Earthquakes

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Earthquakes
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Earthquake Pre-Quiz

• What causes earthquakes
• plate movements
• faulting
• magma movements
• liquid injection
• reservoir filling
• deep excavation

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Earthquake Pre-Quiz (continued)

• Name a state with a large number of earthquakes
• California
• Alaska
• Oregon
• Washington
• Hawaii
• Missouri

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Earthquake Pre-Quiz (continued)

• What conditions in that state cause earthquakes?
• California - San Andreas Fault Zone
• Alaska - convergent boundary
• Oregon - convergent boundary
• Washington - convergent boundary
• Hawaii - magma movement
• Missouri - reactivation of ancient rift

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Earthquake Pre-Quiz (continued)

• Which best describes earthquake hazard in PA?
• a) earthquakes never happen here
• b) gentle, non-damaging about once every
  100 years
• c) slightly damaging earthquakes about
  every 100 years
• d) slightly damaging about every 10 years
• e) slightly damaging every 10, injuring
  possible
• f) slightly damaging every 10, injuring
  every 100 years
• g) injuring every 10, killing every 100
• h) killing every 10, extreme every 100
• i) extreme every 10

lt-- at least
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Earthquake Pre-Quiz (continued)

• Which best describes earthquake hazard in CA?
• a) earthquakes never happen here
• b) gentle, non-damaging about once every
  100 years
• c) slightly damaging earthquakes about
  every 100 years
• d) slightly damaging about every 10 years
• e) slightly damaging every 10, injuring
  possible
• f) slightly damaging every 10, injuring
  every 100 years
• g) injuring every 10, killing every 100
• h) killing every 10, extreme every 100
• i) extreme every 10

lt-- about here
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Earthquake Frequency

• Over 1,000,000 per year (worldwide)
• most small - unfelt by people
• "instrument" earthquakes
• 1,000 cause damage
• 20 "Killers"

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The U.S. has been lucky

• 1906 San Francisco - 500 killed
• 1946 Alaska - 150 killed
• 1964 Alaska - 114 killed
• 1556 China - 830,000 killed
• 1737 India - 300,000 killed
• 1976 China - 240,000 killed

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Earthquakes are difficult to study

• Short duration (2 seconds - 2 minutes)
• Leave little or no trace
• Occur below the ground

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Earthquakes are not solitary events

• Foreshocks
• small earthquakes before a major event
• possible predictive value
• Aftershocks
• smaller earthquakes after a major event
• damage during rescue
• used to map subsurface faults

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Earthquake Anatomy
Epicenter
Fault
Focus
Seismic Waves
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Elastic Rebound Theory

• How it was discovered
• Change in building lot geometry after
• San Francisco Earthquake of 1906

Before
After

• Sudden release of stress changed shape

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Elastic Rebound Theory
building stress
unstressed
deformation
breaking point
"Snap," vibrations released
Unrestrained positions
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Earthquake Definition

• Ground shaking caused by the sudden release of
  energy stored in subsurface rock units.

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Earthquake Definition

• Vibrations caused by the breakage or movement of
  subsurface rock units.

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Measuring Earthquake Strength

• Mercalli Intensity Scale
• (effects on structures and humans)
• Richter Magnitude
• (based upon the amount of energy
  released)
• Other Scales

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Magnitude Scales

• Used for news reports
• Related to the amount of energy released
• Require a seismograph
• One magnitude per earthquake

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Richter Magnitude

• -3 -2 -1 0 1 2
  3 4 5 6 7 8 9
  10 11

architectural damage
structural damage
total destruction
"instrument earthquakes"
"felt"
Greater than about 9 - unlikely (rocks can't
store that much energy)
Each step on the scale is 10x the ground motion
/ 33x the energy
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Seismograph
rotating drum
spring
weight