Quaking, Shaking, Earth

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Quaking, Shaking, Earth

• All about Earthquakes

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What is an earthquake?

• Simply put
• An earthquake is the shaking of the earth.

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Many buildings in Charleston, South Carolina,
were damaged or destroyed by the large earthquake
that occurred August 31, 1886.
Picture from the United States Geological Service
www.usgs.gov
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San Francisco, California, Earthquake April 18,
1906. East side of Howard Street near Seventeenth
Street. All houses shifted toward the left. The
tall house dropped from its south foundation wall
and leaned against its neighbor. 1906. Picture
from USGS.GOV
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San Fernando, California, Earthquake February
1971. Collapsed overpass connecting Foothill
Boulevard and the Golden State Freeway. Feb 10,
1971. Photo by R.E. Wallace, USGS. www.usgs.gov
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What causes an earthquake?

• Earthquakes are the Earth's natural means of
  releasing stress.
• Due to the constant motion of the Earth plates,
  this put stress on the edges of the plates.
• To relieve this stress, the rocks tend to bend,
  compress, or stretch.

An aerial view of the San Andreas fault in the
Carrizo Plain, Central California. Picture from
www.usgs.gov
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Locatin' the Shakin'

• Focus the place on the Earths crust where the
  pressure was released.
• Epicenter the spot on the Earths surface
  directly above the focus.

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Faults

• If the force is great enough, the rocks will
  break.
• An earthquake is the vibrations produced by the
  breaking of rock.
• Most earthquakes occur near plate boundaries.

The Hanshin expressway in Kobe, Japan collapsed
due to an earthquake in 1995. Picture from
http//www.ce.washington.edu/liquefaction/html/qu
akes/kobe/kobe.html
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Normal Fault

• Rock above the fault surface moves downward in
  relation to rock below the fault surface.

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Reverse Fault

• Reverse faults result from compression forces
  that squeeze rock.
• If rock breaks from forces pushing from opposite
  directions, rock above a reverse fault surface is
  forced up and over the rock below the fault
  surface.

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Strike-slip Fault

• At a strike-slip fault, rocks on either side of
  the fault are moving past each other without much
  upward or downward movement.
• The San Andreas Fault is the boundary between two
  of Earths plates that are moving sideways past
  each other.

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How does energy created by an earthquake move
through the Earth?

• Seismic waves are energy waves that travel
  outward from the source of the earthquake.

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Surfin the Waves

• When earthquakes occur, three different types of
  seismic waves are produced.

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P Waves

• Primary waves (P-waves) cause particles in rocks
  to move back and forth in the same direction that
  the wave is traveling.
• P-Waves are the fastest waves and are felt first,
  usually as a bang or a thump.

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S Waves

• Secondary waves (S-waves) move through Earth by
  causing particles in rocks to move at right
  angles to the direction of wave travel.
• These waves are slower than P-Waves.

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L Waves

• Surface waves cause most of the destruction
  resulting from earthquakes.
• Surface waves (L-Waves) or land waves move rock
  particles in a backward, rolling motion and a
  side-to-side, swaying motion.

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How are seismic waves measured?

• Seismic waves from earthquakes are measured with
  an instrument known as a seismograph.
• Seismographs register the waves and record the
  time that each arrived.

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Measuring Earthquake Magnitude

• Magnitude is a measure of the energy that is
  released during an earthquake.

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Richter Scale

• The Richter magnitude scale is used to describe
  the strength of an earthquake and is based on the
  height of the lines on the seismogram.

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Richter Scale

• For each increase of 1.0 on the Richter scale,
  the height of the line on a seismogram is ten
  times greater.
• However, about 32 times as much energy is
  released for every increase of 1.0 on the scale.

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Earthquake Severity

• Richter Earthquake Magnitudes Effects
• Less than 3.5 Generally not felt, but recorded.
• 3.5-5.4 Often felt, but rarely causes damage.
• Under 6.0 At most slight damage to well-designed
  buildings. Can cause major damage to poorly
  constructed buildings over small regions.
• 6.1-6.9 Can be destructive in areas up to about
  100 kilometers across where people live.
• 7.0-7.9 Major earthquake. Can cause serious
  damage over larger areas.
• 8 or greater Great earthquake. Can cause serious
  damage in areas several hundred kilometers
  across.