Earthquake Basics

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Earthquake Basics
Basics
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What was the biggest earthquake EVER?

• Thanks to the extensive records of the United
  States Geological Service (USGS), it is quite
  easy to learn about the biggest earthquake ever
  recorded, which occurred near Santiago, Chile, in
  1960. Known as the Great Chilean Earthquake, the
  quake measured an impressive 9.5 on the Richter
  scale. The death toll of the Great Chilean
  Earthquake is not known, but estimates go as high
  as 6,000, and the earthquake may have caused as
  much as 800 billion US Dollars in damage.
• Pictures to follow from http//www.aggleton.name/
  001820Chilean20Earthquake/earthgqake01.htm

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Why do earthquakes happen?

• Rocks deform plastically when stressed.
• When a rocks elastic limit is exceeded- it
  fractures- releasing ENERGY.

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• Earthquake sudden motion caused by the abrupt
  release of energy that is stored in rocks.

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WAVES

• Waves transmit energy
• Seismic waves waves that travel through rock
• Seismology study of earthquakes and Earths
  interior using seismic waves

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Focus vs. Epicenter?

• Focus rupture point where energy is released
• shallow-focus earthquakes occur within 70 km
  (crust) of the surface
• intermediate-focus occur 70-300 km (mantle) below
  the surface
• deep-focus occur 300-600 km below the surface at
  subduction zones
• Epicenter point on Earths surface directly
  above the focus

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• WHY DONT EARTHQUAKES OCCUR ANY DEEPER THAN
  600km???

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• Ans High pressure and temp. at those depths
  cause the rocks to exhibit plastic behavior (flow
  rather than fracture)

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Types of waves

• Body waves waves that travel through the
  Earths interior (from focus to surface)
• P wave compressional elastic wave (primary),
  travel through solids and liquids
• S wave shear wave (secondary), solids only!
• Surface waves slower waves that travel through
  the crust, radiate from epicenter
• Rolling (up/down) waves
• Side-to-side vibrations

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S waves ?
P waves ?
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Surface Waves
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Measuring seismic waves

• Seismometer the instrument
• vs.
• Seismogram/graph the record it makes

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Measuring Earthquakes

• Mercalli scale measures damage (or intensity)
• Richter scale measures energy released
• Moment-magnitude measures energy as a function
  of movement and fault surface area (most accurate)

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Locating earthquakes

• P-S waves travel at different speeds
• This allows us to calculate distance (time-travel
  curve)
• Distances from multiple observations finds a
  location (triangulation)

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Where do earthquakes occur?

• Plate boundaries!

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Transform boundaries

• Ex. San Andreas
• Strike-slip fault
• Fault is vertical
• Movement is along the line of the fault
• Fault creep small, slow movements along fault

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1906 San Francisco earthquake
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Hazard map Probability of an earthquake w.
magnitude greater than 5 in the next 30 yrs.
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Convergent boundaries

• When one plate subducts under another
• Benioff zone friction along the down-plunging
  contact zone
• Ex. Chilean
• earthquakes,
• Alaskan
• earthquake

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Divergent boundaries

• Divergent boundaries spreading centers
• Friction along sliding blocks
• Mainly shallow

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Middle of plates?

• Sometimes
• 1811-12, New Madrid, MO
• Rumored to have rung church bells in Boston!
• Fault formed 600 million years ago- N. Am. Plate
  tried to split but didnt
• TODAY- N. Am. Plate is being compressed b/c of
  seafloor spreading- this may activate fault zone.

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• Why would an earthquake in the Midwest cause more
  damage than one is San Francisco?

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• Ans Building codes not upgraded for earthquake
  standards