Inside Earth: Chapter 2- Earthquakes

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Inside Earth Chapter 2- Earthquakes

• Section 2 Measuring Earthquakes

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Guide For Reading

• What are the different kinds of seismic waves?
• How does the energy of an earthquake travel
  through Earth?
• What are the scales used to measure the strength
  of an Earthquake?

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There are about ______ earthquakes per day,
worldwide.

• 8,000

4
Most earthquakes begin below the earths surface
in the _______ within ______ kilometers from
Earths surface.

• Lithosphere
• 100

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Focus

• The point beneath Earths surface where rock
  breaks under stress and causes an earthquake

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Epicenter

• The point on Earths surface directly above an
  earthquakes focus

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Seismic Waves
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Seismic Waves

• A vibration that travels through Earth carrying
  the energy released during an earthquake

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Figure 11 At what point do seismic waves first
reach the surface?

• The epicenter

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What determines how much the ground shakes during
an earthquake?

• How close a location is to the epicenter
• The types of rock and soil surrounding the
  epicenter determines how much the ground shakes

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Guide For Reading What are the different kinds
of seismic waves?

• There are three types of seismic waves
• Primary Waves (P waves)
• Secondary Waves (S waves)
• Surface Waves
• An earthquake sends out two types of waves P
  waves and S waves
• When the waves reach Earths surface at the
  epicenter, surface waves develop.

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Primary Waves
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Primary Waves (P Waves)

• A type of seismic wave that compresses and
  expands the ground
• The first wave to arrive at an earthquake

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Describe how P waves move?

• P waves compress and expand like an accordion
• When P waves arrive they vibrate the particles of
  the crust forward and back along the path of the
  wave

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Secondary Waves
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Secondary Waves (S Waves)

• A type of seismic wave that moves the ground up
  and down or side to side

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How are P waves different from S Waves?

• P waves compress and expand the ground
• These waves cause buildings to contract and
  expand
• P waves travel through solids and liquids
• S waves vibrate from side to side
• These waves vibrate from side to side and thrust
  the ground up and down, or back and forth
• S waves can not move through liquids

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Surface Waves
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Surface Waves

• A type of seismic wave that forms when P waves
  and S waves reach Earths surface

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Why do you think surface waves produce more
severe ground movements than P waves and S waves?

• Surface waves consist of loose soil, sand,
  gravel, mud, small rocks, not solid rock
• These more loose substances are more likely shift
  and slide

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Guide For Reading How does the energy of an
earthquake travel through Earth?

• Seismic waves carry the energy of an earthquake
  from the focus, through Earths interior, to the
  epicenter, and across the surface

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Detecting Seismic Waves
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Seismograph

• A device that records ground movements caused by
  seismic waves as they move through Earth

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Describe how a mechanical seismograph records
ground movement.

• A heavy weight attaches to a frame by spring or
  wire
• A pen connected to the weight rests its point on
  a rotating drum
• During an earthquake the seismic waves cause the
  drum to shake while the pen stays in place
• The pen records lines on the paper around the drum

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Measuring Earthquakes
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There are at least ______ different measures for
rating earthquakes.

• 20

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Magnitude

• The measurement of an earthquakes strength based
  on seismic waves and movement along faults

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The Mercalli Scale
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Mercalli Scale

• A scale that rates earthquakes according to their
  intensity and how much damage they cause
• Developed in the early twentieth century

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An earthquakes ______ is the strength of ground
movement in a given place.

• Intensity

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The Mercalli scale has ______ steps and
describes how an earthquake affects ______,
______, and the ______.

• 12
• People
• Buildings
• Land surface

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Figure 14 How would you rate the damage to the
Foligno city hall on the Mercalli scale?

• The damage would probably rate VII - VIII

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The Richter Scale
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Richter Scale

• A scale that rates seismic waves as measured by a
  particular type of mechanical seismograph

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How does the Richter scale measure an earthquake?

• The Richter scale measures seismic waves using a
  seismograph
• Over time, mechanical seismographs were replaced
  by electronic seismographs

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What is a strength of the Richter scale when
measuring earthquakes? What is a weakness?

• Strength Provides accurate measurements for
  small, nearby earthquakes
• Weakness Does not measure larger and more
  distant earthquakes well

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How are the Mercalli scale and the Richter scale
similar? How are they different?

• Both measure the strength of an earthquake
• The Mercalli scale measures the strength in terms
  of extent people notice the earthquake and the
  amount of damage caused
• The Richter scale measures the size of seismic
  waves

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The Moment Magnitude Scale
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Moment Magnitude Scale

• A scale that rates earthquakes by estimating the
  total energy released by an earthquake

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Why is the moment magnitude scale used today by
geologists to measure earthquakes?

• The moment magnitude scale determines the total
  energy released by an earthquake
• This scale uses a electronic seismograph that can
  measure earthquakes that are big or small, and
  near or far
• Geologist examine movement along the fault and
  the strength of broken rock
• These two measurements give a more accurate
  measurement of an earthquake

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On which scale would an earthquakes strength
vary from one place to another? Explain.

• The Mercalli scale because the amount of shaking
  that people would feel and the damage to objects
  would be greater in a place closer to the
  earthquakes epicenter

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Checkpoint (Page 68)What are three scales for
measuring earthquakes? Explain what each one
measures.

• Mercalli Scale
• Measures earthquakes on how much damage they
  cause
• Richter Scale
• Measures seismic waves using a seismograph
• Measures small and nearby earthquakes
• Moment Magnitude Scale
• Looks at the total energy released
• Measures large and distant earthquakes
• Helps scientists predict how much fault movement
  their was

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Locating the Epicenter
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How do scientists calculate how far a location is
from the epicenter of an earthquake?

• Scientists calculate the difference between
  arrival times of the P waves and S waves
• The further away an earthquake is, the greater
  the time between the arrival of the P waves and
  the S waves

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Figure 17 Use the map scale to determine the
distances from Savannah and Houston to the
epicenter. Which one is closer?

• Houston
• 800 Km
• Savannah
• 900 km