Locating and Measuring Earthquakes

As mentioned in the previous post, a seismograph is used to determine the magnitude of an earthquake and the location of the epicenter. Because earthquakes produce different types of wave motion, a modern seimograph station usually has three seismographs. One rrecords up-and-down motions, another records side-to-side motion in a north-south direction, and a third records side-to-side motion in a east-west direction.

The sheet used to record the motion of an earthquake is called a seismogram. This sheet is placed on a rotating drum that turns slowly by a clock. Above the paper is a pen with its point resting on top of the sheet and drum. The pen rest stationary attached to a heavy weight. When the drum moves due to the shaking of the ground, the paper moves and the pen produces a zig-zag line on the paper.
Interpreting a Seismograph
Because P-waves move faster than S-waves, the P-waves always arrive at a seismograph station before the S-waves. Seismologists use the difference in the speeds of the P- and S-waves to locate the epicenter of an earthquake. The graph "Earthquake P-wave and S-wave Travel Time" found in the earth science reference tables, shows how long it takes for P- and Swaves to travel different distances through the Earth. Using this graph, we will explore how to determine the distance and time these waves travel to locate where the earthquake has occurred.

Homework: Due Wednesday, February 24, 2010
Review Book: read pages 54-58; do questions 1-12.

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