Earthquakes The rupture and sudden movement of rocks along a fault. 6.1 Earthquakes and Plate Boundaries Majority of earthquakes occur in Earth’s crust.

Earthquakes

Earthquakes

• The rupture and sudden movement of rocks along a fault.

6.1 Earthquakes and Plate Boundaries

• Majority of earthquakes occur in Earth’s crust.

• Part of the energy released from earthquakes spreads as complex waves.

• When rocks can no longer change shape—the fault breaks and slips, causing earthquakes.

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Focus & Epicenter

• The focus is the location on the fault where an earthquake begins.

6.1 Earthquakes and Plate Boundaries

• The closer the focus is to the surface, the stronger the shaking will be.

• The point on Earth’s surface directly above the focus is the epicenter.

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Plate Boundaries and Earthquakes• Lithospheric plates interact at different plate boundaries

and produce earthquakes.

6.1 Earthquakes and Plate Boundaries

• Earthquake size and depth and fault type depend on the type of plate boundary.

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Plate Boundaries and Earthquakes (cont.)

6.1 Earthquakes and Plate Boundaries

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Earthquakes Away from Plate Boundaries

6.1 Earthquakes and Plate Boundaries

• Not all earthquakes happen at plate boundaries.

• New Madris Earthquakes of 1911

• Millions of years ago, a long zone of intense faulting was formed when the crust began to pull apart, but did not break completely.

• Today, the crust is being compressed, or squeezed together. (p. 246) 1

Seismic Waves

Seismic Wave

• Waves move outwardfrom the focus in alldirections.

• 3 main types of seismic waves.

• Waves of energy that are produced at the focus of an earthquake.

6.2 Earthquakes and Seismic Waves

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1. Primary Waves (P-waves)

6.2 Earthquakes and Seismic Waves

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•Particles vibrate in same direction as wave

•Fastest seismic wave

•First to be detected by seismographs

•Travel through solids and fluids

• Particles vibrate perpendicular to wave

• Slower than P-waves

• Detected after the p-wave

• Travels through solids only

2. Secondary Waves (S-waves)

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• Slowest seismic wave

• Side-to-side swaying or rolling motion

• Cause the most damage

3. Surface Waves

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MeasuringEarthquakes

Measuring Earthquakes• Scientists determine size of earthquakes by measuring how much the rock slips

along the fault.

• They also analyze the heights of the seismic waves, which indicate how much energy is released by an earthquake.

6.3 Measuring Earthquakes

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Seismograph

• Records size, direction, and the movement time of ground

6.3 Measuring Earthquakes

• Records the arrival times of the P- and S-waves

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Seismogram

• Record of the seismic waves

6.3 Measuring Earthquakes

• Used to calculate the size and locations of earthquakes

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Reading a Seismogram• Wave heights indicate the amount of ground motion

for each type of wave.

• Difference between the arrival times of P-waves and S-waves determines the distance of the seismograph from the epicenter. (p.260)

6.3 Measuring Earthquakes

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Locating anEpicenter

Locating an Epicenter

• Triangulation is used to locate the epicenter.

• This method is based on the speeds of the seismic waves.

• At least three seismographs must record the distances.

6.3 Measuring Earthquakes

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1. Find the arrival time differences.

6.3 Measuring Earthquakes

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2. Find the difference from the epicenter.

6.3 Measuring Earthquakes

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3. Plot the distance on a map. (p. 263)

6.3 Measuring Earthquakes

• The epicenter is where the 3 circles overlap.

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Earthquake Scales

Measuring Earthquake Size• Magnitude measures the amount of energy released

by an earthquake.

• Magnitude scale is based on record of height of ground motion.

• Most values range from 0–9.

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Richter Scale

• First magnitude scale that used mechanical seismographs

• Good for earthquakes between 3.0-7.0 only.

• Not used today

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Moment Magnitude Scale• Used today

• More accurate scale for measuring earthquake size.

• Based on the amount of energy released during an earthquake.

• Uses electronic seismographs

6.3 Measuring Earthquakes

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Earthquake Intensity• Intensity refers to the amount of shaking and damage caused.

• Intensity varies depending on:

– Size of earthquake

– Distance from epicenter

– Type of ground surface

– Construction of building

• Mercalli Scale measures intensity

• Usually, the maximum intensity is found near the epicenter. (p. 264)

6.3 Measuring Earthquakes

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PredictingEarthquakes

Predicting Earthquakes• At this time, geologists cannot predict earthquakes.

• Geologists can, however, determine the seismic risk by locating active faults and where past earthquakes have occurred.

• Geologists create seismic risk maps.

6.3 Measuring Earthquakes

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Avoiding Earthquake Hazards (p.274)

6.4 Earthquake Hazards and Safety

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