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Earthquakes

Earthquakes . Earth Science Fall 2013. Earthquakes & Plate Tectonics . Earthquake- vibration of the earth’s crust Usually occur when rocks under stress suddenly shift along a fault . Elastic Rebound Theory .

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Earthquakes

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  1. Earthquakes Earth Science Fall 2013

  2. Earthquakes & Plate Tectonics • Earthquake- vibration of the earth’s crust • Usually occur when rocks under stress suddenly shift along a fault

  3. Elastic Rebound Theory • Theory that rocks that are strained past a certain point will fracture and spring back to their original shape. • Energy is released by the rocks along the fault in the form of vibrations called seismic waves. • The release of energy usually increases the stress in other rocks along the fault causing aftershocks. • Aftershocks usually follow a major earthquake and are a series of tremors

  4. Elastic Rebound Theory Focus- area along a fault where slippage first occurs; under the surface of the earth Epicenter- point on the earth’s surface directly above the focus When an earthquake occurs seismic waves radiate outward in all directions from the focus

  5. Elastic Rebound Theory • Earthquake depth statistics • Shallow focus quakes occur within 70km of the earth’s surface • 90% of continental earthquakes are shallow focus • Usually cause the most damage • Intermediate focus quakes occur at a depth of 70km – 300 km • Deep focus quakes occur in subduction zones at a depth of between 300km – 650km • Usually occur farther inland from the subduction zone than shallow and intermediate focus earthquakes

  6. Elastic Rebound Theory Video Elastic Rebound Theory Earthquakes 101

  7. Major Earthquake Zones • Most earthquakes occur on or near the edges of the earth’s lithospheric plates • Some plate boundaries contain groups of interconnected faults called fault zones • Ex: San Andreas Fault

  8. 3 Major Earthquake Zones • Pacific Ring of Fire • Includes the west coasts of North & South America, east coast of Asia, and the western Pacific islands of the Philippines, Indonesia, New Guinea, and New Zealand • Subduction is occuring • Mid-ocean ridges • Oceanic crust is pulling away from both sides of the ridge • Eurasian-Melanesian mountain belt • Collision of plates

  9. Recording Earthquakes • Seismograph- instrument used to detect and record seismic waves • Consists of 3 separate sensing devices: • One records vertical motion • One records horizontal motion in an east-west direction • One records horizontal motion in a north-south direction

  10. Types of Seismic Waves • The major types of seismic waves: • S waves • P waves • Surface waves

  11. Three Types of Seismic Waves • P waves (primary waves) • Move the fastest • First to be recorded by a seismograph • Can travel through solids & liquids • The more rigid the material the faster they travel • Compression waves (cause rock particles to move together and apart along the direction of the waves)

  12. Three Types of Seismic Waves • S waves (secondary waves) • Second waves recorded by the seismograph • Only travel through solid material • Cannot be detected on the side of the earth opposite the earthquake’s epicenter. Why? • Shear waves ( they cause rock particles to move at right angles to the direction in which the waves are traveling)

  13. Three Types of Seismic Waves • Surface waves • When P waves and S waves reach the earth’s surface their energy is converted in surface waves. • Slowest-moving waves • Last to be recorded by a seismograph • Move similar to that of ocean waves • Cause the earth’s surface to rise & fall • Particularly destructive when traveling through loose earth

  14. Locating an Earthquake • To find the epicenter of an earthquake scientists analyze the difference between the arrival time P waves and S waves • P waves travel 1.7 times faster than S waves • So is S waves arrive soon after P waves the earthquake originated really close and vice versa • To determine how far an earthquake is from a given seismograph • Scientists plot the difference between the arrival times of the two waves • Need data from at least three seismograph stations to do this

  15. Earthquake Measurement • Magnitude- amount of ground motion • Seismologists report the magnitude of an earthquake by using a magnitude scale like the Richter scale or the moment of magnitude scale • Scientists prefer the moment of magnitude scale because it is related more to cause of the earthquake, where the Richter scale is more related to the effect of the earthquake

  16. Earthquake Measurement • Major earthquake • Magnitude of 7 or above • Moderate earthquake • Magnitude between 6 and 7 • Minor earthquake • Magnitude between 2.5 and 6 • Microquake • Magnitude less than 2.5 • Usually not felt by people

  17. Earthquake Measurement • Mercalli Scale • Expresses the intensity of an earthquake or the amount of damage it causes • Expressed as Roman numerals from I to XII and a description • Ex: rating of II (low intensity) Felt only by a few persons at rest, especially on the upper floors of buildings. Delicately suspended objects may swing. • Ex: rating of XII Total Destruction

  18. Earthquake Damage A moderate earthquake that continues for a long time often causes more damage than an earthquake of high magnitude that only lasts for a short time.

  19. Earthquake Damage • Destruction of Buildings & Property • Type of ground beneath a building can affect the way a building responds to seismic waves. • Loose soil & rock is much more likely to be damaged during an earthquake than one built on solid ground

  20. Earthquake Damage • Tsunamis • Giant ocean wave caused by a major earthquake with an epicenter on the ocean floor • Most are caused by 2 events related to under sea earthquakes • Faulting • Underwater landslides • Pacific Tsunami Warning Center (PTWC) • Network of seismograph stations around and in the Pacific Ocean that alerts scientists to the location and magnitude of earthquakes

  21. Earthquake Safety • Safety Rules • Before an earthquake occurs, be prepared. • During an earthquake, stay calm. • After an earthquake, be cautious

  22. Earthquake Warnings & Predictions • Identified faults in populated areas • Along some faults there are zones of immobile rock called seismic gaps • A seismic gap is an area where the fault is locked and unable to move • Scientists believe that seismic gaps are likely the location of future earthquakes

  23. Earthquake Warnings & Predictions • Sometimes there is slight tilting of the ground shortly before an earthquake • Some earthquakes are preceded by a decrease in the speed of local P waves • Can last for several days or several years • Longer the decrease lasts the stronger the earthquake will be • Japan Tsunami • Earthquake & Tsunami Relationship

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