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10_quakes_09_post - 10: Earthquakes & Earth’s...

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Unformatted text preview: 10: Earthquakes & Earth’s Interior Structure 09/30/2009 Indonesia Earthquake Fig. 10-29c: 1999 6.0 earthquake, Armenia; Photo: Reuters Earthquakes: 1 09/30/2009 Indonesia Earthquake Earthquakes: 3 Earthquakes: Earthquakes: 2 Earthquakes: 09/30/2009 Indonesia Earthquake Earthquakes: 4 Earthquakes: Why this is important • Earthquakes are a destructive, lethal natural hazard Why this is important • Distribution of earthquakes defines plate boundaries Fig. 10.20 Photo: AP Earthquakes: 5 Earthquakes: 6 Earthquakes: Why this is important What is an earthquake? • Earthquake waves are like X-rays, allowing us to decipher internal structure of Earth Fig. D.10 Earthquakes: 7 Earthquakes: • Ground vibrations (waves) produced by sudden movement along a fault Earthquakes: 8 Earthquakes: What is an earthquake? What is an earthquake? • Elastic rebound theory: stress builds up, rock bends, reaches a critical value, and then breaks and moves suddenly. • Focus (Hypocenter): point of initial rupture or slip • Epicenter: point on Earth’s surface above focus Fig. 10.4 Fig. 10.1 For large faults, entire fault surface does not slip at once; rather, smaller patches may slip at different times. Two slip patches (red) each have a focus and an epicenter; seismic waves only shown for earthquake A. Earthquakes: 9 Earthquakes: 10 Earthquakes: Seismic waves Seismic waves • Primary (P or push-pull) waves travel by alternately compressing and expanding rocks in the direction of propagation of the wave; similar to sound waves. • Primary (P or push-pull) waves travel by alternately compressing and expanding (dilating) rocks in the direction of propagation of the wave; similar to sound waves. Propagation direction Fig. 10.12 Earthquakes: 11 Earthquakes: Earthquakes: 12 Earthquakes: Fig. 10.12 Seismic waves • Secondary (S or shear) waves travel by shearing rock particles sideways relative to the direction of propagation of the wave. Seismic waves • Secondary (S or shear) waves travel by shearing rock particles sideways relative to the direction of propagation of the wave. Fig. 10.12 Propagation direction Fig. 10.12 Earthquakes: 13 Earthquakes: 14 Earthquakes: Seismic waves • P & S waves -- body waves because they travel through Earth • P-waves travel 1.6x faster than S-waves. Seismic waves • Surface waves: seismic waves that travel at or near the Earth's surface, causing it shear from side to side and up and down. Fig. 10.16 side to side shearing up/down shearing Fig. 10.12 Earthquakes: 15 Earthquakes: Earthquakes: 16 Earthquakes: Recording seismic waves Recording seismic waves Fig. 10.14 Fig. 10.13 • Seismograph: instrument that measures seismic waves. • Seismograph: instrument that measures seismic waves. 1. Instrument is bolted to ground and shakes with ground. 2. Heavy weight attached to spring does not shake. 3. Pen attached to weight marks vibrations on a recording drum that moves up/down and side-to-side during shaking. Earthquakes: 17 Different designs are needed to record up-down and side-to-side shaking. Earthquakes: 18 Earthquakes: Recording seismic waves Fig. 10.14 Review Questions 10-1. The point on the fault surface where slip is initiated during an earthquake is the: A. epicenter B. fault plane C. focus (hypocenter) 10-2. The point on the Earth's surface above the location where slip is initiated during an earthquake is the: A. epicenter B. fault plane C. focus (hypocenter) 10-3. Which of the following types of waves are compressional waves like sound waves? A. S but not P B. P and S C. P but not S D. surface waves 10-4. Which of the following types of waves are body waves? A. S but not P B. P and S C. P but not S D. surface waves 10-5. A. True / B. False: S waves are speedier than P waves. • Seismogram: recording of earthquake on seismograph Earthquakes: 19 Earthquakes: 10-6. A. True / B. False: A seismograph is the recording produced by the seismogram. Earthquakes: 20 Earthquakes: Analyzing seismic waves Arrive 1st Locating epicenter Arrive 3rd Arrive 2nd • Vp>Vs • P arrives before S • S-P increases w/ increasing distance to epicenter Highest amplitude; produce most damage Earthquakes: 21 Locating epicenter • Vp>Vs • P arrives before S • S-P increases w/ increasing distance to epicenter Fig. 10.16 Earthquakes: 23 Earthquakes: Fig. 10.16 Earthquakes: 22 Earthquakes: Locating epicenter Earthquakes waves radiate out from epicenter (green box), reaching closest seismograph station (yellow) first and reaching farther stations later. This applies to both P waves and S waves. P waves arrive first, S waves arrive later. Difference in arrival time between S and P waves increases with increasing distance from epicenter. Earthquakes: 24 Earthquakes: Locating epicenter Locating epicenter