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Forces Inside the Earth

Forces Inside the Earth. Earthquakes. California, USA. The red line is the San Andreas Fault, dividing two crustal plates. Everything to the left (west) of the fault line is trying to move northwest. Everything to the right (east) of the fault line is trying to move southeast. The result:

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Forces Inside the Earth

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  1. Forces Inside the Earth Earthquakes

  2. California, USA • The red line is the San Andreas Fault, dividing two crustal plates. • Everything to the left (west) of the fault line is trying to move northwest. • Everything to the right (east) of the fault line is trying to move southeast. • The result: earthquakes

  3. Major Earthquakes in History • April 18, 1906 at 5:12 am an earthquake struck with the magnitude of 8.3 in San Francisco • Most of the city was destroyed by fire. • October 17, 1989 just after 5 pm an earthquake with the magnitude of 7.1 struck again. • The Nimtiz Freeway in Oakland collapsed

  4. Pressure • Like paper clips, pencils or anything else rocks can only bend to a certain point. Beyond that point they can break • Elastic limit: • The limit to how much pressure an substance can withstand and still return to its original shape • Beyond the elastic limit, the substance will be permanently deformed or will break • It’s the same with rock layers! • If rock layers break, a fault is formed. • Further pressure causes movement along the fault.

  5. Forces and Faults • Along an earthquake fault, rocks can move in only three ways: • Rocks can be pulled apart • Rocks can be pushedtogether • Rocks can be forced to slide past each other • So, movement along a fault results from three different kinds of forces: • Tension • Compression • Shear

  6. Normal Fault • Normal faults result from tension, or the force pulling rock layers apart. • The top block moves down relative to the bottom block • There is vertical motion of rock layers along the fault • Examples: • Grand Teton Mts., Wyoming • Sierra Nevada Mts., California

  7. If the block above the inclined fault line moves down it is called a normal fault (mostly because if moves with gravity not because it is the most common fault).

  8. Reverse Fault • Reverse faults result from compression, or force pushing rock layers together. • The top block moves up relative to the bottom block. • There is vertical motion of the rock layers along the fault. • Examples: • Himalayas, Nepal and India

  9. The block above the inclined fault line moves up it is called a reverse fault (the reverse of the normal).

  10. Strike-Slip Fault • Strike-slip faults result from shear, or the pushing rock layers from different directions. • Both blocks move sideways past each other. • There is no vertical motion of rock layers along fault, only sideways motion. • Examples: • San Andreas Fault, California

  11. If the block opposite an observer moves to the left it is termed left lateral, if the block opposite the observer moves to the right it is termed right lateral.

  12. Wave Type • All vibrations travel though matter as waves, but in different ways… • Sound waves are examples of compression waves: • Matter is squeezedand stretchedin the same direction the wave is traveling • Ocean waves are examples of transverse waves: • Matter moves up and down at right angles to the direction the wave is traveling

  13. Seismic Wave Types • Vibrations from earthquakes travel through solid earth as three types of seismic waves: primary waves, secondary waves, surface waves. • Primary waves (P-waves): • Compression waves (squeezed and stretched) • Travel fastest of all waves • Secondary Waves (S-waves) • Transverse waves (up and down) • Travel slower than (P-waves) • Surface Waves • Transverse waves(up and down, and side to side) • Travel the slowest of all waves • Travel along the Earth’s surface only

  14. Focus and Epicenter Focus The point in the Earth’s interior where the slip (earthquake) occurs P-waves expand out S-waves expand out Epicenter The point on the Earth’s surface directly above the focus Surface waves expand out Where is the most dangerous place to be? We must locate this point accurately.

  15. P-wavestravel faster thanS-waves. At which location do you have to wait the longest time for theS-waveto arrive after the P-wave? (P-waves outrun the S-waves) The time difference between the arrival of theP-waveand theS-waveis called thelag time.

  16. Seismograms • Seismic waves are recorded far away from the epicenter by using a seismograph. Vibrations are recorded on a rotating drum. • The record on paper, a seismogram, records the arrival of P-waves,S-waves, and surface waves.

  17. Seismograms The time between the P-wave arrival and S-wave arrival is the lag time. Lag time depends on the distance: the greater the distance, the greater the lag time will be. This is how we precisely locate earthquakes.

  18. Travel Time Graphs • The travel times of P-waves and S-waves can be shown on a graph • It takes 5 minutes for P-waves to travel 2,500 km, and 9 minutes for S-waves to travel the same distance. • What’s the lag time?

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