The Earth motion produced by a natural (earthquake) or synthetic disturbance on the surface or underground; utilized in the seismic method of geophysical exploration and for investigating the Earth's interior. Three types of waves are produced: (1) longitudinal or P waves; (2) traverse or S waves; and (3) surface or Raleigh and L waves. The speed of propagation is characteristic for each type of rock, depending largely on its compactness. In sandy clay, the speed of the P wave is about 4,000 ft/s (1.22 km/s); in sandstone, 10,000 ft/s (3.05 km/s); and in igneous rock up to 22,000 ft/s (6.71 km/s).