The Geology Behind Earth

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The Geology Behind Earths Features

• Natural Disasters

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Essential Questions

• What is an earthquake?
• What is the way to detect an earthquake?
• What is a Tsunami?
• What are the ways to prepare for a natural
  disaster?

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What is an earthquake?
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Bill Nye - Earthquakes

• http//learning.aliant.net/Player/ALC_Player.asp?P
  rogIDDEP_BN027
• Complete the Question sheet provided.
• 24mins

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• Earthquake
• Occur at the boundaries between tectonic plates
• Occur as a result of the forces of stress,
  strain, and strength

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• Stress
• Is the local force per unit area that causes
  rocks to deform
• Strain
• Is the relative amount of deformation, expressed
  as the percentage of distortion (eg. compression
  of a rock by 1 of its length)
• Strength
• Rocks fail that is they lose cohesion and break
  into two or more parts when they ae stressed
  beyond a critical value

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Seismic Waves

• There are three types of seismic waves
• P-waves
• S-waves
• Surface waves

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P-waves

• Primary waves, also called P waves or
  compressional waves
• P waves arrive first at any surface location
• can travel through solid, liquid and gas
• Can travel through rock at 6km/s
• are waves that have the same direction of
  vibration along their direction of travel, which
  means that the vibration of the medium (particle)
  is in the same direction or opposite direction as
  the motion of the wave
• As they travel through rock, the waves move tiny
  rock particles back and forth -- pushing them
  apart and then back together
• http//www.geo.mtu.edu/UPSeis/images/P-wave_animat
  ion.gif

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S-waves

• Secondary waves, also called S waves or shear
  waves
• As these waves move, they displace rock particles
  outward
• S waves don't move straight through the earth
• only travel through solid material
• the ground is displaced perpendicularly to the
  moves alternately to one side and then the other
• http//www.geo.mtu.edu/UPSeis/images/S-wave_animat
  ion.gif

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Surface Waves

• sometimes called long waves, or simply L wave
• are responsible for most of the damage associated
  with earthquakes, because they cause the most
  intense vibrations
• stem from body waves that reach the surface
• are something like the waves in a body of water
  -- they move the surface of the earth up and down
• L waves are the slowest moving of all waves

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• Both P and S waves refract or reflect at points
  where layers of differing physical properties
  meet. They also reduce speed when moving through
  hotter material. These changes in direction and
  velocity are the means of locating
  discontinuities.
• Seismic discontinuities (a surface at which
  velocities of seismic waves change abruptly) aid
  in distinguishing divisions of the Earth into
  inner core, outer core, D", lower mantle,
  transition region, upper mantle, and crust
  (oceanic and continental).

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Earthquakes

• Did you know
• According to the United States Geological Survey,
  more than three million earthquakes occur every
  year. That's about 8,000 a day, or one every 11
  seconds!

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How does an Earthquake occur?
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How do Earthquakes Occur?

• When brittle rocks being stressed suddenly fail
  along a geologic fault
• Most large earthquakes are caused by ruptures of
  pre-existing faults, where past earthquakes have
  already weakened the rocks on the fault surface
• The two blocks of rock on either side of the
  fault slip suddenly, releasing energy in the form
  of seismic waves
• When the fault slips, the stress is reduced,
  dropping to a level below he rock strength

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• A fault rupture does not happen all at once
• It begins at the focus and expands outward along
  the fault surface 2-3km/s
• It stops where the stresses become insufficient
  to continue breaking the fault or where the
  rupture enters ductile material in which it can
  no longer propagate as a fracture
• Fault ruptures in the largest earthquakes can
  extend for more than 1000km and the fault slip
  can be as large as 20m

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Earthquake Terminology

• Focus
• The point at which fault slipping begins
• Epicenter
• The geographic point on Earths surface directly
  above the focus
• Focal depth
• In continental crust is 2-20km
• Below 20km is rare because under the high
  temperatures and pressures the crust behaves as a
  ductile material
• However in subduction zones where cold oceanic
  crust plunges into the mantle earthquakes can
  originate at depths as great as 690km

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Earthquake Terminology continued

• Foreshock
• A small earthquake that occurs near, but before,
  a mainshock
• Aftershock
• Large earthquakes trigger smaller earthquakes
• Follow the mainshock
• Their foci are distributed in and around the
  rupture plane of the mainshock
• Happen where that stress exceeds he rock strength
• The number and sizes depend on the magnitude of
  the mainshock
• P 348 fig 13.6

