What are Earthquakes?

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What are Earthquakes?

• The shaking or trembling caused by the sudden
  release of energy
• Usually associated with faulting or breaking of
  rocks
• Continuing adjustment of position results in
  aftershocks

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What is the Elastic Rebound Theory?

• Explains how energy is stored in rocks
• Rocks bend until the strength of the rock is
  exceeded
• Rupture occurs and the rocks quickly rebound to
  an undeformed shape
• Energy is released in waves that radiate outward
  from the fault

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Earthquakes Building and Releasing Stress
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• The Focus and Epicenter of an Earthquake

• The point within Earth where faulting begins is
  the focus, or hypocenter
• The point directly above the focus on the surface
  is the epicenter

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Where Do Earthquakes Occur and How Often?

• 80 of all earthquakes occur in the
  circum-Pacific belt
• most of these result from convergent margin
  activity
• 15 occur in the Mediterranean-Asiatic belt
• remaining 5 occur in the interiors of plates and
  on spreading ridge centers
• more than 150,000 quakes strong enough to be felt
  are recorded each year

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What are Seismic Waves?

• Response of material to the arrival of energy
  fronts released by rupture
• Two types
• Body waves
• P and S
• Surface waves
• R and L

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SEISMIC WAVES
Body Waves Travel outward in all directions from
the focus and have the capacity to travel through
the Earth's interior. P (primary) waves
-compression/expansion S (secondary) waves -
shear Surface Waves Travel around but not
through the Earth they are guided by the Earth's
surface.
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Body Waves P and S waves

• Body waves
• P or primary waves
• fastest waves
• travel through solids, liquids, or gases
• compressional wave, material movement is in the
  same direction as wave movement
• S or secondary waves
• slower than P waves
• travel through solids only
• shear waves - move material perpendicular to wave
  movement

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BODY WAVES
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P- (Compressional) Waves
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S- (Shear) Waves
Note S-waves cannot propagate through liquids
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Surface Waves R and L waves

• Surface Waves
• Travel just below or along the grounds surface
• Slower than body waves rolling and side-to-side
  movement
• Especially damaging to buildings

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• Seismographs record earthquake events

At convergent boundaries, focal depth increases
along a dipping seismic zone called a Benioff zone
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Seismometers(seismographs)
Inertia (Newtons 2nd law of gravity) governs
the movement of earthquakes and the functioning
of instruments to measure them seismometers.
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How is an Earthquakes Epicenter Located?

• Seismic wave behavior
• P waves arrive first, then S waves, then L and R
• Average speeds for all these waves is known
• After an earthquake, the difference in arrival
  times at a seismograph station can be used to
  calculate the distance from the seismograph to
  the epicenter.

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How is an Earthquakes Epicenter Located?

• Time-distance graph showing the average travel
  times for P- and S-waves. The farther away a
  seismograph is from the focus of an earthquake,
  the longer the interval between the arrivals of
  the P- and S- waves

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How is an Earthquakes Epicenter Located?

• Three seismograph stations are needed to locate
  the epicenter of an earthquake
• A circle where the radius equals the distance to
  the epicenter is drawn
• The intersection of the circles locates the
  epicenter

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Quantifying Earthquake Intensity
Richter magnitude scale calculated based on the
amplitude (height) of the recorded
waves modified Mercalli intensity scale
estimated based on damage relative to set
criteria (Roman numerals) moment magnitude
calculated based on the amount of energy released
(distance of slip x area of slip x stiffness of
rock)
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How are the Size and Strength of an Earthquake
Measured?

• Magnitude
• Richter scale measures total amount of energy
  released by an earthquake independent of
  intensity
• Amplitude of the largest wave produced by an
  event is corrected for distance and assigned a
  value on an open-ended logarithmic scale

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Can Earthquakes be Predicted?

• Earthquake Prediction Programs
• include laboratory and field studies of rocks
  before, during, and after earthquakes
• monitor activity along major faults
• produce risk assessments

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This photograph by Arnold Genthe shows Sacramento
Street and approaching fire.
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The Great fire as seen from a ferry boat in the
bay
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Refugees search for souvenirs on sidewalk in
front of the Stanford mansion on Powell Street at
California.
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This photograph, taken from a tethered balloon
five weeks after the great earthquake
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PHOTOGRAPHS OF DAMAGE ASSOCIATED WITH THE 1964
ALASKA EARTHQUAKE
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Government Hill Elementary School in Anchorage
was one of many schools severely damaged by
quake-triggered landslides.
These thick trees near Port Valdez splintered
into pieces against the force of massive tsunami
waves.
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Secondary Effects
Natural - Landslides/mudflow  -
Liquifaction - Subsidence - Tsunamis
Man-made - Dam failures - Secondary fires
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Landslides/mudflow
Earthquake induced landslide - Madison River,
Montana, 1959 - 80 million tons of rock released
as a result of the quake- River was dammed
creating Quake Lake
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Tsunamis - small amplitude, long wavelength
in open ocean - speed 350-500 mph         -
near shore the waves begin to break and the
enormous quantity of water piles up        -
breaking wave may be 15 - 65 m high - Aleutian
Islands, 1946 10 hours later hits Hawaii
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Earthquake Effects Tsunami
A tsunami is a giant wave or sequence of waves
produced by large water displacements. These are
most commonly caused by underwater earthquakes.
Tsunamis can be destructive long distances from
their sources.
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Tsunami
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Locations of past tsunamis around the world
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Liquefaction
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Liquefaction Features
Loma Prieta, CA,1989
El Centro, CA,1979
Water loaded with suspended sediment that blasts
through to the surface creates sandboils.
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Earthquakes and Seismology
Taiwan, September 1999
Earthquakes have devastated human civilization
sporadically through time, yet the information
gained through studies of earthquake vibrations
is fundamental to our understanding of the inner
workings of the Earth.
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Damage from Earthquakes
The extent of damage from an earthquake commonly
hinges on the geology rock/soil of contrasting
mechanical properties or degree of water
saturation
(as demonstrated in the Turnagain Heights, AK
quake of 1964).
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Seismic Risk
If earthquakes usually come from movement on
faults, and faults are most active at plate
margins, then why are their such seismic hazards
within the North American plate?
movement on old faults
answer
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Earthquakes and Plate Tectonics
It should be clear that the pattern of earthquake
foci on the globe is not random. Earthquakes are
concentrated on plate margins.