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Transcript
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://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) 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! • 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 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. 1-10c, 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/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 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. Earth’s Interior 3 Shadow Zones 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. http://www.youtube.com/watch?v=a7QqrFkiE7g http://www.youtube.com/watch?v=yOGoKCK17a4&feature=related http://www.forgefx.com/casestudies/prenticehall/ph/seismic/seismic-wavessimulator.htm http://aspire.cosmic-ray.org/labs/seismic/index.htm Seismic Waves The sudden release of energy within the Earth produces waves. http://www.youtube.com/watch?v=FW-TkpvKPl0 http://www.youtube.com/watch?v=0lAXStQCjr8&feature=related 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 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.