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The Elastic Rebound Theory

• Proposed by Henry Fielding Reid of John Hopkins
  University in 1910
• Explains why earthquakes recur on active faults
• P345 Fig 13.1

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• P 347 fig 13.5

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What is the way to detect an earthquake?
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Detection

• Detecting an earthquake is much easier than
  predicting one.
• a powerful earthquake can be felt by people in
  the area, and the damage it causes can be seen.
• Seismograph
• An instrument that records the seismic waves
  generated by earthquakes
• Seismogram
• A record, graphed or digital, of the seismic
  activities of an area

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Ideal Seismographs

• It would be a device affixed to a stationary
  frame not attached to Earth when the ground
  shook, the seismograph would measure the changing
  distance between the frame which did not move and
  the vibrating ground

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Current Seismographs

• Attach a dense mass, such as a piece of steel to
  Earth so loosely that the ground can vibrate up
  and down or side to side without causing much
  motion of the mass
• Attachment is usually a spring (for vertical
  movement) or hinge (for horizontal movement)
• When seismic waves move the ground, the mass
  tends to remain stationary because of its
  inertia, but the mass and the ground move
  relative to each other because the spring
  compresses or stretches or the hinge swings left
  and right
• Record bthe movements automatically

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Reading a Seismogram

• When you look at a seismogram, there will be
  wiggly lines all across it. These are all the
  seismic waves that the seismograph has recorded.
• Most of these waves were so small that nobody
  felt them.

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Reading a Seismogram continued

• The P wave will be the first wiggle that is
  bigger than the rest of the little ones (the
  microseisms).
• Because P waves are the fastest seismic waves,
  they will usually be the first ones that your
  seismograph records.
• The next set of seismic waves on your seismogram
  will be the S waves.
• These are usually bigger than the P waves.
• If there aren't any S waves marked on your
  seismogram, it probably means the earthquake
  happened on the other side of the planet. S waves
  can't travel through the liquid layers of the
  earth so these waves never made it to your
  seismograph.
• The surface waves are the other, often larger,
  waves marked on the seismogram.
• They have a lower frequency.
• Surface waves travel a little slower than S
  waves.

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Finding the Epicenter

• Measure the distance between the first P wave and
  the first S wave. In this case, the first P and S
  waves are 24 seconds apart.
• Find the point for 24 seconds on the left side of
  the chart below and mark that point. According to
  the chart, this earthquake's epicenter was 215
  kilometers away.
• Measure the amplitude of the strongest wave. The
  amplitude is the height (on paper) of the
  strongest wave. On this seismogram, the amplitude
  is 23 millimeters. Find 23 millimeters on the
  right side of the chart and mark that point.
• Place a ruler (or straight edge) on the chart
  between the points you marked for the distance to
  the epicenter and the amplitude. The point where
  your ruler crosses the middle line on the chart
  marks the magnitude (strength) of the earthquake.
  This earthquake had a magnitude of 5.0.

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Magnitude

• Is related to the total area of the fault rupture
• Most earthquakes are very small and the rupture
  never breaks the ground surface
• In large earthquakes surface breaks are commonb

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The Richter Scale

• The Richter Scale is the best known scale for
  measuring the magnitude of earthquakes.
• The energy released by an earthquake increases by
  a factor of 30 for every unit increase in the
  Richter scale.
• An earthquake that measures 4.0 on the Richter
  scale is 10 times larger than one that measures
  3.0

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Richter scale no. No. of earthquakes per year Typical effects of this magnitude
lt 3.4 800 000 Detected only by seismometers
3.5 - 4.2 30 000 Just about noticeable indoors
4.3 - 4.8 4 800 Most people notice them, windows rattle.
4.9 - 5.4 1400 Everyone notices them, dishes may break, open doors swing.
5.5 - 6.1 500 Slight damage to buildings, plaster cracks, bricks fall.
6.2  6.9 100 Much damage to buildings chimneys fall, houses move on foundations.
7.0 - 7.3 15 Serious damage bridges twist, walls fracture, buildings may collapse.
7.4 - 7.9 4 Great damage, most buildings collapse.
gt 8.0 One every 5 to 10 years Total damage, surface waves seen, objects thrown in the air.
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What is a Tsunami?
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What are the ways to prepare for a natural
disaster?