding, 'landslides', and fires caused by ruptured fuel and power lines. In addition, earthquakes may trigger 'tsunamis' or 'seiches'. Structural damage or collapse may be caused by any of these effects, which may be local or may occur hundreds or even thousands of miles from the 'epicenter' of the earthquake. A 'Federal Emergency Management Agency' study considered just capital (damages to buildings and their contents) and income-related costs, and provided an estimate of $4.4 billion as the minimum average annualized loss due to earthquakes in the United States. Most earthquakes in the United States occur in Alaska and California, although Hawaii, Nevada, Washington, and Idaho also experience many earthquakes. Four of the five 'largest earthquakes in the United States' occurred in Alaska. The USGS 'Earthquake Hazards Program maintains detailed records on historical earthquakes and continuously monitors earthquake activity around the world. While we can't prevent earthquakes or even accurately predict when they will occur, we can take steps to lessen their human impact. In the United States and many other countries, building codes take into account the local earthquake risk so that buildings and other structures can be designed to withstand all but the most severe earthquakes. In addition, those who live in earthquake-prone areas should know 'how to be prepared for an earthquake and what to do if one occurs'