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Chapter Six Earthquakes 6.1

Chapter Six Earthquakes 6.1. An Earthquake is any vibration in the Earth’s crust. An Earthquake is any vibration in the Earth’s crust. The Elastic Rebound Theory says that “rocks along a fault will move suddenly to release energy built up by compression, tension or shearing”.

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Chapter Six Earthquakes 6.1

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  1. Chapter Six Earthquakes 6.1

  2. An Earthquake is any vibration in the Earth’s crust

  3. An Earthquake is any vibration in the Earth’s crust

  4. The Elastic Rebound Theory says that “rocks along a fault will move suddenly to release energy built up by compression, tension or shearing”

  5. These vibrations are usually followed by aftershocks

  6. The above seismogram was recorded at 40 samples per second from a GS-13 short period vertical (sz) seismometer in the walk in vault near Leonard, Oklahoma. The first P and S are from a magnitude 4.2 (mbLg) earthquake which was felt widely in Garvin County. The P and S which occurred 170.5 seconds later was apparently a small movement along part of the fault plane which slipped to cause the mbLg 4.2 earthquake. The difference in mbLg magnitude between the two earthquakes is 1.3. Because magnitudes increase one unit for each increase of 10 (ten) in seismic wave amplitude, we expect the amplitude of the main shock to be more than 10 times larger than the aftershock amplitudes (actually a magnitude difference of 1.3 should produce an amplitude difference of 20 because 10 raised to the power 1.3 is 20). In the displayed seismogram the mainshock/aftershock amplitude ratio is 14.3 for P and 15.0 for S. However mbLg, by definition, is determined from 1.0 hertz S waves. When this seismogram is filtered with a 0.6 to 1,5 Hertz bandpass, the measured S ratio is 20.0.

  7. The FOCUS is where the land crust starts to slip.

  8. The EPICENTER is the point on the surface of the Earth that is directly above the focus. Here the land would mostly bounce up and down.

  9. There are three main places (or zones) where there will be a lot of Earthquakes. 1. The Pacific Ring of Fire

  10. There are three main places (or zones) where there will be a lot of Earthquakes. • The Pacific Ring of Fire • The Mid-Ocean Ridge

  11. There are three main places (or zones) where there will be a lot of Earthquakes. • The Pacific Ring of Fire • The Mid-Ocean Ridge • Eurasian-Melanseian Mountain belt

  12. Chapter Six Earthquakes 6.2

  13. The energy of an earthquake is carried by seismic waves, and the seismic waves are recorded by a seismograph.

  14. The energy of an earthquake is carried by seismic waves, and the seismic waves are recorded by a seismograph.

  15. The first waves to hit are called “P” waves, or primary waves. • They move through solids or liquids • They move material in the same direction as the wave travels.

  16. The first waves to hit are called “P” waves, or primary waves. • They move through solids or liquids • They move material in the same direction as the wave travels.

  17. The second wave to hit is the “S” wave, or secondary wave. • Second wave to arrive • Wave particles move at right angles to the wave direction.

  18. When “P” waves and “S” reach the surface, they may combine and make “L” waves, or “long”. This may look like two different things. The ground moves left and right, as it moves up and down

  19. When “P” waves and “S” reach the surface, they may combine and make “L” waves, or “long”. This may look like two different things. The ground moves left and right, as it moves up and down It may “roll” like an ocean wave.

  20. Earthquakes are located by triangulation from at least three different seismographs.

  21. Measuring how big an earthquake is. Richter Scale measures how much ENERGY is released by an earthquake. Each number on the scale is 31.7 times as much as the number below it.

  22. Measuring how big an earthquake is. Richter Scale measures how much ENERGY is released by an earthquake. Each number on the scale is 31.7 times as much as the number below it. A “7” on the Richter scale releases 31.7 times as much energy as a “6”, and almost 1,000 times as much energy as a “5”.

  23. Measuring how big an earthquake is. Richter Scale measures how much ENERGY is released by an earthquake. Each number on the scale is 31.7 times as much as the number below it. A “7” on the Richter scale releases 31.7 times as much energy as a “6”, and almost 1,000 times as much energy as a “5”. Anything less than a 2.5 is called a microquake, and no one will even feel it.

