1. Loma Prieta Earthquake
Earthquakes (Ch )
Loma Prieta Earthquake
October 17th 1989

2. What is an Earthquake?
Vibrations of the ground, caused by movement along fractures in the Earth’s crust.
Earthquakes occur when too much stress builds up and the rock gets strained.

3. Stress and Strain
Stress is the forces placed on rocks by the pressures of plate motion.
Tensional stress pulls the rocks apart.
Compressional stress pushes them together.
Shear stress forces rocks to the side (like scissors)

4. Elastic Limit
As stress builds up, rocks store energy.
Stronger rocks can store more energy.
The elastic limit is the maximum stress that can be stored before the rock ruptures.

5. Earthquakes and Plate Tectonics
Stress builds up along plate boundaries, as the forces in the mantle put pressure on the plates.
The types of stress relate to the types of boundary.
When the rock finally breaks, long cracks form in the crust. These are called faults.

6. Types of Earthquake Faults
There are three basic types of faults
1. Normal Faults
2. Reverse Faults
3. Strike-slip Faults

7. Normal Faults
form as a result of tensional forces pulling the crust apart
hanging wall shifts lower
often occur at divergent plate boundaries
Found across Nevada, and in the African Rift Valley

8. Reverse Faults (also known as Thrust Faults)
form as a result of compressional forces
hanging wall shifts higher
often occur at convergent plate boundaries

9. Example of a reverse Faults – one rock layer pushed over another rock layer
Reverse faults are steep!

10. Strike-Slip Fault formed as a result of shearing forces
movement along strike slip faults is mostly horizontal
often occur at Transform boundaries
San Andreas Fault
right strike-slip fault
(not a left strike-slip fault)

11. Earthquake Focus and Epicenter
The Focus is the point of strain at which the earthquake originates!
The focus is always beneath the surface.
The earthquake’s focus can be shallow or deep.
The epicenter is on Earth’s surface, directly above the focus.

12. Deep and Shallow Earthquakes
Deep earthquakes occur in subduction zones
Trenches can be deeper than Mt. Everest is high!

13. Shallow earthquakes occur at transform boundaries or in rift zones at diverging boundaries.

14. Seismic Waves When the fault ruptures, energy is released
This energy moves outward in the form of seismic waves
“seismos” is Greek for earthquake

15. Types of Seismic Waves
P-Waves – Primary waves. Rock moves parallel to direction of wave motion.
S-Waves – Secondary waves. Rock moves perpendicular to direction of travel.
Surface Waves – Circular waves traveling along Earth’s surface Very destructive.

16. P-Waves Fastest (450+ km/min.) therefore the first to arrive
Push-Pull motion of the rock
also called “longitudinal” motion
Move though solids, liquids, even gases

17. S-Waves Slower (240 km/min) so they show up second
Up-and-Down motion of the rock
also called “transverse” motion
Only move through solids

18. Clues to Earth’s Interior using Seismic Waves
S-waves travel through solids only, so when they hit the liquid core they are absorbed. This shows us the outer core is liquid.
P waves speed up when they hit the inner core, which supports the idea that the inner core is solid.