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Earthquake Properties

Earthquake Properties. http://www.worldphoto360.com/wp-content/uploads/2009/12/Earthquake.jpg. Physical Properties. Focus – point of rock failure  where the earthquake originates Epicenter - The point on the earth's surface directly above the earthquake's focus. Seismic Waves.

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Earthquake Properties

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  1. Earthquake Properties http://www.worldphoto360.com/wp-content/uploads/2009/12/Earthquake.jpg

  2. Physical Properties • Focus – point of rock failure  where the earthquake originates • Epicenter - The point on the earth's surface directly above the earthquake's focus

  3. Seismic Waves • Vibrations generated during an earthquake

  4. Types of Earthquake Waves • Surface waves • Travel on Earth’s surface outward from the epicenter in a circular motion pattern • Complex motion • Slowest velocity of all waves

  5. Types of Earthquake Waves • Body waves - travel through interior of Earth 1. Primary (P) waves • Push-pull (compressional) motion • Travel through solids, liquids, and gases • Greatest velocity of all earthquake waves

  6. Types of Earthquake Waves • Body waves - travel through interior of Earth 2. Secondary (S) waves • "Shake" motion • Travel only through solids • Slower velocity than P waves

  7. A seismogram records wave amplitude vs. time http://ww2.lafayette.edu/~malincol/Geol120/seismogram.GIF

  8. The machine that produces seismograms is called a seismograph. http://www.cartage.org.lb/en/themes/Sciences/Earthscience/Geology/Earthquakes/MeasuringEarthquakes/Seismographs/seismograph.GIF http://upload.wikimedia.org/wikipedia/commons/0/0f/Kinemetrics_seismograph.jpg

  9. Locating an epicenter requires 3 seismograph stations.

  10. Distance from the epicenter to a station is determined by the difference in arrival time between P and S waves.

  11. Calculate distances from 3 stations and see where the circles intersect. 300 km 500 km 100 km

  12. Intensity – qualitative; determined by effects on people, structures, and the environment Magnitude – quantitative; measures energy released at the source of the earthquake Measuring The Size of An Earthquake

  13. Magnitude Scales • Richter scale - measures earthquake magnitude  based on size of largest wave • Moment magnitude scale - accounts for size of fault rupture, amount of movement at fault, and rocks’ stiffness  more precise • Each step represents a 10-fold increase in wave size, or a 30-fold increase in energy released • A magnitude 6 earthquake is 10 times bigger than a magnitude 5 earthquake and releases 30 times more energy. • A magnitude 7 earthquake is 100 times bigger than a magnitude 5 earthquake and releases 900 times more energy.

  14. Moment = µ A D µ = shear modulus A = LW = areaD = average displacement during rupture Source: USGS The seismic moment is a measure of the size of an earthquake based on the area of fault rupture, the average amount of slip, and the force that was required to overcome the friction sticking the rocks together that were offset by faulting. Moment Magnitude

  15. Magnitude From Amplitude http://www.earthquakes.bgs.ac.uk/earthquakes/education/eq_booklet/dia_richter_scale.jpg

  16. Let’s try it! • An earthquake has been recorded on 3 seismograms. • Can you locate its epicenter? • Can you determine its magnitude? http://www.divinationpower.com/UserFiles/2009/12/1/Earthquake%20measuring%203.7%20in%20magnitude%20jolts%20northeastern%20Bulgaria.jpg

  17. Intensity • Witness observations I • Felt by very few people; barely noticeable II • Felt by a few people, especially on upper floors. III • Noticeable indoors, especially on upper floors, but may not be recognized as an earthquake. Hanging objects swing. IV • Felt by many indoors, by few outdoors. May give the impression of a heavy truck passing by. V • Felt by almost everyone, some people awakened. Small objects move. Trees and poles may shake. VI • Felt by everyone. Difficult to stand. Some heavy items of furniture move, plaster falls. Slight damage to chimneys possible. VII • Slight to moderate damage in well-built, ordinary structures. Considerable damage to poorly built structures. Some walls may fall. VIII • Little damage in specially built structures. Considerable damage to ordinary buildings, severe damage to poorly built structures. Some walls collapse. IX • Considerable damage to specially built structures, buildings shifted off foundations. Noticeable cracks in ground. Wholesale destruction. Landslides. X • Most masonry and frame structures and their foundations destroyed. Ground badly cracked. Landslides. Wholesale destruction. XI • Total damage. Few, if any, structures standing. Bridges destroyed. Wide cracks in ground. Waves seen on ground. XII • Total damage. Waves seen on ground. Objects thrown up into air. Abbreviated Modified Mercalli Intensity Scale

  18. Intensity • Intensity  when focus depth  • Intensity  when distance from epicenter  • Also depends on what ground is made of – why? • Let’s see it!

  19. Did You Feel It? http://earthquake.usgs.gov/eqcenter/dyfi.php

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