For natural earthquakes, we do not know location of epicenter or time when quake began. We do know difference in arrival times of S- and Pwaves. Now we plot the same circles as in previous slide but centered around seismograph stations. They intersect at a common point, the epicenter. These circles represent time delay between S- and P-wave arrivals converted to distance using average velocities of S- and P-waves. Earthquakes: 25 Earthquakes: 26 Earthquakes: Locating epicenter • Triangulation: using at least 3 seismic stations to determine location of epicenter In-Class Activities / iClicker Q1. Refer to the seismogram below. Determine the difference in arrival times between the P- and S-waves. A. ~2 minutes B. ~8 minutes C. ~10 minutes D. ~18 minutes E. ~28 minutes Fig. 10.16 Earthquakes: 27 Earthquakes: Earthquakes: 28 Earthquakes: In-Class Activities / iClicker Q2. Determine the distance to the epicenter. A. ~2700 km B. ~5600 km C. ~7200 km Earthquakes: 29 In-Class Activities / iClicker Q4. Which type of waves generally produce the most shaking? A. P-waves B. S-waves C. Surface waves Earthquakes: 31 Earthquakes: In-Class Activities Q3. The seismograms below each recorded the same earthquake. Label the P-, S-, and surface-waves on each diagram. Earthquakes: 30 Earthquakes: In-Class Activities / iClicker Q5. Which seismograph station was located farthest from the epicenter? A. Station X B. Station Y C. Station Z Earthquakes: 32 Earthquakes: In-Class Activities / iClicker Q6. Which seismograph station was located closest to the epicenter? A. Station X B. Station Y C. Station Z Earthquake magnitude Based on amplitude of largest earthquake wave recorded at seismograph station and its distance from epicenter Richter magnitude is logarithmic: e.g., magnitude 6 is 10x more intense than magnitude 5. Fig. 10.18 Earthquakes: 33 Earthquakes: 34 Earthquakes: Earthquake magnitude & intensity Fig. 10.19 Earthquakes: 35 Earthquakes: Earthquake magnitude • Gutenberg-Richter relationship: describes the number of earthquakes per year of a given magnitude. • Number of quakes decreases with increasing magnitude. Q7. How many magnitude 2 earthquakes occur per year? A. 103 B. 104 C. 105 D. 106 E. 107 Earthquakes: 36 Earthquakes: Fig. 10.19 Review Questions Seismic waves & rays 10-7. Which of the following statements about seismic waves is most correct? A. S-waves travel through liquids. B. S-waves exhibit a push-pull type of motion. C. P and S waves are surface waves. D. P-waves travel faster than S-waves. Fig. d.3 10-8. A. True / B. False: The location of an epicenter is based on the difference in arrival times between S- and P-waves. 10-9. Earthquake A has a Richter magnitude of 5, and earthquake B has a Richter magnitude of 7. How much more intense was B than A? A. 2 times B. 20 times C. 100 times D. 1000 times 10-10. A. True / B. False: As the Richter magnitude decreases, the number of earthquakes decreases. Slide 29 • Velocity of seismic waves increases with increasing density. 10-11. This question is based on Slide 20. The time interval between the arrival of Pwaves and the arrival of S-waves at a seismograph is 5 minutes. How far away is the epicenter from the seismograph? A. 1200 km B. 2500 km C. 3400 km Earthquakes: 37 Earthquakes: 38 Earthquakes: Ray paths Seismic waves & rays • Rays are perpendicular to wave fronts. Earthquakes: 39 Earthquakes: Fig. D.2 • Direct ray: seismic ray that travels in a straight line from source to receiver. Earthquakes: 40 Earthquakes: Ray paths • For constant seismic velocity throughout Earth --> straight ray paths Ray paths Focus • Reflection: when a seismic ray encounters a boundary between two layers across which the velocity changes, the ray may bounce off the boundary; the angle of incidence equals the angle of reflection. !i = !r Earthquakes: 41 !r Higher velocity Earthquakes: 42 Earthquakes: Ray paths • Refraction: when a seismic ray encounters a boundary between two layers across which the velocity changes, the ray is bent Ray paths: refraction Ray bends upward when velocity increases across boundary. Fig. C.5 Photo: S. Schwartzenberg Earthquakes: 43 Earthquakes: Lower velocity !i Earthquakes: 44 Earthquakes: Ray paths: refraction Ray paths: refraction • Continuous increase in velocity with depth--> seismic rays follow smooth, upward curving paths. Ray bends downward when velocity decreases across boundary. Fig. D.8 Earthquakes: 45 Earthquakes: 46 Earthquakes: Earth’s internal structure Ray paths: refraction • Continuous increase in velocity with depth--> seismic seismic rays follow smooth, upward curving paths. Discovery of crust-mantle boundary • For distances <800 km, direct arrives before refracted Curved rays in a mantle whose density increases gradually with depth Fig. D.6 • For distances >800 km, refracted arrives before direct Fig. D.8 Earthquakes: 47 Earthquakes: Earthquakes: 48 Earthquakes: