Dynamic Earth

1
Dynamic Earth

• Class 8
• 2 February 2006

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Any Questions?
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NoteExam 1Date shown on hardcopy syllabus
asFebruary 14th, but said February 15th.The
correct date is February 14th (On-line version is
correct)
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What Plate TectonicsTheory explains

• Age distribution of oceanic crust(and why the
  oceans are MUCHyounger than the continents)
• Distribution of earthquakes and volcanoes

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Seismology

• Study of the propagation of mechanical energy
  released by earthquakes.
• When energy is released in this fashion, waves of
  motion (like the effect of a pebble tossed into a
  pond) are set up in the Earth.

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Earthquakes

• earthquake movement of rock bodies past other
• fault locus of the earthquake movement
• faults come at all scales, mm to separation of
  lithospheric plates (e.g., San Andreas).

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Earthquake terms

• focus site of initial rupture

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Focus point where an earthquake originates
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Earthquake terms

• focus site of initial rupture
• epicenter point on surface above the focus

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Seismic Waves Radiate from the Focus of an
Earthquake
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Earthquakes
Most damage from ground shaking
Eric Marti/AP Photo
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Chile, May 22, 1960

• Magnitude 9.6 earthquake, the largest earthquake
  ever recorded
• The city of Valdivia suffered catastrophic damage
• Severe shaking lasted for over 15 minutes
• Coastal areas subsided
• A 10-meter tsunami was generated

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Chile, May 22, 1960

• The earth ruptured for 450 miles along the Chile
  coast
• The area of rupture was the size of California

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Elastic Rebound Theory
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Elastic Rebound Theory
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Elastic Rebound Theory
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Elastic Rebound Theory
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1906 San Francisco Earthquake
Fig. 18.2
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1906 San Francisco Earthquake
Fault Offset (2.5m)
Fault Trace
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Seismic waves

• Waves are started because of initial tension or
  compression in the rock.
• Instruments used to measure these waves are
  called seismographs.

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Seismographs Record Vertical or Horizontal Ground
Motion
Vertical
Horizontal
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Modern Seismograph
Kinematics
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Seismograph Record and Pathway of Three Types of
Seismic Waves
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Two kinds of waves from earthquakes

• P waves (compressional) 68 km/s. Parallel to
  direction of movement (slinky), also called
  primary waves. Similar to sound waves.
• S waves (shear) 45 km/s. Perpendicular to
  direction of movement (rope) also called
  secondary waves. Result from the shear strength
  of materials. Do not pass through liquids.

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Comparison of P-wave and S-wave Motion
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Two Types of Surface Waves
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Time Lag Between S and P waves with Distance from
Epicenter
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Seismic Travel-time Curve
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Locating the Epicenter
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Locating an epicenter

• The difference between the arrival times of the P
  and S waves at a recording station is a function
  of the distance from the epicenter.
• Therefore, you need three stations to determine
  the location of an epicenter.

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Measuring the force of earthquakes

• 1. Surface displacement
• 1964 Alaska earthquake displaced some parts of
  the seafloor by 50 ft.
• 1906 San Francisco earthquake moved the ground
  8.5 ft.
• 2. Size of area displaced
• Alaska 70,000 sq. miles

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Measuring the force of earthquakes

• 3. Duration of shaking
• Up to tens of seconds
• 4. Intensity scales
• Based on damage and human perception
• 5. Magnitude scales
• Based on amount of energy released

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Modified Mercalli Intensity Scale

• I Not felt
• II Felt only by persons at rest
• IIIIV Felt by persons indoors only
• VVI Felt by all some damage to plaster,
  chimneys
• VII People run outdoors, damage to poorly built
  structures
• VIII Well-built structures slightly damaged
  poorly built structures suffer major damage
• IX Buildings shifted off foundations
• X Some well-built structures destroyed
• XI Few masonry structures remain standing
  bridges destroyed
• XII Damage total waves seen on ground objects
  thrown into air

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Richter scale

• Richter scale amount of energy received 100 km
  from epicenter
• Largest quake ever recorded 9.6 (rocks not
  strong enough for more).
• Earthquakes less than M 2 are not felt by
  people.
• Scale is logarithmic
• Increase 1 unit 10 times greater shaking
• Increase 1 unit 30 times greater energy

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Maximum Amplitude of Ground Shaking Determines
Richter Magnitude
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Richter Magnitude Versus Energy
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Earthquake prediction

• Long termimprecise (can be done)
• Short termprecise (very difficult)
• We can't stop earthquakes, so we have to be
  prepared for them.

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New Housing Built Along the 1906 Trace of the San
Andreas Fault
R.E. Wallace, USGS
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Seismic Hazard Map
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Distribution of earthquakes

• Not random
• Focused in linear zones

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World Seismicity, 19632000
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Earthquake distribution defines plate boundaries
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The Earth has a number of layers, just like an
egg or an apple
Core
Mantle
Crust
Yoke
Core
?
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Earths layers
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Lithosphere
Asthenosphere
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Plates

• Rigid Lithosphere with definite boundaries
• Can have both oceanic and continental crust or
  just one kind.

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Note that crust under continentsis thicker (45
km)than under oceans (8 km).
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The Earths Major Plates
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Types of plate boundaries

• Divergent mid-ocean ridges
• Convergent collision zones
  volcanic arcs
• Strike-slip San Andreas Fault
  (California) Anatolian Fault
  (Turkey)

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Divergent (Spreading)
Convergent (Subduction Zone)
Transform
Three Types of Plate Boundaries
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Earthquakes Associated with Divergent and
Transform Margins
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Strike-slip Faults
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Subduction Zones
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Earth-quakes in subduction zones
Benioff- Wadati Zone
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Convection within the Earth
Lithosphere is created at spreading centers and
destroyed at Trenches (Subduction Zones)
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(No Transcript)
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Subduction zones
Ocean-continent convergence
Ocean-ocean convergence
Continent-continent collision
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Growth of Continents

• Addition of volcanic arc material to continent
  (e.g. Andes).
• Collisions Continents
• Collisions Exotic terranes

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Exotic terranes approaching arcor microcontinent
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Collision
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Accreted Terrane
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The West Coast of North America
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Exotic Terranes plastered to the continent at
subduction zones
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Any Questions?
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Tuesday

• Finish reading Chapter 3 in the text

Thursday

• Homework 3 due