Ch' 19 Earthquakes

1
Ch. 19 - Earthquakes

• Earthquakes are caused gigantic fractures in the
  Earths crust, which produce ground vibrations.
• The pressure (force/area) acting on the rocks
  involved forms fractures when this stress exceeds
  the strength of the rocks.
• The deformation of materials in response to
  stress is called strain.
• 3 Types ? compression, tension, and shear.
• 1. Compression occurs with a decrease in volume
  of the material.
• 2. Tension occurs when the material pulls apart.
• 3. Shear occurs when the material twists.
• Fig. 19-1 (pg. 496)

2

• Fault movement of the Earths crust causing a
  fracture or system of fractures.
• 3 Types of Faults
• 1. Reverse fault occurs when land compresses
  together horizontally
• 2. Normal fault occurs when land separates from
  one another horizontally.
• 3. Strike-slip fault occurs when land slides
  left and right of one another horizontally.
• Earthquakes are caused by movements along faults.
  As stress continues to build in these rocks,
  they reach their elastic limit, break, and
  produce an earthquake.

3
Types of Seismic Waves

• Seismic waves are vibrations of the ground during
  an earthquake.
• 1. Primary waves (P waves) rocks are squeezed
  and pulled in the same direction in which the
  waves are traveling.
• 2. Secondary waves (S waves) rocks move at
  right angles in relation to the direction of the
  waves.
• 3. Surface waves move in two directions as they
  pass through rock. Similar to an ocean wave.

4

• The point were an earthquake originates is the
  focus.
• The focus is usually several kilometers below the
  Earths surface.
• The epicenter is the point directly above the
  focus at the Earths surface.
• What is the study of earthquake waves?
• Seismology
• Seismic waves provide us with information to
  understand the Earths interior.
• What are seismometers or seismographs?
• Instruments used to detect and record vibrations
  sent out from earthquakes. They can shake the
  entire Earth.

5

• The seismogram is the paper portion of the
  seismograph that records the vibrations
• Fig. 19-8
• A travel-time graph displays the distance and
  time it takes for p-waves and s-waves to travel.
• Fig. 19-9
• P S-waves travel through the crust and mantle.
  Only the P-wave travels through the outer and
  inner core of the Earths interior.
• Fig. 19-10
• The disappearance of S-waves allows scientists to
  theorize that the Earths outer core must be a
  liquid.

6

• Studies of seismic waves shows that the
  lithosphere (crust upper mantle) is primarily
  igneous rocks granite, basalt, (crust) and
  peridotite (liquid mantle).
• The Earths core and mantle are made up similar
  components to that of a meteorite iron and
  nickel. Scientists study meteorites to
  indirectly study the Earths core and mantle.

7
Measuring Locating Earthquakes

• Approximately 1 million earthquakes occur
  annually.
• Little if any cause damage and are not felt.
• Magnitude is the amount of energy released during
  an earthquake. The magnitude is rated on a scale
  called the Richter scale based on the size of the
  largest seismic wave generated by the quake.
• 1-10
• A magnitude of 8 is ten times larger than 7 and
  100 times larger than 6.

8

• Moment magnitude scale is rated on the size of
  the fault rupture, the amount of movement along
  the fault, and the rocks stiffness
• Most seismologist use this scale.
• The values are estimated from several seismic
  waves rather than the largest generated.
• Modified Mercalli scale rates the earthquake
  based on the amount of damage done to the
  structures involved. It uses Roman numerals I to
  XII.
• Table 19-1 pg. 507
• MiniLab page 508

9

• Locating an earthquake
• Locations of several seismic stations are used by
  plotting the radius or epicentral, which is the
  distance away from the station which detects the
  greatest seismic intensity with calculations.
  The time it takes for the seismic waves to get to
  the station can be used to calculate this also.
• Fig 19-14
• Seismic belts are the global distribution of
  these epicenters. Most of the seismic belts are
  narrow regions.
• Fig. 19-15 (pg. 510)
• Are there any trend or patterns on the map?

10
Earthquakes Society

• Most of the damage produced by an earthquake is
  to buildings and deaths may occur because of it.
  Brick, stone, and concrete buildings damage
  easily compared to wooden buildings.
• Some buildings even rest on large rubber
  structures that absorb most of the vibrations.
• Some major earthquakes include the following
  Mexico City (1985) destroyed lots of buildings,
  Turkey (1999), Peru (1970) 7.8 magnitude due
  to landslides from the quake, Taiwan (1999) - 7.6
  magnitude vertical offsets up to 10 m height.

11

• Tsunami large ocean waves generated by vertical
  motions of the seafloor during an earthquake.
• The Indian Ocean tsunami of December 26, 2004
  with a 9.0 magnitude.
• The death toll exceeded 225,000.
• Most of the earthquakes that are predicted are
  based on probability from the history of
  earthquakes in an area and the rate at which
  strain builds up in the rocks.
• Seismic gaps are active faults that havent
  experience significant earthquakes for a long
  period of time.
• Lasers ? Fig. 19-20