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Earthquakes
6.1 Earthquakes and Plate Boundaries
Earthquakes
• When rocks can no longer change shape—
the fault breaks and slips, causing
earthquakes.
• The rupture and sudden movement of
rocks along a fault.
• Majority of earthquakes occur in
Earth’s crust.
• Part of the energy released from
earthquakes spreads as complex waves.
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6.1 Earthquakes and Plate Boundaries
Focus & Epicenter
• The focus is the location on the fault
where an earthquake begins.
• 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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6.1 Earthquakes and Plate Boundaries
Plate Boundaries and Earthquakes
• Lithospheric plates interact at different plate
boundaries and produce earthquakes.
• Earthquake size and depth and fault type
depend on the type of plate boundary.
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6.1 Earthquakes and Plate Boundaries
Plate Boundaries and Earthquakes (cont.)
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6.1 Earthquakes and Plate Boundaries
Earthquakes Away from
Plate Boundaries
• Not all earthquakes happen at plate
boundaries.
• New Madrid, Missouri Earthquakes of 1811
• 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)
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Seismic Waves
6.2 Earthquakes and Seismic Waves
Seismic Wave
• Waves of energy that are produced at the
focus of an earthquake.
• Waves move outward
from the focus in all
directions.
• 3 main types of
seismic waves.
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6.2 Earthquakes and Seismic Waves
1. Primary Waves (P-waves)
•Particles vibrate in same direction as wave
•Fastest seismic wave
•First to be detected by seismographs
•Travel through solids and fluids
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2. Secondary Waves (S-waves)
• Particles vibrate perpendicular to wave
• Slower than P-waves
• Detected after the p-wave
• Travels through solids only
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3. Surface Waves
• Slowest seismic wave
• Side-to-side swaying or rolling motion
• Cause the most damage (pg. 254)
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Measuring
Earthquakes
6.3 Measuring Earthquakes
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.
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6.3 Measuring Earthquakes
Seismograph
• Records size,
direction, and
the movement
time of ground
• Records the
arrival times of
the P- and Swaves
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6.3 Measuring Earthquakes
Seismogram
• Record of the seismic waves
• Used to calculate the size and locations
of earthquakes
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6.3 Measuring Earthquakes
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 Swaves determines
the distance of the
seismograph from
the epicenter.
(p.260)
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Locating an
Epicenter
6.3 Measuring Earthquakes
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.
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6.3 Measuring Earthquakes
1. Find the arrival time differences.
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6.3 Measuring Earthquakes
2. Find the difference from the
epicenter.
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6.3 Measuring Earthquakes
3. Plot the distance on a map. (p. 263)
• 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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6.3 Measuring Earthquakes
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
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Earthquake Intensity
6.3 Measuring Earthquakes
• 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)
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Predicting
Earthquakes
6.3 Measuring Earthquakes
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.
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6.4 Earthquake Hazards and Safety
Avoiding Earthquake Hazards (p.274)
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