Chapter 4
EARTHQUAKES OUTLINE
One million in last century only (all over the world)
Early Explanations of Earthquakes
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Culturally & Scientifically
e.g., Earth rested on some SUPER ORGANISM whose
movement caused earthquake
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In Japan, the organism was GIANT CATASH
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Mongolia - A GIANT FROG
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China - An OX
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India - A GIANT MOLE
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S. America - A WHALE
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Algonquin Indians & N. America - A TORTOISE
Others: A punishment from God reinforced by quake in Lisbon, Portugal on
ALL SAINTS DAY, Nov. 1, 1775 - 70,000 people died.
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A Greek philosopher - Aristotle first explained quake
by natural causes
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Today, geologists believe that quake is caused
by faulting related to PLATE TECTONICS
Study of Earthquakes
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SEISMOLOGY using instrument SEISMOGRAPH which records, detects & measures
vibrations produced by earth quake
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Energy is released from FOCUS or HYPOCENTER in Earth crust
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Point at surface vertically above Focus Is EPICENTER (newspaper)
Categories of Earthquakes
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3 broad types
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SHALLOW FOCUS - focal length < 70 km
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INTERMEDIATE FOCUS - between 70 - 300 km
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DEEP FOCUS - >300 km
LOCATION
Most quakes associated with plate boundaries
Shallow Focus Earthquakes: Found mainly at divergent & transform
boundary.
Intermediate & Deep Focus quakes mainly at Convergent boundaries
Seismic Waves:
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Earthquakes cause ground shaking by producing 2 main types of waves
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Body Waves: Travel through earth
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Surface Waves: Travel along surface
Body Waves: - Types of Body waves
- Produce sharp shaking
a. P-Wave or Primary Waves:
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Fastest seismic waves
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Can travel through solids, liquids, gases
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Compressional, or push-pull waves
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Move material forward & backward in direction of wave movement
b. S-Waves or Secondary or Shear Wave
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Slower than P waves
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Travel through solid only
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Move material perpendicular to the direction of wave movement
Surface Waves: - Produce rolling or swaying motion
- 2 main types
a. Rayleigh Waves (R- waves)
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Behave like water waves
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Move forward while particles of material move in elliptical path
in vertical plane in direction of wave
b. Love Wave (L-waves)
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Move like S-waves
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But individual particles of material move back & forth in horizontal
plane to the direction of wave travel
Locating an Earthquake
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P, S, R, L waves are used
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P are fastest, S travel ½ velocity
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P & S travel from focus to the surface
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L & R follow the P & S at surface
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L & R are last to arrive at seismograph
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By collecting data on all 4 waves
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The difference in arrival time of P & S waves is function of
the distance from the focus to seismograph -> lab
Measurement of Intensity of Magnitude
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Int. & magnitude measure the strength of a quake
INTENSITY: A qualitative measure of type of damage -> it is subjective
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The common scale is (US) Mercalli Intensity Scale (Mercalli-Italian
seismologist)
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Has values I to XII
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Modified for use in US by Wood & Neuman - Cal. Tech. 1931
e.g. I - Not felt except for very favourable conditions.
VII - Every body runs out door. Damage is negligible…
XII - Damage is total, Waves seen at surface…
Magnitude: A measure of energy releases by quake
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quantitative
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A scale developed F. Ritcher, 1935
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a seismologist at Cal. Tech.
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Open ended scale beginning at 1.
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Largest measured so far is 8.6
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To determine scale
Rit. Scale is logarithmic.
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