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

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

EarthquakesEarthquakes

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• 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)

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

EarthquakesEarthquakes

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• 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.

EarthquakesEarthquakes

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

Energy Release

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/Earthquakes/rayleighlove_lrg.gif

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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

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

• 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

Fig. 1-14, p. 18

Three types of plate boundaries

1. 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?

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

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.

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• When pressure dominates, the inner core remains solid, even at high temperatures.

• 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 InteriorEarth’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. 2-23, 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