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Features of Earthquakes Chapter 11, Section 2

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Focus The point on a fault where rocks break and energy is released.

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Epicenter Point on the surface of the Earth directly above the focus.

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Seismograph Instruments that register seismic waves and record horizontal and vertical ground movement. Produce a paper record of the seismic event that is called a seismogram.

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Magnitude The height (or amplitude) of the lines on a seismogram indicate the magnitude or strength of an earthquake. Most commonly measured with the Richter magnitude scale. For each increase of 1.0 on the scale: The height of the line on the seismogram is 10x greater. The amount of released energy is 32x greater. A magnitude 9.5 earthquake is the largest ever recorded (southern Chile – 1960).

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Locating an Epicenter Because s and p waves travel at different speeds, they each reach a seismograph station at different times. The difference in time can be converted into a distance. Using three seismograph stations, the location of the epicenter can be determined through triangulation.

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Tips for Your Assignment: Finding amplitude of S-wave: The distance between the center line and the highest peak on the seismogram.

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Tips for Your Assigment: Finding S-P lag time: Time between the start of the p-wave and the start of the s-wave.

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Tips for Your Assignment: Finding distance: Use the chart to convert s-p lag time into distance.

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Tips for Your Assignment: Finding magnitude: Line up distance and s- wave amplitude to determine magnitude.

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Tips for Your Assignment: Finding epicenter location: Draw a circle from each station with a radius equal to the distance of the station from the epicenter. Epicenter is located where the three circle cross.

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