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Earthquakes

Learn about the causes of earthquakes, the dangers they pose, and methods for predicting them. Discover how earthquakes can result in falling buildings, fires, tsunamis, and other destructive events. Explore terms like focus, epicenter, and seismograph, and understand the Richter and moment magnitude scales. Find out about earthquake prediction techniques, such as studying P-wave velocities and land elevation changes.

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Earthquakes

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  1. Earthquakes Ch-10

  2. What is an Earthquake • The shaking of the Earth’s crust caused by the release of energy • More than 1,000,000 happen per year • 1 every 30 seconds • 3000 strong enough to move sections of Earth • 20 strong enough to cause severe damage

  3. Falling buildings Broken pipes Gas lines Fires and explosions Water lines no way of putting out the fires Electrical service no lights or needed electrical devices Sewer lines disease spreads Destroyed roadways cuts off supplies food and water medical cuts off escape cuts off help destroys runways creates tsunamis Dangers

  4. What causes an Earthquake • Anything that causes the ground to shake • Ex) Volcanic eruption, Collapses of some kind, Impacts of meteors • Stress building up between lithospheric plates • Most are caused this way • Elastic-rebound theory • The pressure between the two plates builds up then is released and they go back to the original shape • The boundary between the plates is called a fault plane

  5. Normal fault One side drops down Two sides pulled apart (diverging) The footwall is the part below the fault plane The hanging wall is the part above the fault plane Types of Faults F H

  6. Reverse fault One side driven up over the other side Two side coming together (collision) H F

  7. Thrust fault One side driven up over the other side (same as reverse) Two sides coming together (same as reverse) Angle of fault plane is 45 degrees or less from the horizon H F

  8. Strike-slip fault Two plates move past each other with no up or down motion Example is the San Andreas fault Looking down

  9. Earthquake Terms • Focus • The actual point of movement • Usually kilometers below the surface • The depth of the focus determines the amount of damage • Shallow = more damage • Epicenter • The point on the land directly above the focus

  10. Earthquakes waves • Body waves • Travel from the focus through the Earth • Two types of body waves • #1 P-waves • Push pull motion • First to arrive • Can travel through any material • #2 S-waves • Side to side motion • Second to arrive • Can travel through solids but not liquids or gases

  11. Surface waves • only travel on the surface • last to arrive • causes the most damage • sometimes called L-waves • two types • Love waves • Rayliegh waves • motion is up and down as well as side to side

  12. Recording an Earthquake • Seismograph • Anchored into the bedrock • Two types • horizontal motion • vertical motion • Records information on a seismogram • Shows three groups of zig-zags for an earthquake (P, S, L, waves)

  13. Locating the epicenter • Waves of an earthquake do not travel at the same speeds • P fastest, surface or L the slowest • The difference in time between the wave arrivals will tell you how far away it was • Will not tell you the direction • Use a time- travel graph to give you the distance • Need three stations to pin point the location • This process is called Triangulation

  14. The size of an earthquake • The strength is called the magnitude • The deeper the focus the more wide spread the damage • Richter scale • Measures intensity of ground movement • Increase one number is an increase of 31 times as much energy • Ex) a 4.0 releases 31 times more energy than a 3.0 • Ex) a 5.0 releases 961 times more energy than a 3.0

  15. Increase one number increases the amplitude of the largest surface wave by 10 times • Ex) a 4.0 has 10 times larger surface waves than a 3.0 • Ex)a 5.0 has 100 times larger surface waves than a 3.0 • Moment magnitude • Measures energy released at the earthquakes source • Ex) 1906 - 8.3, 1964 – 8.5 (Richter scale) but the 1964 released twice as much energy at the source that the 1906 so the moment magnitudes were 1906 – 7.9, 1964 – 9.2

  16. Earthquake Dangers • Liquefaction • The soil when shook becomes like a liquid and the buildings sink. • Aftershocks • Smaller earthquakes that happen after the large one • Movement has loosened the plate boundary so it is easier for it to move • Can cause almost as much damage because buildings are already unstable. • Can be as many as 100 per day

  17. Tsunami’s • Can be tall up to 30 meters • Speed depends on the depth • 750 km/hr in deep water • Slower in shallow • Now have tsunami detection stations around the oceans

  18. Earthquake Prediction • Must forecast When – Where – Magnitude • How to Predict • Study P-wave velocities • slow 10% – 15 % before a large earthquake • longer the velocity stays low the stronger the quake • Study land elevation • land will sometimes rise before a large quake • Japan land rose 10 years before the quake

  19. Study electrical resistance in the ground • increase in resistance before an earthquake • Study radon concentrations in well water • increases before quake • Seismic gaps • plot the foci of earthquakes and find where it has not moved in a long period of time

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