Geology Earthquakes pptx Science

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					Earthquakes
A Violent Interlude
Definition of earthquake
  -vibration of the Earth's by shock
  waves generated by energy released
  from rocks rupturing under stress
  -most (not all) earthquakes and
  ruptures occur along faults
  -most faults and stress occurs along
  active plate tectonic boundaries
            Fault Types

Dip-Slip [normal]   Strike-Slip [right-lateral]
Elastic rebound theory

Crustal blocks move through time
  -“locked” segment along the fault:
   no slip

Rocks are strained: deformation of
shape
  -the strain can be released
   catastrophically

Energy released causes earthquake
Analogy: breaking rubber band
Other causes of earthquakes:

  Landslides, rockslides, or slumping of
    rocks.

  Movement of magma, gases, or rocks
    associated with volcanism
Morphology of earthquakes and related
terms

  Focus or Hypocenter
    -exact location of the earthquake disturbance-with few
    exceptions is located below the surface of the Earth

  Epicenter
    -the position on the
    surface of the Earth
    directly above the focus

    Slip: the distance of
    displacement along the
    fault
Foreshock(s)

 -Earthquake(s) generated at or very near
 the focus of the main earthquake prior to
 the main shock

 -smaller magnitude than the main shock
 and precede main quake by a short or
 substantial time interval
Main shock
  -is the main earthquake disturbance
  generated at the focus

Aftershock(s)
  -Earthquake(s) generated at or very near
  the focus of the main earthquake following
  the main shock
  -smaller in magnitude than the main shock.
  Follow the main earthquake by a short or
  substantial time interval
Seismic waves

-waves which travel within the Earth

-P (primary) wave-moves in a straight line
path with alternating compression and
expansion

-S (secondary) wave-moves in a sinusoidal
motion along its path of movement
Principles of P and S Waves
      P-Wave Propagation




P waves
Compressional waves – similar to sound waves
– Series of contractions and relaxations
– Fastest, ~5 km/sec (depends on rock type)
– Travel through solid, liquid and gas
         S-Wave Propagation




S waves
Shear waves – motion is right angles to direction of wave
– Half speed of P waves
– Travel only through solid
P-wave




S-wave
Surface Waves
-move along the Earth’s
surface radiating out from
epicenter
 – two types:
          Rayleigh & Love
Seismographs
-seismograph is the machine that detects
seismic waves.
-seismogram is the printed record.
Seismograms from multiple Seismographs around
San Francisco Bay Area
Locating a focus/epicenter:

1. Need a seismogram from at least 3 different seismic
stations.

2. Examine the seismograms and determine the elapsed
time between the arrival of the first P-wave and the first
S-wave.
3. Use a travel-time graph

-knowing the S- & P-wave travel-
time, you can determine the
distance to the focus/epicenter
from the seismic station.
4. On a map, draw a circle around the
seismic station.

5. Repeat for two other seismic
stations. (Triangulation.)
Measuring Earthquakes
Several different scales exist to measure earthquakes.

   Modified Mercalli Scale

   Measures earthquake intensity.
   -describes damage to structures.
   -ranges from I (felt by only a few) to XII (total destruction).

   Intensity depends on:
       -Distance to epicenter
       -Building materials and design
       -Type of ground material - soil, rock, etc.
Modified Mercalli Intensity Scale

   I. Not felt except by a very few under especially favorable conditions.
   II. Felt only by a few persons at rest, especially on upper floors of buildings.
   III. Felt quite noticeably by persons indoors, especially on upper floors of buildings.
   IV. Felt indoors by many, outdoors by few during the day. At night, some awakened.
   Dishes, windows, doors disturbed; walls make cracking sound.
   V. Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects
   overturned.
   VI. Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen
   plaster.
   Damage slight.
   VII. Damage negligible in buildings of good design and construction; slight to moderate in
   well-built ordinary structures; considerable damage in poorly built or badly designed
   structures.
   VIII. Damage slight in specially designed structures; considerable damage in ordinary
   substantial
   buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys,
   factory stacks, columns, monuments, walls. Heavy furniture overturned.
   IX. Damage considerable in specially designed structures; well-designed frame structures
   thrown out of plumb. Damage great in substantial buildings, with partial collapse.
   Buildings shifted off foundations.
   X. Some well-built wooden structures destroyed; most masonry and frame structures
   destroyed with foundations. Rails bent.
   XI. Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly.
   XII. Damage total. Lines of sight and level are distorted. Objects thrown into the air.
Richter Scale

-Measures earthquake magnitude.
-determined by measuring the amplitude of the
largest waves on the seismogram.

-logarithmic scale.
   -each number on the Richter Scale is ten times greater in
   wave amplitude.

-each number on the Richter Scale involves an energy
release about 32 times as great.
Moment magnitude scale
-measures the amount of energy released
  – Designed to differentiate large earthquakes
  – May be used to calculate energy of old events by
    slip along fault
  – Moment = µ A D
  – µ = shear modulus A = LW = area
    D = average displacement during rupture
Moment
Magnitude for
Indonesian
Quake [2004]
Locations of Earthquakes

Most occur along tectonic plate boundaries:
   -around the Pacific Ocean (circum-Pacific belt is major site of
   large earthquakes)
   -Mediterranean Sea area through Iran and on to the Himalayas
   -Asia (Indonesia, Himalayan region)
   -Mid-ocean ridges

Some occur far from plate edges
-intraplate earthquakes

				
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posted:3/16/2011
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