Introduction to Seismology:

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Introduction to Seismology

• Jesse F. Lawrence

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Earthquakes

• Earthquakes have long excited the minds of man.

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Fundamental Idea of Seismology

• The Earth ruptures, releasing vibrational energy.
• Vibrational energy travels through the Earth.
• A seismometer records the vibrations some
  distance away.

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Ray Theory

• Seismic waves can be idealized as rays.
• They fan out in all directions from an earthquake.

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Ray Theory Examples

• Seismic waves can be idealized as rays.
• The rays fan out in all directions from an
  earthquake.
• The rays can bounce off of objects in the Earth.
• The surface
• The core

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Ray Theory Examples

• Sometimes the waves make multiple bounces up and
  down through the Earth.

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Ray Theory Examples

• Energy can also bounce off of smaller interfaces
• Like interfaces between sediment layers
• The crust-mantle boundary

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Seismic Reflection Survey

• Man made sources and receivers can be placed at
  regular distances.
• Energy bounces off of interfaces at shallow
  depths.

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First Motion Fault Orientation

• Energy leaving an Earthquake can have positive or
  negative amplitude
• Motion toward (positive) or away (negative) from
  the receiver.
• We can back out the direction of motion of the
  fault.

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Inversion

• Based on the data, we develop a model of the
  Earth.
• Calculate synthetic data based on the input
  model
• Reduce misfit with the real data.

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Data v. Uncertainty

• As the amount of data goes up, the uncertainty
  goes down

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Fault Planes Plate Boundaries
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Fault Planes Plate Boundaries
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Fault Planes Plate Boundaries
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Earthquake Frequency v. Magnitude
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Earthquake Frequency v. Magnitude

• There are fewer large Earthquakes than small, but
  the large earthquakes release more energy than
  the small earthquakes.

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Notable Earthquakes
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Earthquakes Need not be Disastrous

• In 1996 it was determined that on average in the
  US more people died of skateboarding than
  Earthquakes
• Building codes
• Earthquake preparedness
• Search rescue

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Likelihood of Building Collapse EQ Intensity
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Most Active Region in the US
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San Andreas Fault
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Seismic Hazard

• Seismic hazard is effected by
• Closeness to Earthquake
• Likelihood of Flood
• Likelihood of Landslide
• Likelihood of Tsunami
• Ground Stability

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Seismic Energy and Distance
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Seismic Hazard
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Seismic Hazard
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Seismic Gaps
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Vibrational Energy Attenuates
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Damage in Valdez after 1964 Alaska EQ
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Displacement after 1906 San Andreas
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Loma Prieta, 1989 Liquefaction Effects
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Bay Bridge after Loma Prieta, 1989
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Van Norman Dam after 1971 San Fernando EQ

• Liquefaction made the dam sink

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Bridge After Northridge
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Bridge After Northridge
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Liquefaction During Northridge
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Tsunami, 1964, Alaska
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Landslide From Loma Prieta, 1989
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Animals Earthquakes?

• Elephants communicate over kilometers at
  frequencies similar to earthquakes.
• Some animals may sense the first-arriving P-wave
  and act up.
• Humans only notice the later arriving surface
  waves that knock over buildings.

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Nuclear Sources
Lots of Energy coupled to the ground!!!!
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Nuclear Sources
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Sizes of Nuclear Tests
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Earthquakes v. Nuclear Sources

• Nuclear sources tend to have only higher
  frequencies

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Earthquakes v. Nuclear Sources

• Nuclear explosions have less long-period energy,
  so estimations of source size based on surface
  waves is much lower.