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Earthquakes

Earthquakes. Chapter 16. Outline. Introduction to Earthquakes and Earth’s Interior Why Should You Study Earthquakes? What is the Elastic Rebound Theory? What is Seismology? Where Do Earthquakes Occur and How Often? What Are Seismic Waves? How Is an Earthquake’s Epicenter Located?

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Earthquakes

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  1. Earthquakes Chapter 16

  2. Outline • Introduction to Earthquakes and Earth’s Interior • Why Should You Study Earthquakes? • What is the Elastic Rebound Theory? • What is Seismology? • Where Do Earthquakes Occur and How Often? • What Are Seismic Waves? • How Is an Earthquake’s Epicenter Located? • How are the Size and Strength of an Earthquake Measured? • What Are Destructive Effects of Earthquakes? • Can Earthquakes Be Predicted? Controlled? • What Is Earth’s Interior Like? • Earth’s Internal Heat

  3. Objectives • Energy is stored in rocks & released when they fracture, producing various types of waves that travel outward in all directions from their source. • Most earthquakes (EQs) take place in well-defined zones at transform, divergent, and convergent plate boundaries. • An EQ’s epicenter is found by analyzing EQ waves at, at least 3 seismic stations. • The amount of damage and people’s reactions to an EQ are used to determine an EQ’s intensity on the Modified Mercalli Intensity Scale. • The Richter Magnitude Scale is used to express the amount of energy released during an EQ. • Great hazards are associated with EQs such as ground shaking, fire, tsunami and ground failure.

  4. Introduction • Earthquake the shaking or trembling of part of Earth’s surface caused by a sudden release of energy, usually by slippage of rocks along a fracture (i.e., faulting). • Aftershocks continuing adjustments along the fracture may generate a series of generally smaller quakes (occur after an EQ) • Earthquakes are an indication that Earth is an internally active planet.

  5. Some Significant Earthquakes • YEAR LOCATION MAGNITUDE DEATHS • 1556 China (Shanxi Province) 8.0 1,000,000 • 1755 Portugal (Lisbon) 8.6 70,000 • 1811-1812 USA (New Madrid, Missouri) 7.5 20 • 1886 USA (Charleston, SC) 7.0 60 • 1906 USA (San Francisco, CA) 8.3 700 • 1923 Japan (Tokyo) 8.3 143,000 • 1964 USA (Alaska) 8.6 131 • 1970 China (Yunnan Province) 7.7 15,621 • 1976 China (Tangshan) 8.0 242,000 • 1985 Mexico (Mexico City) 8.1 9,500 • 1988 Armenia 7.0 25,000 • 1989 USA (Loma Prieta, CA) 7.1 63 • 1990 Iran 7.3 40,000 • 1993 India 6.4 30,000 • 1994 USA (Northridge, CA) 6.7 61 • 1995 Japan (Kobe) 7.2 5,000 + • 1997 Iran 7.3 2,400 + • 1998 Afghanistan 6.1 5,000 + • 1999 Izmit, Turkey 7.4 17,000

  6. Why Should You Study Earthquakes? Earthquakes: • are destructive (take many lives & cause injuries each year). • affect national economies (EQs can disrupt the socio-economic fabric of countries, which has political ramifications of local, regional, & global scale.) • affect individuals(basic knowledge of risk & hazard typesassoc. with EQs is valuable)

  7. What Is the Elastic Rebound Theory? • Elastic rebound theoryexplains how energy is released during earthquakes. • A.) On Earth’s surface, any straight line like a road or a fence crossing a fault would be gradually deformed • B.) or bent as rocks on one side of the fault move relative to those on the other side. A B C D

  8. C D C.) When the strength of rock is exceeded, movement occurs along the fault & energy is released, causing an earthquake. D.) After energy is released, the rocks rebound or “snap back” to their original undeformed shape.

  9. How do we measure earthquakes? Epicenter the point on Earth’s surface directly above the focus. Energy released by movement along a fault travels as seismic waves outward in a concentricpattern from the place of movement. Focus point w/in Earth where fracturing first begins (i.e., the point where energy is first released).