  24. Measuring how big an earthquake is. The Mercalli scale measures the intensity of the earthquake, or the amount of damage. II on the Mercalli scale means it was barely noticed XII means that it was “total destruction”

  25. Earthquake Damage 6.3 Or How Not to Die

  26. Things that can go wrong are • Buildings can be built on the wrong type of soil • a. Sand will “liquify” as in Mexico City earthquake of 1985.

  27. Things that can go wrong are • Buildings can be built on the wrong type of soil • a. Sand will “liquify” as in Mexico City earthquake of 1985. • b. Landfill will not support strains, San Francisco 1989

  28. Things that can go wrong are • Buildings can be built on the wrong type of soil • a. Sand will “liquify” as in Mexico City earthquake of 1985. • b. Landfill will not support strains, San Francisco 1989 • c. Poorly built building, most of the world.

  29. Things that can go wrong are • Buildings can be built poorly or on the wrong type of soil • a. Sand will “liquify” as in Mexico City earthquake of 1985. • b. Landfill will not support strains, San Francisco 1989 • c. Poorly built building, most of the world. • Tsunamis • a. means “harbor wave” • b. starts in the ocean floor, always.

  30. Things that can go wrong are • Buildings can be built poorly or on the wrong type of soil • a. Sand will “liquify” as in Mexico City earthquake of 1985. • b. Landfill will not support strains, San Francisco 1989 • c. Poorly built building, most of the world. • Tsunamis • a. means “harbor wave” • b. starts in the ocean floor, always. • c. often the beach will “drain” before the wave, but not always.

  31. Things that can go wrong are • Buildings can be built poorly or on the wrong type of soil • a. Sand will “liquify” as in Mexico City earthquake of 1985. • b. Landfill will not support strains, San Francisco 1989 • c. Poorly built building, most of the world. • Tsunamis • a. means “harbor wave” • b. starts in the ocean floor, always. • c. often the beach will “drain” before the wave, but not always. • d. the SSWWS (Seismic Sea Wave Warning System) tells of tsunamis

  32. Earthquake warnings and predictions Predictions are about as close as “a 50/50 chance of an earthquake in the next 10 years”

  33. Earthquake warnings and predictions • Predictions are about as close as “a 50/50 chance of an earthquake in the next 10 years” • Some indicators are, however, • Ground might tilt a little

  34. Earthquake warnings and predictions • Predictions are about as close as “a 50/50 chance of an earthquake in the next 10 years” • Some indicators are, however, • Ground might tilt a little • Rocks might have a bit more strain on them, might be noticed

  35. Earthquake warnings and predictions • Predictions are about as close as “a 50/50 chance of an earthquake in the next 10 years” • Some indicators are, however, • Ground might tilt a little • Rocks might have a bit more strain on them, might be noticed • Small changes in the Earth’s magnetic field, or electric grounding

  36. Earthquake warnings and predictions • Predictions are about as close as “a 50/50 chance of an earthquake in the next 10 years” • Some indicators are, however, • Ground might tilt a little • Rocks might have a bit more strain on them, might be noticed • Small changes in the Earth’s magnetic field, or electric grounding • Gases in the Earth may seep upwards

  37. Earthquake warnings and predictions • Predictions are about as close as “a 50/50 chance of an earthquake in the next 10 years” • Some indicators are, however, • Ground might tilt a little • Rocks might have a bit more strain on them, might be noticed • Small changes in the Earth’s magnetic field, or electric grounding • Gases in the Earth may seep upwards • Changes in the speed of local “P” waves, from far away earthquakes • a. first they go more slowly • b. then they speed up again fast.

  38. Seismic Gaps are places where faults are “locked” • The fault has not moved for a while, so strain has built up.

  39. Seismic Gaps are places where faults are “locked” • The fault has not moved for a while, so strain has built up. • The longer it is locked, the more strain that is built up.

  40. Seismic Gaps are places where faults are “locked” • The fault has not moved for a while, so strain has built up. • The longer it is locked, the more strain that is built up. • So…as weird as it may seem…the place to look for earthquakes, is where they have not happened for a while

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