Earthquakes Chapter 2

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Earthquakes Chapter 2

• What is an earthquake?
• Vibration of Earth caused by sudden release of
  energy
• Usually displacement along faults (fracture)
• Also due to volcanoes, meteorite impacts, nuclear
  explosions
• Stress builds up until rock breaks/fault moves
• Shock waves sent through Earth are detected by
  seismometers

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Fig 2.7
1963-1988 M 5 EQs
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Rock Behavior

• Rock behavior ? how it responds to stress (force)
  placed on it
• Stress is force applied over an area
• Pounds per square inch
• Newtons / meter2
• Response is termed strain (deformation)
• Measured as a percentage change in rock length or
  volume
• Three stages or types of strain

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Strain or deformation

• Rocks subjected to stress change shape, size, and
  volume
• Three stages of strain
• Elastic deformation (reversible deformation)
• Ductile deformation (flow)
• Brittle deformation (fracture)

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Fig 2.4
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Evidence of Deformation

• Folds
• (ductile)
• Faults
• (brittle)

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Faults

• Fractures in the Earth on which movement has
  occurred
• Fault orientation is defined by strike and dip
• Blocks displaced by fault
• hanging wall
• footwall

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Types of faults
Strike slip Thrust Normal
Fig 2.9
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Dip-Slip Faults

• Movement occurs mainly in dip direction

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Strike-slip Faults

• Movement occurs mainly in strike direction
  little vertical offset
• Stand looking across fault notice movement of
  block on the other side which way do you look to
  find the same unit?
• If block moves to your left left-lateral
• If block moves to your right right-lateral

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Seismic Waves

• Seismic waves propagate in all directions outward
  from the focus
• Body waves
• P and S
• Surface waves
• Rayleigh
• Love

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Fig 2.13 a Simple seismometer
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Fig 2.12 a and b
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Fig 2.12c
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Measure of EQ size

• Magnitude
• Estimate of relative size or energy release of an
  EQ
• Reported by using Richter scale which was
  developed in 1935 for shallow EQs in CA
• Value determined by logarithmic value of
  seismogram ML
• Each unit increase means 10-fold increase in
  ground motion energy increase is about 32 - 40x

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Other magnitude scales

• mb body wave value uses P-wave amplitudes
• Ms Rayleigh surface wave amplitudes
• Mw - Moment magnitude depends on the amount of
  movement along the fault function of amount of
  strain energy released along entire fault rupture
  
• Largest recorded EQs
• 1960 Chile EQ Ms 8.5 Mw 9.5
• 1964 Alaska EQ Ms 8.3 Mw 9.2

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Square man-made events Circle seismic
events Triangle other sources
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Intensity scale

• Modified Mercalli Scale
• Established in 1902 by Mercalli Ital. and
  modified by Wood and Neumann in 1931 for
  California construction
• Subjective scale that measures observed damage
  inflicted on people and property
• Values are I-XII Roman numerals
• Values depend on EQ magnitude, distance from
  source, bedrock, bldg material and style,
  duration of shaking

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Earthquakes dont kill people -buildings do!

• Many deaths in older regions of earth due to poor
  construction especially through
  trans-Mediterranean/Asiatic belt
• Secondary events aftershocks destroy already
  weakened buildings
• Surface waves produce a great deal of damage