  10. What Is Seismology? • Seismologyis the study of earthquakes. • The passage of these waves through Earth materials is detected, recorded, & measured by seismographs (A). B A The record made is a seismogram (B).

  11. 3 Types of Earthquakes(based on focus-depth): • shallow-focusdepths <70km • intermediate-focusdepths 70 to 300 km • deep-focusdepths >300 km • 90% of all EQs foci depths <100 km • Only 3% are deep-focus.

  12. Nearly (95%) all EQs occur in seismic belts along plate boundaries, but no place on Earth is immune.

  13. Where Do Earthquakes Occur? • Majority (80%) occur in the circum-Pacific belt. (Along this belt, most EQs are assoc. with convergent plate boundaries but also occur along divergent & transform boundaries.) • 15% of EQs occur along theMediterranean-Asiatic belt. (along a predominantly convergent zone) • 5% occur in plate interiors & along divergent margins. (Intraplate EQs take place along ancient faults commonly assoc. with buried plate margins.) • ~150,000 EQs strong enough to be felt are recorded each year. • Another 900,000 are recorded, but are too small to be cataloged individually. How Often Do Earthquakes Occur?

  14. What Are Seismic Waves? Oakland, Calf.,1989 • Seismic wavesproduced as energy is released by movement along a fault. • Seismic waves produce vibrations that cause EQs.

  15. Categories of Seismic Waves • Body wavesseismic waves that travel through the solid body of Earth (much like sound waves). Two types of Body Waves: P-waves(primary waves) S-waves(secondary waves) • Surface waves travel along the ground surface (are similar to waves on water). Two types of Surface Waves: Rayleigh waves (vertical motion) Love waves (horizontal motion)

  16. Body Waves P-waves: -- are the fastest seismic waves. -- travel in solids, liquids, and gases -- are compressional (b) -- cause material to vibrate back-&-forth in the direction of travel. S-waves: -- are slower than P-waves -- travel only in solids -- move material perpendicular to the direction of travel Travel velocity of body waves is proportional to the elasticity of the Earth material. -

  17. Surface Waves (esp. damaging to building foundations) 1. Rayleigh waves: (like water waves) -- move material in an elliptical path (a) -- are slower thanLove waves 2. Love waves: -- motion is similar to S-waves (back & forth motion) except movement is horizontal(b)

  18. How does one locate an EQ’s epicenter? • based on the difference in the arrival time at seismograph stations of the first P- & S-waves (P-S time interval) • P-waves arrive first, followed in order by S-waves, Love waves, & Rayleigh waves.

  19. How Is an EQ’s Epicenter Located? Seismologists: -- estimate ave. speeds of P- & S-waves from data -- determine P- & S-wave travel times for distances b/w focus & seismograph -- produce Time-distance graphs (P-S time interval vs. travel distance) that are used to locate epicenters

  20. How do you locate the epicenter of an EQ? Must know: “P-S time interval” for 3 seismograph stations • Use each P-S time interval & determine travel distance from the time-distance graph. A circle with a radius equal to the travel distance is drawn for each of the three seismograph stations. The intersection of the3 circles is the locationof the epicenter.

  21. How Are the Size & Strength of an EQ Measured? Magnitude the amount of energy released by an EQ at its source. It’s determined based on the amplitude of the largest seismic wave recorded for a given EQ. *Magnitude is unchanging!!(It doesn’tvary at each recording station.) Intensitysubjective measure of the damage done by an EQ, as well as what people felt *Intensity can VARY with each recording station.

  22. How Are the Size & Strength of an EQ Measured? • The Richter Magnitude Scale(ranges from 1 to 9) quantitative measure of EQ magnitude • An increase of one unit on the Richter Scale (5 to 6) is equivalent to a 10-fold increase in amplitude of the largest seismic wave produced. • In terms of energy released, a one unit increase on the Richter Scale equals a 30-fold increase in energy released at the focus. (It would take ~30 EQs of 5.5 magnitude to release as much energy as one 6.5 EQ.)

