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Seismic Waves
Mechanical waves that travel
through the Earth.
Cause
• Any physical disturbance that
causes the Earth to vibrate
– Earthquakes (most commonly)
– Volcanoes
– Landslides (terrestrial or undersea)
– Extraterrestrial impacts (asteroids
– and meteorites)
Barringer Meteor Crater,
Arizona
Iron-nickel meteorite
49,000 years old
50 m in diameter
Impact speed 12.8 km/s
1.186 kilometers (.737 miles) in diameter
170 m in depth
Earthquakes
• Earthquakes occur when built-up
stress is suddenly released.
• Rupture or slippage of rock within the
Earth produce seismic waves
http://quake06.
stanford.edu/c
entennial/tour/s
top11.html
Earthquakes
2
Deformation
• Moving plates place stress on the earth
(1)compressive stress (push together)
(2) a tension stress (pull apart)
(3) a shear stress (moving past)
(4) torsion stress (twisting)
Earthquakes
2
Earthquake Waves
• Earthquake waves
travel out in all
directions from a point
where strain energy is
released. This point is
the focus.
• The point on Earth’s
surface directly above
the focus is the
epicenter.
Earthquakes
2
Energy Release
• When stress leads to strain, energy is released
suddenly, and it causes rock to lurch to a new
position.
• A fault is a crack along which movement
has taken place.
• The sudden energy release that goes with
fault movement is called elastic rebound.
The Earth’s Surface is in constant
motion!
• The Theory of Plate Tectonics explains
that the Earth’s surface is composed of
several brittle lithospheric plates that
move.
• Most earthquakes are caused by the
motion of the lithospheric plates.
Fig. 9-5, p. 191
Fig. 2-14, p. 38
Surface Waves
http://whs.moodledo.co.uk/file.php/1610/Ea
rthquakes/rayleighlove_lrg.gif
2
Body Waves—
seismic waves that pass through the Earth
Primary waves, also called P-waves, are
longitudinal waves (compressional).
• P-waves pass through solids and liquids
• P-waves are faster than s-waves.
Secondary waves, also called S-waves are
transverse waves.
• S-waves can travel through solids but
not liquids
• S-waves are slower than p-waves
Body
Fig. 9-8, p. 194
Longitudinal or
compressional
Or rarefactions
transverse
Fig. 9-9, p. 195
Fig. 9-10, p. 196
Gases emitted from the interior during this process are likely the source
for the formation of the atmosphere and oceans.
Fig. 1-10, p. 14
Internal Temperature of Earth
Crust-mantle boundary 800-1200 C
Core-mantle boundary
3500-5000 C
Temperature of the Earth increases with depth (25
degrees C per km, closer to the surface)
Fig. 1-10c, p. 14
Sources of Earth’s Internal Heat
• Heat from Earth’s formation (gravitational
contraction increases temperature of the
interior)
• Heat from extraterrestrial impacts (kinetic
energy to thermal energy)
• Heat from ongoing decay of radioactive
nuclides (radioactive particles and energy
increase temperature)
Fig. 1-11, p. 15
The Earth’s Layers
• Earth layers result from density differences between the
layers caused by variations in composition, temperature, and
pressure.
• Core: metal (Fe and small amount of Ni) [10-13 g/cm3]
• Outer liquid core
• Inner solid core
• Mantle: iron-rich rock (FeMg-Peridotite) [3.3–5.7 g/cm3]
• Crust: aluminum and magnesium rich rock
• Continental Crust: SiAl (rock) less dense [2.7 g/cm3]
• Oceanic Crust: SiMa (rock) darker, more dense [3.0 g/cm3]
Lithosphere and Asthenosphere
• Lithosphere is the solid, brittle outer layer of the
Earth composed of:
– Oceanic and continental crust
– Top part of the mantle
• Asthenosphere is the plastic layer of the mantle
directly below the lithosphere over which the
lithospheric plates move.
• The lithosphere is broken into many pieces called
plates.
Plate Boundaries
• Divergent Plate Boundary (oceanic ridges and
undersea volcanoes—see the Atlantic Ocean)
spread apart
• Convergent Plate Boundary (trenches and
volcanic mountain chains—see the Andes
Mountains) come together
• Transform plate boundary (side-by-side plate
motion—see the San Andreas Fault)--move past
Three types of plate boundaries
1.
Divergent plate boundary
2. Convergent Plate Boundary
3. Transform Plate boundary
Fig. 1-14, p. 18
The Mechanism for Plate Motion is Convection in the Mantle
Heat from the interior flows outward toward the crust
Fig. 1-12, p. 15
What is the evidence that the
Earth’s outer core is liquid?
P-waves
and Swaves
provide
seismic
evidence
that the
outer core is
liquid and
the inner
core is solid
Refraction:
the bending
of a wave as
it passes from
one medium
to another
Caused by
changes in
wave speed
Fig. 9-21, p. 210
Earth’s Interior
3
Solid Inner Core
• The fact that P-waves pass through the
core, but are refracted along the way,
indicates that the inner core is denser
than the outer core and solid.
• When pressure dominates, the inner
core remains solid, even at high
temperatures.
Earth’s Interior
3
Shadow Zones
• P-waves and S-waves travel through Earth
for 105 degrees of arc in all directions.
• Between 105 and 140 degrees from the
epicenter, nothing is recorded.
• This “dead zone” is termed the shadow
zone.
• This seismic pattern indicates that the
outer core is liquid.
Benioff Seismic Zone
(associated with a subduction zone
at a Convergent Plate Boundary)
Pattern of
earthquake
occurrences
indicates the
location of
the
subducted
limb of the
lithospheric
plate
Fig. 2-13, p. 37
Fig. 2-23, p. 46
• http://videos.howstuffworks.com/howstuffw
orks/230-how-tsunamis-work-video.htm
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