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Earthquakes in our Community

Earthquakes in our Community. Basic Information You Need to Know. An Earthquake is……. A sudden movement or shaking of the earth. Caused by rocks breaking apart. Along a big surface inside the earth.

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Earthquakes in our Community

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  1. Earthquakes in our Community Basic Information You Need to Know

  2. An Earthquake is……. • A sudden movement or shaking of the earth • Caused by rocks breaking apart • Along a big surface inside the earth • This release of energy as the rocks break apart cause big vibrations called seismic waves or earthquake waves

  3. How it works…… • A fault is a surface between two large blocks or regions of rock Fault Line between rocks

  4. How it works…… • Forces in the earth push the fault blocks in different directions Most of these movements are caused by the movement of the Earth’s plates

  5. Earthquakes happen this way….. • When the plates move the blocks of earth, the rocks bend, but they can’t bend forever…. • Just like when you bend a stick, it will break when it is bent too far….. • The amount of force needed to break the rocks is called the shear strength of the rocks

  6. Earthquakes happen this way….. • If the plate forces are larger than the shear strength of the rocks, the rocks break apart • Like the bent stick, the ends snap back.. • This snapping back is called the elastic rebound of the rock.

  7. So why do rocks bend and snap? • The surface between the rocks is rough • Just like 2 pieces of sandpaper pressed together, they get stuck • The friction between the rocks stops them from moving

  8. So why do rocks bend and snap? • And the fault line between the rocks fills up…. • Sand and water and other rock dust get into the fault line • The dust and water act just like cement locking the fault in place

  9. So where do earthquakes happen? • They usually happen below the surface of the earth…. • The place on the fault underground where the rocks break is called the earthquake focus… • The epicenter is the place on the surface of the earth directly above the focus

  10. So where do earthquakes happen? • The movement of the fault can cause a sharp step in the land’s surface This step is called a fault scarp • Movement along the earth can be as much as 10 to 15 meters (30-45 feet)

  11. So where does the shaking come from? • When the rupture happens, waves of energy go out from the focus of the earthquake

  12. What is a wave? • A wave is a disturbance that transmits energy through matter and space No – not that kind of wave…...

  13. Wave direction Wave direction What is a wave? • Waves move either up and down or back and forth and are named by the type of motion • For Longitudinal waves, the ground moves in the same direction as the wave travels • For Transverse waves, the ground moves perpendicular to the motion of the wave • When an earthquake hits, these waves travel through the earth 3 ways and are called by special names…..

  14. What are the 3 kinds of waves? • The first kind of waves are compression or longitudinal waves • These are called primary waves or “P” waves that compress and expand the ground as they travel by • They reach us first and can travel through solids and liquids • And they travel at 5 kilometers per second

  15. What are the 3 kinds of waves? • The second kind is a shear or transverse wave • These are secondary waves or “S” waves and they look like ocean waves but they go through solid rock • They reach us after the “P” waves and can’t go through liquids • And they travel at 3 kilometers per second

  16. What are the 3 kinds of waves? • The last kind of waves to reach us are surface waves • These waves travel along the surface of the earth and are the last to reach us • These are a combination of transverse and longitudinal waves • These are the waves we feel the most when an earthquake hits

  17. What are the 3 kinds of waves? • The last kind of waves to reach us are surface waves • There are 2 kinds of surface waves • The first kind creates an up-and-down rolling motion • The second kind shakes the ground sideways and does the most damage

  18. So, lets review…… • Some combination of up-and-down and side-to-side motion • What kinds of motion would you expect to feel in an earthquake? • Damage or total destruction of buildings, twisting of roads, dropping of bridges, etc. • What effects would earthquake motions have on buildings, roads and houses? • Of the types of waves discussed, which do you think are the most dangerous and why? • Surface waves because P and S waves travel through the earth’s center

  19. Questions?

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