6-1    Earthquake:

1. Earthquake:

   • Motion of Earth due to sudden release of energy. Fig. 6.3.

   • Focus: the source of energy release.

   • Epicenter: surface projection of focus.

2. Elastic rebound: Fig 6.5.

   • When force is put on an object, it deforms elastically until failure, which causes rupture.

   • When rupture occurs, energy release suddenly. Feel like "spring back" -- Elastic rebound.

   • A fault: large fracture in Earth's crust.

3. Foreshocks and aftershocks:

   • before and after a major quake.

   • Small magnitude quakes..

6-2    Earthquake waves:

1. Seismology: The science to study earthquake.

2. Seismographs: instruments to record earthquake.

3. Seismograms: a record of earthquake in terms of lines to indicate amplitude. Fig. 6.7

4. Two types of waves:

   • Surface waves: waves travel along Earth's outer layer, not transmitted by liquid.

   • Body waves: travel through the interior.

   1. Primary (P waves) push-pull. Fig 6.9. Change the volume temporarily.

   2. Secondary (S waves) shake. Change the shape of materials.

   • Traveling speed: Fig. 6.8. P fast, S second, surface last.

   • Damage: surface wave is the strongest.

6-3    Epicenter:

On a seismogram, measure the time difference between S and P waves. the larger the difference the farther the epicenter, e.g., Fig. 6.10. Distance can be determined but not directions. To find the direction, 3 stations are needed. Fig. 6.11.

6-4    Intensity and Magnitude:

6-4-1    Intensity: Mercalli intensity scale: I-XII, used to assess the earthquake damages.

6-4-2    Magnitude:

• Is a measure of the amplitude of the largest wave on seismogram.

• Uses logarithmic scale, i.e., increase in 1 magnitude = 10 times increase in amplitude

• 1 magnitude increase = 30 times energy increase.

6-5    Destruction from earthquakes:

6-5-1    Direct destruction: by earthquake waves. It is a function of 

• amplitude

• duration

• structural design and material

6-5-2    Indirect:

• Trunami: strong sea waves produced by earthquake. Magnitude high, but not detected deep in the sea. But once the depth < the magnitude, the wave shows up.

• Landslides and ground subsidence: Liquification: solid materials saturated with water. Upon a shock wave, the solid sudden change into a liquid form.

• Fire:

6-6    Prediction:

Sometimes successful. Other times not.

Long term: maybe more successful, but not exactly a date. Thus, you can't evacuate a city saying a major earthquake in 20 years.

6-7    More Earth's interior:

6-7-1    Lithosphere: Curst +  uppermost mantle. Materials are rigid solid. In crust: continental crust vs. oceanic crust.

6-7-2    Asthenosphere: 100-700 km deep. Materials show fluid behavior, i.e. flow and plastic.

6-7-3    Mohorovicic discontinuity or Moho: Boundary between Crust and Mantle ~ 50 km. Above it, velocity of seismic waves is slow, below it the velocity of seismic waves sudden increase.

6-7-4    Gutenberg boundary: Boundary between Mantel and Outer Core, where P waves speed up.

6-7-5    Shadow zone: about 35 degree belt where no P waves will be detected. Why? Materials difference.

6-7-6    Earth's composition:

• As density of material increases, velocity increases

• At surface, v low, thus material lighter, made of granitic rocks - continental crust.

• At ocean bottom: basaltic composition, a little denser, v faster - oceanic crust.

• Upper mantle: even denser, peridotite results.

• Fe/Ni Core: compared to composition of meteorites, magnetism, compared to average density of the earth.

Homework:

• Read chapter summary on p.177.

• Use your own word to explain the key terms on page 178.

• Answer the review questions on page 178.