Earthquakes

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Earthquakes

• Earthquake the shaking or trembling caused by
  the sudden release of energy as faulting
  displaces rocks along fractures
• Aftershocks continuing adjustments along a fault
  following an earthquake
• Most earthquakes occur along tectonic plate
  boundaries and are often associated with volcanism

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Earthquakes

• Elastic rebound theory In an earthquake, rocks
  are subjected to stresses which are released as
  rocks snap back to their original shape and
  position along a fault plane
• Earthquake focus the point where fracturing of
  rocks begin (also known as hypocenter)
• Epicenter the point on the earths surface
  directly above the focus

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ELASTIC REBOUND
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Earthquake Focus and Epicenter
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Earthquake Focus Depths

• Earthquakes classified according to depth
  shallow focus occur at lt 70 km, intermediate at
  70-300 km, and deep focus at gt 300 km below this
  depth rocks do not fracture
• 90 of all earthquakes occur at lt 100 km

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Earthquake Focus Depths
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Earthquakes

• Seismology the study of earthquakes
• Seismic waves energy waves that radiate out from
  the point of stored energy release when an
  earthquake occurs
• Seismograph an instrument that detects, records,
  and measures the vibrations produced by an
  earthquake
• Seismogram the record of an earthquake produced
  by a seismograph

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Seismograph
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Seismogram
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Earthquakes

• Earthquake duration reflects the length of the
  fault and the time it takes for the stressed rock
  to snap back to its undeformed shape
• 95 of earthquakes occur at plate margins
• 80 of all earthquakes on earth occur in the
  Pacific Ring of Fire
• 5 occur within crustal plates, primarily from
  localized stresses related to compression along
  margins
• 150,000 earthquakes recorded each year

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Pacific Ring of Fire
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Seismic Waves

• Two main types of seismic waves
• Body waves waves that travel through the solid
  body of the earth
• Surface waves waves that travel along the
  Earths surface, much like waves on water
• Body waves occur as either
• P-waves- (primary waves), compressional waves,
  the fastest seismic waves, travel through solids,
  liquids, and gases
• S-waves- slower moving than P-waves, are shear
  waves, moving material perpendicular to the
  direction of travel, and can not travel through
  non-solids that do not have shear strength
• Since P-waves move faster they always arrive at
  seismic recording stations before S-waves

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Body Waves P Waves
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Body Waves S Waves
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Body Waves
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Surface Waves

• Surface waves travel along the surface causing a
  rolling or swaying motion, in two different
  patterns
• Rayleigh waves move in an elliptical pattern
  much like ocean waves
• Love waves move in an S-shaped lateral direction
  along the surface

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Surface Waves
Love Waves
Rayleigh Waves
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Locating Earthquakes

• Because P-waves move faster than S-waves,
  measuring the difference between arrival times
  (known as the P-S interval) at recording stations
  can provide the distance from an earthquake
• Comparing the difference in arrival times at
  three different stations allows determination of
  the epicenter of an earthquake this is known as
  triangulation

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Earthquake Triangulation
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Measuring earthquakes

• Intensity a qualitative assessment of earthquake
  damage, uses the Modified Mercalli Intensity
  Scale, which provides descriptions of earthquake
  characteristics at ground level to describe
  different levels of intensity
• Magnitude measurement of release energy,
  determined by measuring the amplitude of the
  largest seismic wave recorded on a seismogram,
  and plotting that data along with the distance
  from the epicenter on the Richter Magnitude Scale

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Estimated Intensity of San Francisco
Earthquake, 1906
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RichterMagnitude Scale
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Earthquake effects

• Ground shaking
• Liquefaction
• Fire San Francisco, 1906, 90 of damage
• Seismic sea waves Hilo, 1946, 16.5 m
• Ground failure rockslides, avalanches

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Liquefaction
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Earthquake prediction

• Although much is now known about how earthquakes
  occur and where faults are located, the ability
  to predict when they will occur is still not very
  well-developed
• Seismic risk maps show where earthquakes are
  likely and their potential severity help in
  planning and preparation

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Seismic Risk Map of U.S.