Mechanical waves that travel through the Earth. 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 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

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

Moving plates place stress on the earth 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)

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.

Figure 9.5: The relationship between earthquake epicenters and plate boundaries. Approximately 80% of earthquakes occur within the circum-Pacific belt, 15% within the Mediterranean-Asiatic belt, and the remaining 5% within plate interiors or along oceanic spreading ridges. Each dot represents a single earthquake epicenter. Fig. 9-5, p. 191

Figure 2.14: A map of the world showing the plates, their boundaries, relative motion and rates of movement in centimeters per year, and hot spots. Fig. 2-14, p. 38

Surface Waves http://whs.moodledo.co.uk/file.php/1610/Earthquakes/rayleighlove_lrg.gif

seismic waves that pass through the Earth 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

Active Figure 9. 8: Seismic waves Active Figure 9.8: Seismic waves. (a) Undisturbed material for reference. (b) and (c) show how body waves travel through Earth. (b) Primary waves (P-waves) compress and expand material in the same direction they travel. (c) Secondary waves (S-waves) move material perpendicular to the direction of wave movement. (d) P- and S-waves and their effect on surface structures. Body Fig. 9-8, p. 194 14

Longitudinal or compressional Or rarefactions transverse

Figure 9.9: (a) A schematic seismogram showing the arrival order and pattern produced by P-, S-, and L (surface)-waves. When an earthquake occurs, body and surface waves radiate out from the focus at the same time. Because P-waves are the fastest, they arrive at a seismograph first, followed by S-waves, and then by surface waves, which are the slowest waves. The difference between the arrival times of the P- and S-waves is the P–S time interval; it is a function of the distance of the seismograph station from the focus. (b) Seismogram for the 1906 San Francisco earthquake, recorded 14,668 km away in Göttingen, Germany. The total record represents about 26 minutes, so considerable time passed between the arrival of the P-waves and the slower-moving S-waves. The arrival of surface waves, not shown here, caused the instrument to go off the scale. (c) A time–distance graph showing the average travel times for P- and S-waves. The farther away a seismograph station is from the focus of an earthquake, the longer the interval between the arrivals of the P- and S-waves, and hence the greater the distance between the curves on the time–distance graph as indicated by the P–S time interval. Fig. 9-9, p. 195 16

Active Figure 9.10: Three seismograph stations are needed to locate the epicenter of an earthquake. The P–S time interval is plotted on a time–distance graph for each seismograph station to determine the distance that station is from the epicenter. A circle with that radius is drawn from each station, and the intersection of the three circles is the epicenter of the earthquake. Fig. 9-10, p. 196 19

Differentiation More dense material sinks (molten iron and nickel) leaving the less dense material (silicates) near the surface Figure 1.10: (a) Early Earth was probably of uniform composition and density throughout. (b) Heating of the early Earth reached the melting point of iron and nickel, which, being denser than silicate minerals, settled to Earth’s center. At the same time, the lighter silicates flowed upward to form the mantle and the crust. (c) In this way, a differentiated Earth formed, consisting of a dense iron-nickel core, an iron-rich silicate mantle, and a silicate crust with continents and ocean basins. 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 Figure 1.10c: In this way, a differentiated Earth formed, consisting of a dense iron-nickel core, an iron-rich silicate mantle, and a silicate crust with continents and ocean basins. 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)

Figure 1.11: A cross section of Earth, illustrating the core, mantle, and crust. The enlarged portion shows the relationship between the lithosphere (composed of the continental crust, oceanic crust, and solid upper mantle) and the underlying asthenosphere and lower mantle. 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 Figure 1.14: An idealized cross section illustrating the relationship between the lithosphere and the underlying asthenosphere and the three principal types of plate boundaries: divergent, convergent, and transform. Three types of plate boundaries 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 Figure 1.12: Earth’s plates are thought to move as a result of underlying mantle convection cells in which warm material from deep within Earth rises toward the surface, cools, and then, on losing heat, descends back into the interior. The movement of these convection cells is thought to be the mechanism responsible for the movement of Earth’s plates, as shown in this diagrammatic cross section. 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?

Caused by changes in wave speed 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 Active Figure 9.21: (a) P-waves are refracted so that little P-wave energy reaches the surface in the P-wave shadow zone. (b) The presence of an S-wave shadow zone indicates that S-waves are being blocked within Earth. Fig. 9-21, p. 210

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, the inner core remains solid, even at high temperatures.

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

Figure 2.13: The age of the world’s ocean basins established from magnetic anomalies demonstrates that the youngest oceanic crust is adjacent to the spreading ridges and that its age increases away from the ridge axis. Fig. 2-13, p. 37

Active Figure 2.23: The Emperor Seamount–Hawaiian Island chain formed as a result of movement of the Pacific plate over a hot spot. The line of the volcanic islands traces the direction of plate movement. The numbers indicate the ages of the islands in millions of years. Fig. 2-23, p. 46

http://videos. howstuffworks 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