Measuring Earthquakes

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Measuring Earthquakes

• Ch. 2 Section 2

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Measuring Earthquakes

• Every day there are about 8,000 earthquakes.
• Most too small to notice
• After an earthquake the two most common questions
  are how big was it? and where was the center
  of the quake?

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Measuring Earthquakes

• Most earthquakes begin in the lithosphere within
  100 km of Earths surface.
• Focus the point beneath Earths surface where
  rock that is under stress breaks, triggering an
  earthquake.
• Epicenter The point on the surface directly
  above the focus.

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Seismic Waves

• Seismic Waves vibrations that travel through
  Earth carrying the energy released during an
  earthquake.
• The first seismic waves first reach the Earths
  surface at the epicenter.
• The epicenter also has the greatest amount of
  energy from an earthquake.

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3 Types of Seismic Waves

• Primary Waves P waves, first waves to arrive to
  the surface.
• Compress and expand the ground.
• Travel through solids and liquids.

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3 Types of Seismic Waves

• Secondary Waves S waves, arrive after the P
  waves.
• Vibrate from side to side and thrust the ground
  up and down, or back and forth.
• Shake structures violently.
• Travel through solids only.

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3 Types of Seismic Waves

• Surface Waves created from P waves and S waves.
• Move more slowly than P and S waves.
• Produce the most severe ground movements.

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Detecting Seismic Waves

• Seismograph - An instrument used by geologists
  to record the ground movements of an earthquake.

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Measuring Earthquakes

• There are at least 20 different measures for
  rating earthquakes.
• Most common ways are the Mercalli scale, the
  Richter scale, and the moment magnitude scale.

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The Mercalli Scale

• Developed to measure the earthquakes intensity,
  or the strength of the ground motion.
• Describes how earthquakes affect people,
  buildings, and the land surface.
• Tells roughly how much damage may be caused.

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Richter Scale

• Rating of the size of seismic waves.
• Developed in the 1930s.
• Used by geologists all over the world for over 50
  years, but has been replace by electronic
  seismographs.
• Work well for small nearby earthquakes, not for
  large distant ones.

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The Moment Magnitude Scale

• Used by todays geologists.
• Estimates the total energy released by an
  earthquake.
• Use on earthquakes of all sizes at any location.
• Scientists use data from electronic seismographs

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Magnitude TNT Equivalent Example
1.0 30 lb Construction site blast
2.0 1 ton Large quarry or mine blast
3.0 29 ton  
4.0 1 kiloton Small atomic bomb
5.0 32 kiloton Nagasaki atomic bomb
6.0 1 megaton Double Spring Flat, NV Quake, 1994
7.0 32 megaton Largest thermonuclear weapon
8.0 1 gigaton San Francisco, CA Quake, 1906
9.0 32 gigaton Indian Ocean Quake 2004
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Locating the Epicenter

• Geologists use seismic waves to locate an
  earthquakes epicenter.
• Measure the difference between the arrival times
  of the P and S waves.
• Use the data from 3 different seismographs.
• Draw circles of the radius from each station to
  the epicenter.
• Where the 3 circles come to together is the
  epicenter.