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This is similar to a p wave but with the extra up-down motion. The ground rolls forward and up and then down and backwards. Rayleigh waves displace the ground like rolling ocean waves. Love waves shake the ground side-to-side like an S wave. Surface Waves - travel along theEarth's surface. S waves shear the rock that they pass through. They cause matter to oscillate side-to-side, perpendicular to the motion of the wavefront. S, Secondary, or Shear waves are somewhat slower (~3.5 km/sec in the upper crust). P waves push (compress) and pull (dilate) the rock that they pass through. They cause the matter to oscillate forward and backward, parallel to the motion of the seismic wavefront. It lies directly over the focus.īody Waves - travel through theinterior of the Earth P, Primary, or Compressional waves travel the fastest (~6 km/sec in the upper crust). The epicenter is the map position of the Earthquake. The focus or hypocenter is the exact position on the fault, including the depth, where the slippage began. Seismologists can determine the point on a fault where the slippage began, the area (length and depth) of the fault that slipped, the amount of slippage or fault throw (how far the crust moved), and the time it took for the slippage to occur. The whole fault doesn't move at one time only the part of the fault around which the stress exceeded the strength. Seismic waves travel outward from the portion of the faultthat broke, like expanding ripples from a pebble dropped in stillwater. The ground shakes asthe stress energy is released and the rocks lurch to their newposition in a matter of seconds. Then,sudden slippage of rock along a fault occurs. Those stresses (compression,tension, shear) build up in the crust until the stress exceeds the strength of the rockor the friction along a preexisting fault. As the lithospheric plates of the Earthcontinue their slow motions, stresses build up in the crust,especially near the plate boundaries.