Earth’s internal structure Earth’s internal structure Fig. D.6 • S-wave shadow zone: absence of S-wave arrivals located from +103° to -103° of arc distance from epicenter. • Crust overlies higher-velocity mantle • Boundary is Mohorovicic discontinuity (Moho) Fig. D.11 Earthquakes: 49 Earthquakes: 50 Earthquakes: Earth’s internal structure Earth’s internal structure Fig. D.11 Fig. D.4 • S-waves can’t travel thru liquids • (Outer) core is liquid. Earthquakes: 51 Earthquakes: • P-wave shadow zone: absence of Pwave arrivals located from 103° to 143° of arc distance from the epicenter. • Ray A just skims the core-mantle (C-M) boundary. • Ray B undergoes extreme refraction at C-M boundary. • Ray C does not undergo as much refraction because it intersects CM boundary at a higher angle. Earthquakes: 52 Earthquakes: Fig. D.10 Earth’s internal structure • Continental crust: felsic (granitic) composition ("= 2.7 g/cm3); 30-70 km thick. • Oceanic crust: mafic (basaltic) composition ("=2.9 g/cm3); 8-10 km thick. • Upper mantle: ultramafic composition ("=3.3 g/cm3) Earth’s internal structure • Low-velocity layer: region in mantle where seismic waves decrease in velocity. • Lithosphere: outer region of Earth above low-velocity layer. • 100 km thick in oceanic regions • 150 km thick in continental regions Fig. D.7 Earthquakes: 53 • Asthenosphere: region that extends from low-velocity layer to ~ 200 km depth Earthquakes: 54 Earthquakes: Earth’s internal structure • Lithosphere forms the “plates” of plate tectonics • Lithospheric plates ride over asthenosphere Earth’s internal structure Fig. D.12 • Core: region extending from 2900 km to 6370 km depth • Outer Core: region extending from 2900 km to 5155 km depth; no Swaves; liquid Fig. D.7 Earthquakes: 55 Earthquakes: • Inner Core: region extending from 5155 km to 6371 km depth; P- and S-waves; solid Earthquakes: 56 Earthquakes: Fig. D.7 Earth’s internal structure Seismic-reflection profiling http://jersey.uoregon.edu/~mstrick/ Artificial “earthquakes” used to explore for oil and gas. ~2.75 g/cm3 3-6 g/cm3 Whole-Earth density (5.55 g/cm3) is twice that of rocks in crust, suggesting higher-density materials in mantle & core. Fig. D.12 10-12 g/cm3 12-13 g/cm3 Earthquakes: 57 Seismic-reflection profiling Earthquakes: 59 Earthquakes: Earthquakes: 58 Earthquakes: Seismic-reflection profiling Earthquakes: 60 Earthquakes: Review Questions 10-12. Which of the following best describes how seismic ray paths travel through a spherical body in which density increases uniformly with depth? A. Ray paths are continuously downward curved. B. Ray paths are continuously upward curved. C. Ray paths are straight. D. Ray paths do not exist for such a body. 10-13. The acute angle between an incident ray and a layer boundary is 60°. What is the angle between the reflected ray and the layer boundary? A. 30° B. 60° C. 90° D. 120° 10-14. A. True / B. False: A refracted ray bends downward across a boundary in which velocity increases. 10-15. A. True / B. False: The oceanic crust is thinner than the continental crust. 10-16. A. True / B. False: Oceanic crust is composed mainly of ultramafic rocks. Earthquakes: 61 Review Questions 10-21. Which of the following statements is true? A. The P-wave velocity of the lower mantle is greater than the outer core. B. The P-wave velocity of the lower mantle is the same as the outer core. C. The P-wave velocity of the lower mantle is less than the outer core. 10-22. Which region in the diagram at right of the Earth does not transmit S waves? A. region A B. region B C. region C 10-23. Which of the following statements about the diagram at right is true? A. all regions are solid B. regions B and C are both liquid C. region C is liquid and region A is solid D. region C is solid and region B is liquid Earthquakes: 63 Earthquakes: Review Questions 10-17. Which statement concerning the Earth's internal structure is least correct? A. Seismic velocities increase across the Moho. B. Seismic velocities decrease going from the lithosphere into the upper asthenosphere. C. The P-wave shadow zone is larger than the S-wave shadow zone. D. The S-wave shadow zone reflects the inability of S-waves to travel through liquids. 10-18. The molten layer immediately beneath the mantle is called the ___. A. asthenosphere B. inner core C. outer core 10-19. What type of seismic wave is being recorded by the seismograph in the diagram below showing a simplified view of Earth’s major layers (crust is imperceptibly thick)? A. P wave B. S wave C. surface wave 10-20. A. True / B. False: The average density of the Earth is less than the density of the inner core. Earthquakes: 62 Earthquakes: ...
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This note was uploaded on 09/25/2011 for the course GEOLOGY 100 taught by Professor Lepre during the Fall '11 term at Rutgers.

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