Seismic waves are waves of force that travel through the Earth or other elastic body, for example as the result of an earthquake, explosion, or some other process that imparts forces. Seismic waves are studied by seismologists, and measured by a seismograph, which records the output of a seismometer, or geophone. For seismic studies of oil reservoirs, hydrophones may give additional information.
The propagation velocity of the waves depends on density and elasticity of the medium which is penetrated. The velocities range from approx. 3-8 km/s in the Earth's crust up to 13 km/s in the deep mantle.
Earth quakes create various types of waves with different velocities; when reaching seismic observatories, their different travel time enables the scientists to locate the hypocenter. In geophysics the refraction or reflection of seismic waves is used for research of the Earth's interior, and artificial vibrations to investigate subsurface structures.