Predicting Earthquakes

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Predicting Earthquakes

• Short-Range Predictions
• Identifying possible precursors such as changes
  in stress and strain in rocks along a fault.
• Recognizing foreshocks

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October 17,1989 Loma Prieta Earthquake

• Two foreshocks occurred along the same section of
  the San Andreas Fault system as the mainshock
• ML 5.3 in June, 1988
• ML 5.4 in August, 1989
• State Office of Emergency Services issues
  short-term advisory for a possible larger
  earthquake (expired 2 months before the main
  shock).
• Increased probability of ML 6.5 earthquake for
  that particular section of the San Andreas Fault
  system.

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Predicting Earthquakes

• Long-Range Forecasts
• Give the probability of a certain magnitude
  earthquake occurring.
• Given on a time scale of 30 to 100 years or more.
• Important for updating building codes for
  designing earthquake-resistant structures.
• Important for insurance companies.

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• Research in 1970s Denali Fault could produce a
  MW 8 earthquake with 6 m of displacement.
• 2002 MW 7.9 earthquake occurred with 5 m of
  displacement.
• No environmental disaster due to proper
  engineering.

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Predicting Earthquakes

• Long-Range Forecasts
• Based on the premise that earthquakes are
  repetitive or cyclical.
• As soon as quake is over, plates will continue to
  move - strain will build in rocks again.
• Compile a chronology of earthquake events along a
  fault.

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If major earthquakes occur every 200 years along
the San Andreas Fault, which regions would be at
the greatest risk?
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Seismic Gaps occur where earthquakes have not
recently released accumulating elastic energy
likely to be regions for future earhquakes.
A recent subduction zone earthquake occurred in a
seismic gap that had not had a major earthquake
since 1877.
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Mw 9.0, 1952
Mw 8.3, 2006
Mw 8.5, 1963
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Mw 9.1, 2004
Mw 8.6, 2005
Mw 8.5, 2007
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Long-Term Forecasting

• Paleoseismology study of timing, location, and
  size of prehistoric earthquakes.
• Often achieved by digging trenches across a fault
  zone.

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Earthquake Waves and Earths Internal Structure
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• In 1909, Yugoslavian seismologist, Andrija
  Mohorovicic presented the first convincing
  evidence for layering within the Earth.
• Found that seismic waves speed up below 50 km
  depth boundary between crust and mantle.
• Greater wave velocity Denser rock.
• Boundary named the Mohorovicic discontinuity
  surface (Moho).

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Shadow Zone belt around the Earth where no
direct P and S waves are detected - evidence that
the Earths outer core is liquid.
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Earthquake Review Questions
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What type of motion is associated with primary
seismic waves?

1. Horizontal bending or shearing.
2. Rolling motion like that of ocean waves.
3. Compression and expansion of material.
4. Vertical bending or shearing.

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Secondary waves cannot travel through the Earths
outer core? What does this indicate?

1. Very dense and solid.
2. Soft, but still solid material.
3. Cannot indicate the properties of the material.
4. Completely liquid.

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Which of these statements best describes the
cause of most earthquakes along faults?

1. Magma migrating upward along the fault.
2. Rocks deform and store elastic energy until they
   rupture and release the energy.
3. Fault creep in the absence of friction.
4. Tidal forces from the Moon and Sun cause movement
   along the fault.

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The exact point of origin for an earthquake along
a fault.

1. Hypocenter
2. Epicenter
3. Seismic Center
4. Fault Plane
5. Ground Zero

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The magnitude of an earthquake can be determined
on the Richter scale if which of the following is
known?

1. Amount of damage to human structures.
2. Distance to the epicenter.
3. Amplitude of the largest wave recorded on the
   seismogram.
4. Both 1 and 2.
5. Both 2 and 3.

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How much more energy is released during a
magnitude 9 earthquake than during a magnitude 8?

1. 10 times more
2. 32 times more
3. 100 times more
4. Twice as much
5. None of the above

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An earthquakes moment magnitude can be
accurately determined from

1. Amount of damage to human structures.
2. Duration of an earthquake.
3. Amount of displacement along a fault and the
   surface area of the rupture.
4. Only the amplitudes of the largest seismic waves.
5. Both 1 and 2.

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Which is an earthquake-induced phenomenon during
which water-saturated sediments turn into a
mobile fluid?

1. Wave amplification
2. Tsunami
3. Elastic Rebound
4. Liquidation
5. Liquefaction

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Which of the following would not likely produce a
tsunami?

1. Horizontal displacement of the ocean floor along
   a transform fault.
2. Vertical displacement of the ocean floor along a
   subduction zone.
3. Giant underwater landslide.
4. The collapse of a section of seafloor during a
   major volcanic eruption.
5. A large portion of a landmass sliding into the
   ocean.

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Along what type of plate boundary do a majority
of the largest recorded earthquakes occur?

1. Divergent
2. Transform
3. Convergent

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Which of the following locations would be the
safest when the next big earthquake occurs in
Oregon?

1. A concrete/brick structure built on sediment
   deposits.
2. A steel-frame building on solid bedrock.
3. A wood-frame house in a low elevation area along
   the coast.
4. A wood-frame house on steep bedrock slopes in the
   Coast Range.
5. Any structure built on water-saturated sediment.