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Earthquakes
DEF.: earthquake _ a trembling or shaking of the ground caused by the sudden release
of strain
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Seismic Waves
DEF.: the waves or vibrations cause by an earthquake.
Represent the TRANSMISSION OF ENERGY away from the point of displacement.
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Where do earthquakes occur?
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• Plate margins
• The Circum Pacific belt
• From about 5km to 700 km depth
• 90% originate shallower than 100 km
Zones of earthquake foci in 1965 near the Tonga Islands
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Seismology
• The study of earthquake waves, seismology, dates back almost 2000 years to the
Chinese
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Seismographs
Instruments that record seismic waves
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Seismograph for horizontal movement
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Types of seismic waves
1)Body waves
2)Surface Waves
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Types of seismic waves
1) Body waves
–Travel through Earth materials
–Two types, P and S
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Two types of body waves
• Primary (P) waves
– Push-pull motion, changing the volume of the intervening material
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Two types of body waves
• Primary (P) waves
– Travel through solids, liquids, and gases
– about 1.7 times faster than S waves
1
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Two types of body waves
• Secondary (S) (shear) waves
– Slower velocity than P waves
– Slightly greater amplitude
– Only move through solids
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Surface waves
• Complex motion, cause greatest destruction
• Have greatest amplitude and slowest velocity
• Have the greatest periods (time interval between crests)
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Video
Seismogram showing arrival of P, S, and surface waves
LOCATING THE EPICENTER
• Def: focus _ the center of release of earthquake energy
• Def: epicenter _ the point on the Earth's surface directly above the focus
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LOCATING THE EPICENTER
• Time-travel graphs from 3 or more seismograms
• Handout: "Great Fargo Earthquake"
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Measuring the size of earthquakes
• Intensity
• Magnitude
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Modified Mercalli Intensity Scale
Measures effect on people and buildings
Based on human observation
Expressed in Roman numerals I-XII
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Magnitude scales
Richter scale (Charles Richter, 1935)
– DEF.: magnitude _ measure of the total energy released by
an earthquake
– amplitude of largest seismic wave recorded
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2
23
Richter Scale
Logarithmic with regard to:
1)Ground motion (wave amplitude) factor of 10
Ex: Magnitude 2
3 = 10x more shaking than 2
4 = 100x
5 = 1000x
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Richter Scale
• Logarithmic with regard to:
2) Energy released by factor of ~30!
Ex: Magnitude 2
3 = 30x more energy than 2
4 = 900x more energy
5 = 27,000x more energy
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Richter scale
• Largest magnitude recorded on a Wood-Anderson
seismograph was 8.9
• Can measure from –2 to ~ 9 (but no set upper limit)
• Underestimates energy released by great earthquakes
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Table 16.2
Cartoon of seismograph going off scale
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Other magnitude scales
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Moment magnitude (Mw) scale
• Can measure huge earthquakes
• Displacement along a fault
• Ex: Alaska 8.3; Mw = 9.2
Chile 8.5; Mw = 9.6
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Table 16.4
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What are the causes of earthquake destruction?
READ pp. 459-466
• Ground shaking
• Liquifaction
• Seiches
3
• Tsunami
• Landslides and subsidence
• Fire
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End of Chapter 16
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