Transcript seismic waves notes-0 - Fort Thomas Independent Schools

Seismic Waves
Mechanical waves that travel
through the Earth.
http://earthguide.ucsd.edu/eoc/teachers/t_t
ectonics/p_paleomag.html
Main ideas of plate tectonics:
1. Earth’s surface is composed of lithospheric
plates
2. Plates are moving
3. Moving plates change the location of
continents and alter the surface of the earth
4. Sea floor moves to carry the continents
5. Come together (convergence), spread apart
(divergence) and move past (transform)
along plate boundaries
• 6. Earth’s inner core is solid, its outer core
is liquid. The composition of both metal:
iron and some nickel. Even though the
core is hot enough to melt, it is under too
much pressure to melt in the inner core.
• 7. Earth mantle is solid igneous rock.
• 8. The core is hotter than the crust.
• 9. Convection in the mantle (hot mantle
rises, cooler mantle sinks)
• 10. The Decay of radioactive elements
keeps the interior of the Earth warm
Cause of
Seismic Waves
• 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)
The Nature of Waves
1
Seismic Waves
• Forces in Earth’s crust can cause regions
of the crust to shift, bend, or even break.
• The breaking
crust vibrates,
creating seismic
(SIZE mihk)
waves that carry
energy outward.
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.
Seismic Waves
• Seismic waves are produced by earthquakes
when stresses build up by moving plates are
suddenly released.
• Body waves vs. surface waves
• Interior waves produced by this disturbance
include longitudinal waves or p-waves and
transverse waves or s-waves.
• P-waves are faster than s-waves, and can travel
through solids or liquids. S-waves cannot travel
through liquids.
• The epicenter and focus of an earthquake can
be calculated using seismic data from at least
Surface Waves
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
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
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
1.
Heat from Earth’s formation
– gravitational contraction increased
temperature of the interior)
– Heat from extraterrestrial impacts (kinetic
energy to thermal energy)
2. 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?
(See next slide)
P-Waves
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
S-Waves
Caused by
changes in
wave speed
Fig. 9-21, p. 210
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. 2-19, p. 43
Fig. 2-23, p. 46
• http://videos.howstuffworks.com/howstuffw
orks/230-how-tsunamis-work-video.htm
http://www.uwiseismic.com/General.aspx?i http://www.geogateways.com/toolkit/ggima
ges/tsunami1.jpg
d=23
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, atoms are
squeezed together tightly and exist in
the solid state.
• If temperatures are high enough, atoms
move apart enough to exist in the liquid
state, even at extreme pressures.
Earthquakes
2
Earthquake Measurement
• The Modified
Mercalli scale ranks
earthquakes in a
range from I-XII,
XII being the worst
and uses eyewitness
observation and postearthquake
assessments to assign
an intensity value.
Earthquakes
2
Earthquake Measurement
• The Richter magnitude scale uses the
amplitude of the
largest
earthquake wave.
• Richter magnitude
is intended to give
a measure of the
energy released
during the
earthquake.
Earthquakes
2
Earthquake Measurement
• The table shows
the global
frequency of
different
magnitude
earthquakes.
Earthquakes
2
Levels of Destruction
• Research has shown that poor building
methods are the largest contributors to
earthquake damage and loss of life.
Earthquake Proofing
• Although no building can be made entirely
earthquake proof, scientists and engineers are
finding ways to reduce the damage to
structures during mild or moderate
earthquakes.
The Nature of Waves
1
Seismic Waves
• Seismic waves are a combination of
longitudinal (p-waves) and transverse
waves (s-waves). They can travel
through Earth and along Earth’s surface.
• The more the
crust moves
during an
earthquake, the
more energy is
released.
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