Earthquakes

Earthquakes occur when rocks within the Earth rupture. The rocks of the Earth's lithosphere are under stress due to relative motions of tectonic plates. When the stress exceeds the strength of the rocks, they fail. This failure releases energy in the form of heat and seismic waves. The seismic energy propagates away from the earthquake focus, sometimes for thousands of miles, in the form of seismic waves.

 

To simulate a rock rupture click on the GO! button within the image.

There are two main groups of seismic waves that are generated by an earthquake, body waves and surface waves, each traveling with a different speed and with different motions. Body waves travel faster than surface waves, so they are the first seismic waves recorded on a seismogram.

Two types of body waves exist: P-Waves and S-Waves. P-waves ( Primary Waves, or Compressional Waves) are faster, S-Waves, (Secondary Waves or Shear Waves) are slower. Hence, on a seismogram the first recorded seismic waves are P-waves and they are followed by S-waves. Body waves travel through the Earth and are used by scientists to study the internal structure of the Earth.

P-Waves are compressional waves, particles travel along the path of propagation:	S-Waves or shear waves, particles travel perpendicular to the direction of propagation:

Surface waves travel slower than body waves and they are also divided into two types: Rayleigh and Love Waves. Love waves are the faster of the surface waves and they travel across the surface of the Earth.

The body waves are very useful in locating the position of an earthquake. If we know the speed of the P and S waves within the Earth, and measure the time difference between the arrival times of S-waves and P-waves on a seismogram, we can calculate the distance between the recording station and the earthquake. With seismic wave arrival time readings of the same earthquake from several seismic stations, we can accurately determine the earthquake location.

In the next three activities (mapping earthquakes, earthquake depth and earthquake magnitude) students will interactively map thousands of earthquake locations based on student-selected mapping criteria. For example, using QUEST students can study earthquakes by selecting a geographical region, and assigning depth, magnitude, and date criteria. QUEST also allows students to store their maps and compare the results to reach their own conclusions about earth processes.

 

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