Seismic Waves

Mechanical waves that travel through the Earth.

http://earthguide.ucsd.edu/eoc/teachers/t_tectonics/p_paleomag.html

Main ideas of plate tectonics:1. Earth’s surface is composed of lithospheric

plates2. Plates are moving3. Moving plates change the location of continents

and alter the surface of the earth4. Sea floor moves to carry the continents5. 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

49,000 years old

1.186 kilometers (.737 miles) in diameter

170 m in depth

Iron-nickel meteorite

50 m in diameter

Impact speed 12.8 km/s

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/centennial/tour/stop11.html

Earthquakes

2

• Moving plates place stress on the earth(1)compressive stress (push together)

(2) a tension stress (pull apart)

(3) a shear stress (moving past)

Deformation

(4) torsion stress (twisting)

Seismic Waves

The Nature of 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.

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• The point on Earth’s surface directly above the focus is the epicenter.

Earthquakes

2

• Earthquake waves travel out in all directions from a point where strain energy is released. This point is the focus.

Earthquake Waves

• The sudden energy release that goes with fault movement is called elastic rebound.

• A fault is a crack along which movement has taken place.

Earthquakes

2

• When stress leads to strain, energy is released suddenly, and it causes rock to lurch to a new position.

Energy Release

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 three seismic stations.

Surface Waves

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Primary waves, also called P-waves, are longitudinal waves (compressional).

• P-waves pass through solids and liquids

Body Waves—seismic waves that pass through the Earth

• 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

Fig. 9-8, p. 194

Body

Or rarefactions

Longitudinal or compressional

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

Fig. 1-10, p. 14

Gases emitted from the interior during this process are likely the source for the formation of the atmosphere and oceans.

Internal Temperature of Earth

Fig. 1-10c, p. 14

Temperature of the Earth increases with depth (25 degrees C per km, closer to the surface)

Crust-mantle boundary 800-1200 C

Core-mantle boundary 3500-5000 C

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

Fig. 1-14, p. 18

Three types of plate boundaries1. Divergent plate boundary 2. Convergent Plate Boundary 3. Transform Plate

boundary

Fig. 1-12, p. 15

The Mechanism for Plate Motion is Convection in the Mantle

Heat from the interior flows outward toward the crust

What is the evidence that the Earth’s outer core is liquid?

(See next slide)

Fig. 9-21, p. 210

P-waves and S-waves 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

P-Waves

S-Waves

• This “dead zone” is termed the shadow zone.

• This seismic pattern indicates that the outer core is liquid.

Shadow Zones

• P-waves and S-waves travel through Earth for 105 degrees of arc in all directions.

Earth’s Interior

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• Between 105 and 140 degrees from the epicenter, nothing is recorded.

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/howstuffworks/230-how-tsunamis-work-video.htm

http://www.geogateways.com/toolkit/ggimages/tsunami1.jpg

http://www.uwiseismic.com/General.aspx?id=23

• If temperatures are high enough, atoms move apart enough to exist in the liquid state, even at extreme pressures.

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.

Earth’s Interior

3

• When pressure dominates, atoms are squeezed together tightly and exist in the solid state.

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 post-earthquake assessments to assign an intensity value.

Earthquakes

2 Earthquake Measurement

• Richter magnitude is intended to give a measure of the energy released during the earthquake.

• The Richter magnitude scale uses the amplitude of thelargest earthquake wave.

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.

• 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.

Earthquake Proofing

Seismic Waves

The Nature of 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.

Click image to view movie.

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