Seismic Waves

1. Seismic Waves Mechanical waves that travel through the Earth. http://earthguide.ucsd.edu/eoc/teachers/t_tectonics/p_paleomag.html

2. Main ideas of plate tectonics: • Earth’s surface is composed of lithospheric plates • Plates are moving • Moving plates change the location of continents and alter the surface of the earth • Sea floor moves to carry the continents • Come together (convergence), spread apart (divergence) and move past (transform) along plate boundaries

3. 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.

4. 9. Convection in the mantle (hot mantle rises, cooler mantle sinks) • 10. The Decay of radioactive elements keeps the interior of the Earth warm

6. 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)

7. Barringer Meteor Crater, Arizona Iron-nickel meteorite 50 m in diameter Impact speed 12.8 km/s 49,000 years old 1.186 kilometers (.737 miles) in diameter 170 m in depth

8. 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

9. Earthquakes 2 Deformation • Moving plates place stress on the earth • compressive stress (push together) (2) a tension stress (pull apart) (3) a shear stress (moving past) (4) torsion stress (twisting)

10. 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.

11. 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.

12. 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.

13. 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.

14. Surface Waves

15. Body Waves— seismic waves that pass through the Earth 2 • 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

16. Body Fig. 9-8, p. 194

17. Longitudinal or compressional Or rarefactions transverse

18. Fig. 9-9, p. 195

21. Fig. 9-10, p. 196

22. 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.

23. Fig. 9-5, p. 191

24. Fig. 2-14, p. 38

25. Differentiation More dense material sinks (molten iron and nickel) leaving the less dense material (silicates) near the surface 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

26. 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

27. 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)

28. Fig. 1-11, p. 15

29. 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]

30. 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.

31. 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

32. Three types of plate boundaries • Divergent plate boundary 2. Convergent Plate Boundary 3. Transform Plate boundary Fig. 1-14, p. 18

33. The Mechanism for Plate Motion is Convection in the Mantle Heat from the interior flows outward toward the crust Fig. 1-12, p. 15

34. What is the evidence that the Earth’s outer core is liquid?(See next slide)

35. P-Waves 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 S-Waves Fig. 9-21, p. 210

37. 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.

39. 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

40. Fig. 2-13, p. 37

41. Fig. 2-19, p. 43

42. Fig. 2-23, p. 46

43. http://videos.howstuffworks.com/howstuffworks/230-how-tsunamis-work-video.htmhttp://videos.howstuffworks.com/howstuffworks/230-how-tsunamis-work-video.htm

44. http://www.geogateways.com/toolkit/ggimages/tsunami1.jpg http://www.uwiseismic.com/General.aspx?id=23

45. 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.

46. Earthquakes • 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. 2 Earthquake Measurement

47. Earthquakes • The Richter magnitude scale uses the amplitude of the largest earthquake wave. 2 Earthquake Measurement • Richter magnitude is intended to give a measure of the energy released during the earthquake.

48. Earthquakes 2 Earthquake Measurement • The table shows the global frequency of different magnitude earthquakes.

49. 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.

50. 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. Click image to view movie.