  23. Average Number of Earthquakes of Various Richter Magnitudes per Year Worldwide AVERAGE NUMBER MAGNITUDE EFFECTS PER YEAR <2.5 Typically not felt but 900,000 recorded 2.5 -6.0 Usually felt; minor to moderate damage to 31,000 structures 6.1-6.9 Potentially destructive, especially in populated 100 areas 7.0-7.9 Major earthquakes; 20 serious damage results >8.0 Great earthquakes; 1 every 5 years usually result in total destruction

  24. How Are the Size & Strength of an EQ Measured? • Modified Mercalli Scale(ranges from 1 to 12) assesses earthquake intensity(approximates size & strength) • For an earthquake of given Richter magnitude, Intensity WILL VARY with distance from epicenter, local geology, construction practices, etc. II Felt only by a few people at rest, especially on upper floors of buildings. VI Felt by all, many frightened and run outdoors. Some heavy furniture moved, a few instances of fallen plaster or damaged chimneys. Damage slight. IX Damage considerable in specially designed structures. Buildings shifted off foundations. Ground noticeably cracked. Underground pipes broken.

  25. What are the Destructive Effects of EQs? Destructive effects of earthquakes include: ground shaking fire tsunami landslides panic disruption of vital services psychological shock Numbers of deaths & injuries depend on several factors: magnitude duration of shaking local geology population density distance from epicenter construction practices disaster response planning • The time at which an earthquake occurs affects its destructiveness. EQs during working hours in urban areas are most destructive & cause most fatalities & injuries.

  26. Ground Shaking • Effects of ground shaking cause more injuries than any other earthquake hazard. • Along with magnitude & distance to epicenter, Earth materials (bedrock vs. sediment) strongly influence the amplitude & duration of seismic waves. • Amplitude & duration of S-waves is greater in poorly consolidated or water-saturated material than in bedrock. Structures built on bedrock suffer less ground shaking than those built on poorly consolidated or water-saturated material.

  27. Liquefaction process in which the material on which buildings are constructed behaves as a fluid (esp. prevalent in fill & water-saturated sediment. Niigata, Japan, 1964

  28. Ground Shaking Building material & type of construction can affect the amount of damage done by ground shaking. Easily destroyed structures • Adobe & mud-walled buildings • Unreinforced brick & poorly built concrete structures

  29. Fire Severed electrical & gas lines pose a great hazard. • 1902 90% of damage in the San Francisco EQ • 1923  75% of all homes destroyed (mostly of wood) in Tokyo & almost all homes in Yokohama, Japan. • 1989  (Loma Prieta EQ) Large fire in the marina district of San Francisco San Francisco, CA, 1989

  30. Killer Waves -- Tsunamis • Tsunami(tidal wave)an ocean wave produced by an earthquake. Height of tsunamis Open ocean < 1 m high Shallow water of coastal areas 30 m or more Tsunamis travel at several 100 km/hr & can devastate low-lying coastal areas 1000s of miles from their source. Following the tsunami that struck Hilo, HI (in 1946), the U.S. developed an early warning system to predict the arrival of tsunami to coastal areas of the Pacific Ocean. Hilo, Hawaii, 1946

  31. Ground Failure that has caused deaths: • EQ-triggered landslides in mountainous areas(1959 Montana EQ fault scarp  large rock slide dammed the Madison R.) • Collapse of cliffs of wind-laden silt (1920 Ganso, China EQ  100,000 killed). • EQ-induced avalanche(1970 Peru EQ  66,000 killed). Montana, 1959

  32. Can Earthquakes Be Predicted? Successful prediction includes: location strength time frame for occurrence • Seismic risk maps (constructed by historic records & distribution of known faults) are used to assess the likelihood & potential severity of future EQs • This guides EQ preparedness planning.

  33. Earthquake Precursors • Precursors  short- & long-term changes w/in Earth in advance of EQs Precursors include (short-term): tilting of the surface (as rocks deform due to inc. pressure) fluctuations in the water levels in wells changes in locations & frequency of small EQs increased number of foreshocks, etc. Long-term prediction: delineation of seismic gaps • Seismic gaps locked areas along fault zonesLocked areas aren’t releasing energy. • Continued accumulation of pressure could lead to major EQs in the future.

