1. Earthquakes

2. Global Earthquake Distribution
Earthquakes occur in well-defined zones.
These zones coincide with the edges of lithospheric plates.

3. Depth of Focus Plate boundaries determine the depth of earthquakes.
There can be shallow earthquakes that occur near the surface or deep earthquakes that occur under mountains or volcanoes.

4. Causes of Earthquakes Deformation
An earthquake is any seismic vibration of Earth caused by the rapid release of energy.
Deformation
A strain is deformation in response to a stress.

5. Deformation
Stress is the force per unit area that acts on a material.
(1) compressive stress
(2) a tension stress
(3) a shear stress
(4) torsion stress

6. Elastic Deformation
Elastic deformation occurs when a material deforms as a stress is applied, but returns to its original shape when the stress is removed.
Plastic deformation occurs when a material deforms, or changes shape, as a stress is applied and remains in the new shape when the stress is released.

7. Energy Release
When this strain energy is released suddenly, it causes rock to lurch to a new position.
A fault is a crack along which movement has taken place.
The sudden energy release that goes with fault movement is called elastic rebound.

8. Earthquake Waves
Earthquake waves travel out in all directions from a point where strain energy is released. This point is the focus.
The point on Earth’s surface directly above the focus is the epicenter.

9. Body Waves
Primary waves, also called P-waves, cause particles in a material to undergo a push-pull type motion.
The particles do not permanently change location.
Particles can bump into each other, then primary waves can move through it.
P-waves travel through all kinds of matter.

10. Body Waves
Secondary waves (S-waves) are sometimes called shear waves, because of the relative motion of particles as energy is transferred.
S-waves cause particles to move perpendicular to the direction of wave travel.
S-waves can only travel through solids.

11. Surface Waves Surface waves move in a more complex manner.
They can exhibit an up and down rolling motion, and also a side-to-side motion that parallels Earth’s surface.

12. Earthquake Measurement
The Modified Mercalli scale ranks earthquakes in a range from I-XII,
XII being the worst and uses eyewitness observation and post-earthquake assessments to assign an intensity value.

13. Earthquake Measurement
The Richter magnitude scale uses the amplitude of the
largest earthquake wave.
Richter magnitude is intended to give a measure of the energy released during the earthquake.

14. Levels of Destruction Earthquake Proofing
Research has shown that poor building methods are the largest contributors to earthquake damage and loss of life.
Earthquake Proofing
Although no building can be made entirely earthquake proof, scientists and engineers are finding ways to reduce the damage to structures during mild or moderate earthquakes.

15. Earthquakes 101

16. What’s Inside Earth?
Scientists use seismic waves to learn about Earth’s interior.
Refraction of seismic waves as they pass through Earth provides information about Earth’s structure.

17. Earthquake Observations
A boundary that marks a density change between layers is called a discontinuity
One such discontinuity separates the crust from uppermost mantle, and is known as the Mohorovicic (moh huh ROH vee chihch) discontinuity, or Moho.

18. Shadow Zones
P-waves and S-waves travel through Earth for 105 degrees of arc in all directions.
Between 105 and 140 degrees from the epicenter, nothing is recorded.
This “dead zone” is termed the shadow zone.

19. Shadow Zones

20. Solid Inner Core
The fact that P-waves pass through the core, but are refracted along the way, indicates that the inner core is denser than the outer core and solid.
When pressure dominates, atoms are squeezed together tightly and exist in the solid state.
If temperatures are high enough, atoms move apart enough to exist in the liquid state, even at extreme pressures.

21. Composition of Earth’s Layers
The crust and uppermost mantle, which together form the lithosphere, are made of rocky material—mostly silicates.
The asthenosphere is a weaker, plastic-like layer upon which Earth’s lithospheric plates move.
Mantle below the asthenosphere also is composed of silicates.
The cores are made mostly of metallic material like iron and nickel.

22. Earth’s Layers