Seismic Waves

An Earthquake begins at the…  Focus: The point where the energy is released after elastic limit is reached.  Epicenter: The point on the surface of the Earth directly above the focus.

Earthquakes produce three types of waves with different behaviors as they travel through materials.

Primary (P) Waves:  Move out from the earthquake focus.  Travel the fastest of the 3 waves.  Travel twice as fast as secondary waves.  Move by causing particles in rocks to move back and forth in the same direction that the wave is traveling.  Example: slinky  Are bent and slowed when they hit the outer core.  Longitudinal wave Blue-

Secondary (S) Waves:  Move out from the earthquake focus.  Move slower than primary waves.  Move by causing particles in rocks to move at right angles to the direction of wave travel.  Example: rope  Cannot travel through liquids, so they are stopped by the outer core.  Transverse Wave Red-

Surface Waves:  Form when P and S waves reach the surface.  Slowest Waves, Most destructive  Can cause the ground to shake making rock roll and sway from side to side.  Only travel through crust

Seismograph:  instrument used to record the energy released by an earthquake. Recording time of wave arrival.  Produces paper sheet called a seismogram  A stationary pen traces a record of vibrations

Seismograph Stations  P waves arrive first  S waves arrive second  Surface waves arrive last (slowest)  3 or more seismograph stations are needed to determine the location of the epicenter.  When an epicenter is far from a location, the p wave has more time to put distance between it and the s and surface waves.

Finding the epicenter...  Each city determines their distance from the epicenter.  That distance is used as the radius of a circle.  After 3 cities create circles, where they intersect is the actual epicenter.

Because of the behavior of these different waves…  Scientists have indirect evidence for the solid inner core and liquid outer core of the Earth.  Earthquake waves travel faster through the mantle than through the crust.  So, scientists know the mantle is denser than the crust.

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