# Seismic Waves Mechanical waves that travel through the

• Slides: 51

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: //quake 06. stanford. edu/c entennial/tour/s top 11. 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! • 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

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 -10 c, 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/cm 3] • Outer liquid core • Inner solid core • Mantle: iron-rich rock (Fe. Mg-Peridotite) [3. 3– 5. 7 g/cm 3] • Crust: aluminum and magnesium rich rock • Continental Crust: Si. Al (rock) less dense [2. 7 g/cm 3] • Oceanic Crust: Si. Ma (rock) darker, more dense [3. 0 g/cm 3]

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/tsunami 1. 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 Surface Waves • Surface waves move in a more complex manner. • They can exhibit an up and down rolling motion, and also a side-to-side motion that parallels Earth’s surface.

Earthquakes 2 Surface Waves

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.