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Measuring Earthquakes


Measuring Earthquakes Ch. 2 Section 2 Measuring Earthquakes Every day there are about 8,000 earthquakes. Most too small to notice After an earthquake the two most ... – PowerPoint PPT presentation

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Title: Measuring Earthquakes

Measuring Earthquakes
  • Ch. 2 Section 2

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?

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
  • Epicenter The point on the surface directly
    above the focus.

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

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.

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

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.

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

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.

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.

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
  • Work well for small nearby earthquakes, not for
    large distant ones.

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

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
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
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