  34. Earthquake Prediction Programs • United States, Russia, Japan, & Chinaare the only nations that have govt.-funded EQ prediction programs. • EQ prediction is progressing, but unsuccessful short-term predictions may lead to skepticism & disregard (like with hurricane, tornado, & tsunami warnings).

  35. What You Can Do to Prepare For an EQ? 1. Be familiar with geologic hazards of the area where you live & work. 2. Make sure your house is bolted securely to the foundation & that the walls, floors, & roof are all firmly connected together. 3. Heavy furniture such as bookcases should be bolted to the walls; semi-flexible natural gas lines should be used so that they can give without breaking; water heaters & furnaces should be strapped & the straps bolted to wall studs to prevent gas-line rupture & fire. Brick chimneys should have a bracket or brace anchored to the roof. 4. Maintain a several-day supply of freshwater & canned foods. Keep a fresh supply of flashlight & radio batteries & a fire extinguisher. 5. Maintain a basic first-aid kit, & have a working knowledge of first-aid procedures. 6. Learn how to turn off the various utilities at your house. 7. Have a planned course of action for when an EQ strikes.

  36. What is Earth’s Interior Like? • Earth is concentrically layered (crust, mantle, core). Each layer differs in composition & density and they’re separated by distinct boundaries. • The behavior & travel times of P- & S-waves provide info about Earth’s internal structure. • Rock density & elasticity increase with depth in the Earth as do travel velocities of P- & S-waves.

  37. Layers of differing density & elasticity cause seismic waves to be bent like light is when it passes from air to water. (refraction) • Part of the energy of P- & S-waves isreflectedat density-elasticity boundaries (like light from a mirror). Depths to boundaries are calculated by knowing: --wave velocities --time interval it takes for waves to travel from the source to density-elasticity boundaries & back to the surface C

  38. Seismic wave velocity continuously changes with depth, but the changes are abrupt at strong density-elasticity boundaries. Three discontinuities: crust-mantle, mantle-outer core, outer core-inner core

  39. Crust Crust  the Earth’s thin skin Mohorovicic discontinuity (Moho)  the crust-mantle boundary (20-90 km deep beneath continents & 5-10 km deep beneath oceanic crust). Continental crustOceanic crust Average density of 2.65 gm/cm3 Average density of 3.0 gm/cm3 P-wave velocity of 6.75 km/sec P-wave velocity of 7 km/sec

  40. Mantle • Mantle lies b/w the Moho & the discontinuity at 2900km (marks an abrupt velocity decrease). • Low-velocity zone(depth of 100-250 km) Here, decreased seismic wave velocities derive from the decreased elasticityof the asthenosphere. • Composition of the mantle: peridotite (olivine & pyroxene) • Volume of Earth: ~80%

  41. The Core • The 2900 km discontinuity marks the boundary b/w the mantle & outer core. Outer core(molten) composed of Fe & S Inner core(solid) composed of Fe & Ni P-wave velocity drops rapidly at mantle-outer core boundary. P-waves are refracted so that little P-wave energy passes thru Earth to reach the surface between 103o & 143o of an EQ focus. This is theP-wave shadowzone.

  42. The Core • At the mantle-outer core boundary, S-waves are blocked & not transmitted into the outer core. The outer core behaves as a liquid b/c liquids do NOT transmit S-waves. The S-wave shadowzone is larger than the P-wave shadow zone. It includes all areas on the surface greater than l03o from the EQ focus.

  43. Seismic Tomography Seismic tomograghya CAT scan of Earth using seismic waves instead of x-rays • 3-D areas of unusually slow or fast wave travel can be detected. Travel speeds in the mantle: • faster travelcorrespond to cold areas(inactive continental interiors) • slower travel marks hot areas(divergent margins & other volcanic areas) • Distribution of hot & cold areas marks the location of mantle convection cells.

  44. Earth’s Internal Heat • Most of Earth’s internal heat derives from radioactive decay of U, Th & K40. Earth’s temperature increases with depth. (geothermal gradient) Geothermal gradient decreases: • 2.5 oC / km near the surface • 1oC / km in the mantle

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