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**Ch. 6.2 Recording Earthquakes**

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**Seismograph—an instrument used for detecting and recording seismic waves.**

Detected waves are converted into electrical signals, and recorded onto tape, or directly loaded into a computer for analysis.

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**Types of Seismic Waves Three main types:**

Primary waves (P waves)—fastest, so first to be recorded. Can travel through solids and liquids. They are compression waves that move particles together and apart along the direction of the wave’s travel.

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**Secondary waves (S waves)—2nd waves to be recorded on a seismograph**

Secondary waves (S waves)—2nd waves to be recorded on a seismograph. Only travel through solids. They can’t penetrate the liquid outer core. They are shear waves; they move particles at right angles to the direction of the wave’s travel.

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The energy of P and S waves is converted to Surface Waves when they reach the earth’s surface. They are the slowest waves, with movement similar to rolling ocean waves. They cause the greatest amount of damage.

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**Locating an Earthquake**

Since P waves travel about 1.7 times faster than S waves, by analyzing the difference in arrival times of the waves at a seismograph station, the distance to the earthquake can be determined. Info from at least three stations is needed to determine the location of the epicenter.

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**Earthquake Measurement**

The energy released by an earthquake (magnitude) is measured using the Richter scale. Going up the scale, 1 to 10, each whole number represents 10 times greater energy. Major earthquakes have magnitude 7 or above. The Mercalli scale uses roman numerals, I to XII, and expresses the earthquake’s intensity. Uses descriptive phrases to describe effects.

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When calculating energy differences in the Richter scale, remember that each whole number differs from the adjacent whole number by a factor of 10. A magnitude 3 has 10 times more energy than a magnitude 2. A magnitude 4 has 100 times more energy than a magnitude 2 (10 x 10). A magnitude 5 has 1000 times more energy than a magnitude 2 (10 x 10 x 10).

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Calculate the energy difference between a magnitude 3 earthquake and a magnitude 7 earthquake (Richter Scale). 7 – 3 = 4 10 x 10 x 10 x 10 10,000

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