Seismic velocity is the speed of a seismic wave as it moves through the Earth's crust. An earthquake or other seismic event typically produces several types of waves. Each type has its own velocity and the velocity will shift as waves move through different kinds of materials. Researchers collect observations on the movement of seismic waves as part of a larger study of how earthquakes happen, and what occurs under the ground during a quake.
Both body and surface waves can exhibit different velocities. Body waves like P and S waves travel underground, while surface waves like L waves ripple across the Earth's crust. The behaviors of the waves generated can depend on the specifics of the event. Seismic observatories collect information on incoming seismic waves and can compare data with each other to triangulate the location of an event and determine how deep it is.
Deeper waves tend to have a greater seismic velocity. They can be extremely high energy, and have a potential to cause significant damage when they reach the surface. The elasticity and density of materials has an impact on seismic velocity. Knowledge of local rock properties is important for researchers because they need to be able to determine how the substrate of rocks beneath the surface may have affected the behavior of seismic waves.
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Observers usually measure seismic velocity in meters per second. In a substrate like clay, P waves can move as fast as 2,500 meters per second, while the slower S waves can reach speeds of 1,000 meters per second. Basalt formations can allow P waves to reach up to 6,400 meters per second in speed, while P waves move at about half that rate. Charts of common materials and the range of velocities measured are available to allow observers to learn more about conditions under the earth during a quake.
Knowledge of seismic velocity measurements is also important for seismic surveying. In this form of geotechnical testing, geologists create controlled explosions and measure the movement of waves through deposits of rock and other materials. This can reveal water, petroleum, and other materials of interest inside the Earth that may not be visible from the surface. The reflections of the waves, in addition to changes in velocity, also offer data about what may lie beneath. Computer programs can collate the data from a survey to generate maps and other visual representations of data for the benefit of observers.